1. The lobula giant movement detector neurone (LGMD) and its post-synaptic cell, the descending contralateral movement detector neurone (DCMD), show a preference for small-rather than large-area stimuli within their 180° receptive field. Intracellular recordings from LGMD show that two different neural mechanisms are responsible.

  2. Large-field moving stripes cause little decrement in the labile afferent synapses to LGMD. The response to a small moving target is progressively inhibited by large-field moving stripes when these are closer than ca. 20° and moving stripes produce progressively more excitatory response when their area is limited to 40° or less. These observations indicate a phasic lateral inhibitory network between the afferent excitatory pathways to subfield A of LGMD. The responsible units have not been identified, but are probably in the proximal medulla or distal lobula. This network reduces input to LGMD during sustained large-field stimulation and, in addition, protects the decrement-prone small-field neurones from the potentially fatiguing effects of whole-field movements.

  3. Large-field stimuli also produce long-lasting compound IPSPs in LGMD, which limit or prevent spiking. Separate and different IPSPs are generated by ON and OFF stimuli. The neurones mediating OFF IPSPS have been identified anatomically and their function confirmed by selective lesion. Their input dendrites cover approximately 8° × 12°of the proximal face of the medulla. The axons form a dorsal uncrossed bundle (DUB) above the second chiasma, and terminate on the dorsal lobe of the lobula, apparently synapsing on dendritic subfield C of LGMD.

  4. The ON and OFF inhibitory inputs form ‘feed-forward’ inhibitory loops around the ON/OFF excitatory afferents. ‘Feed-forward’ inhibition is necessary in addition to lateral inhibition to prevent a large excitatory response to whole-field transients. Lateral inhibition is necessary in addition to feedforward inhibition to protect the small-field excitatory afferents from depression. This combination may be widespread in sensory systems and could provide a basis for saccadic suppression.

  5. The response of LGMD to a variety of visual stimuli is analysed and shown to be adequately explained as the resultant of small-field excitatory and lateral-and feed-forward inhibitory inputs.

In this paper we deal with one of the most striking features of the movement detector (MD) system (Rowell, 1971) - its preference for stimuli which affect only small areas of the retina. Stimuli which affect all or much of the retina simultaneously evoke only very small responses in the interneurones of the MD system ; stimuli of intermediate size, and those which affect large areas of the retina, but not absolutely synchronously, evoke intermediate levels of response (Palka, 1967 a, 1969, 1972; Hom & Rowell, 1968; Rowell, 1971). The most obvious biological result of this arrangement is that the responses of the MD system increases when the size of the moving target is enlarged to a critical dimension but is suppressed by further enlargement. Changes in the retina brought about by the animal’s own movements (Palka, 1967; Rowell & Horn, 1968), or alterations in ambient illumination, therefore, produce little or no response (Palka, 1967a; Rowell & O’Shea, 1976).

The mechanism of this discrimination has been considered by Palka (1967 a, 1969) and by O’Shea & Rowell (1975 a) with slightly different results, which we now summarize. The most obvious way in which an array of similar afferent channels can discriminate against large-field stimuli is by lateral inhibition between the different channels. The hypothesis of lateral inhibition was therefore the first selected for investigation by Palka (1967 a). He rejected it after demonstrating that the inhibitory effect between two stimuli on the retina was independent of the distance separating them. This is contrary to the effect predicted by a classic lateral inhibition circuit, in which the inhibitory effect of a nearby stimulus is much greater than that of a distant one. Intracellular records have subsequently shown (O’Shea & Rowell 1975 a) that this result was misleading. The light sources used by Palka to produce the stimuli emit some non-directional light, and the eye was in the dark adapted state. Adaptation is so efficient in this eye (Rowell & O’Shea, 1976a) that the onset or offset of non-directional light is in effect a large-field stimulus. More recently we found that such stimuli produce inhibition of the MD neurones mediated by IPSPs in the lobula giant movement detector (LGMD) (O’Shea & Rowell, 1975 a; and this paper), which masks any other effect (lateral inhibition, for example). Palka, working with extracellular techniques, correctly deduced the presence of this second type of inhibition, and discarded the lateral inhibition hypothesis.

Our intracellular recordings from the input fan-like arborization (dendritic subfield A, O’Shea & Williams, 1974) of the LGMD revealed that large-field stimuli elicited IPSPs, which appeared to be the basis of the inhibition described by Palka. As, however, the locus of response decrement to a repeated stimulus precedes the formation of the IPSPs (see O’Shea & Rowell, 1975 a; O’Shea & Rowell, 1976), although the IPSP might prevent the LGMD from spiking in response to whole-field stimuli, it cannot explain all the properties of the preference for small-field stimuli. This is because with the IPSP alone, large-field stimuli might not cause spikes butj would continue to excite the decrementing sites and therefore depress the responsiveness of the system to subsequent small-field stimuli. In fact, large-field stimuli do depress responsiveness slightly, but much less than even a few small-field stimuli would do. We therefore reconsidered lateral inhibition and showed (O’Shea & Rowell, 1975 a) that such a network does precede the labile afferent synapses on to the LGMD, and that it is primarily this which discriminates against large-field stimuli.

The available evidence indicates that the lateral inhibitory network seems to be fed by phasic, ON/OFF units and is not activated at all during tonic illumination. This characteristic distinguishes it from the other, more peripheral, lateral inhibitory network which we have already described from this system (Rowell & O’Shea, 1976b). Given that the area is greater than the optimal size for that particular ΔI (Palka, 1967 a), increasing the area progressively inhibits response up to a maximum of between 30 and 40°. Stimuli affecting areas larger than 40°are all rejected maximally. The units responsible for mediating this second, deeper, lateral inhibition have not yet been identified.

The subsequent part of this paper deals with the IPSPs generated by large-field stimuli in the LGMD, and which are the basis of the inhibition studied in detail by Palka (1967 a, 1969, 1972). We believe the occurrence of this inhibition is a necessary consequence of two conditions: the apparent design requirement that the system discriminate totally against large-field stimuli, and the inherent shortcomings of a classical lateral inhibitory network for accomplishing this. The problem is that lateral inhibition discriminates efficiently against sustained large-field stimuli (given appropriate time constants and extent of the inhibitory projections). In its simplest form (Fig. 1 A), however, it cannot discriminate against large-field transients, such as those which occur during a sudden change in ambient illumination, or at the start of movement of the animal. This is because transmission time to the inhibitory synapse is at least equal to that to the excitatory synapse. The situation is made worse where, as is usually the case, lateral inhibition is mediated not by collateral branches of the afferent neurones but by interneurones (Fig. 1B). To block transient excitation it is necessary to employ a feed-forward loop, by which the inhibitory signal can be made to arrive at the summing point simultaneously with, or even slightly before, the excitatory signal. In principle, this feed-forward arrangement could be made part of the lateral inhibitory network (Fig. 1C), but this requires that the inhibitory signal is derived at two synapses prior to the convergence of the retinotopic projection. An alternative arrangement, which we believe to be found in the MD system, is to leave the lateral inhibition network in its simpler form and to add a separate feed-forward loop which starts one synapse prior to the lateral inhibition circuit and causes IPSPs in the LGMD after the point of convergence of the afferent excitatory synapses (Fig. 1D).

Fig. 1.

Diagrams to show possible circuits mediating lateral inhibition between an array of similar afferents converging on to a fan-shaped interneurone, such as the LGMD. (A) Simple lateral inhibitory network, no interneurones involved in inhibitory connections. (B) Simple lateral inhibitory network, inhibition mediated by interneurones, rather than by collaterals. (C) Lateral inhibitory network incorporating feed-forward inhibition. (D) Simple lateral inhibitory network, with separate feed-forward inhibitory loop around it. (A) and (B) do not discriminate against whole-field transient stimuli, whereas (C) and (D) do. Further discussion in the text. Filled circles, excitatory chemical synapses. Open triangles, inhibitory chemical synapses.

Fig. 1.

Diagrams to show possible circuits mediating lateral inhibition between an array of similar afferents converging on to a fan-shaped interneurone, such as the LGMD. (A) Simple lateral inhibitory network, no interneurones involved in inhibitory connections. (B) Simple lateral inhibitory network, inhibition mediated by interneurones, rather than by collaterals. (C) Lateral inhibitory network incorporating feed-forward inhibition. (D) Simple lateral inhibitory network, with separate feed-forward inhibitory loop around it. (A) and (B) do not discriminate against whole-field transient stimuli, whereas (C) and (D) do. Further discussion in the text. Filled circles, excitatory chemical synapses. Open triangles, inhibitory chemical synapses.

In this paper we elucidate the circuit responsible for the IPSPs in the LGMD and show that it conforms to the model shown in Fig. 1D. There are actually two separate loops which produce feed-forward inhibition around the phasic lateral inhibitory network, and so effectively limit the response of the MD system to large-field transient excitation. Both ON or OFF stimuli produce IPSPs in the LGMD, and the channels mediating these two IPSPs are separate. The neurones mediating the OFF IPSPS have.been identified histologically and their role confirmed by selective lesion. They probably synapse with dendritic subfield C (see O’Shea & Williams, 1974) and their input dendritic fields are located on the inner face of the medulla. Together with the postulated phasic lateral inhibitory circuit, these two (ON and OFF) feed-forward inhibitory loops produce the selective discrimination of the MD neurones against both transient and sustained large-field movement stimuli.

The majority of experiments used Schistocerca nitens (Thunberg), from laboratory culture, and a few used S. americana gregaria (Forskå l), obtained from a commercial supplier. Intracellular recordings were made from the LGMD neurone, usually in dendritic subfield A (O’Shea & Williams, 1974), using electrodes filled with 3 M potassium acetate, as previously described (O’Shea & Rowell, 1976). Extracellular records of the DCMD neurone were obtained from the ventral nerve cord with hook electrodes, and the spike potentials discriminated and counted on the basis of amplitude by a variable window discriminator. Experiments were routinely tape recorded for further analysis.

ON and OFF stimuli were generated by a variety of light sources, including: (a) electronic flash (GEC strobelight) and (b) a light emitting diode (Hewlett Packard), emitting predominantly in the green (in both of these cases the change in light intensity was effectively instantaneous) ; (c) a compur-type shutter, actuating time a few milliseconds ; (d) tungsten filament indicator bulbs, controlled by a transistor switch in the supply (the thermal capacity of the filament slowed the rate of change to 20– 25 ms to 50% value on both ON and OFF) and (e) fluorescent room lighting; here the characteristics of the phosphor luminescence gave 18 ms to half light value at OFF, and about 100 ms on ON. All of these stimuli were highly effective, and the variation in rise time proved unimportant, except when precise latency measurements were required. Light intensity was controlled in various ways, including (a) crossed-polaroid filters followed by a diffuser, (b) distance of light source from diffuser, (c) neutral density filters, and (d) variation in filament voltage. The latter, which results in changes of spectral composition, produced the same results as the former, which do not. Changes in light intensity were monitored with a silicon photocell, and measured with a Tektronix digital photometer. In all experiments, the animal was light adapted; the surfaces viewed by the eye were generally of around 30 cd/m2 in luminance (zero log units).

Moving targets, moving and stationary stripes, and similar stimuli were presented by back projection on to a white perimeter screen formed by a portion of a tabletennis ball surrounding the eye. The other eye and the ocelli were covered with opaque black wax. Movement of the projected patterns was derived from a pen recorder mechanism, which moved opaque templates in the focal plane of the projector. The size of the projected field could be controlled by a diaphragm. In earlier experiments, the pen recorder mechanism was used to move larger patterns which the animal viewed directly by reflected light. The pen-recorder mechanism was driven by a triggered low-frequency waveform generator. Repetitive stimuli were controlled by a digital timing circuit.

(A) The responses to the first group of ten presentations of the single stripe, at an interstimulus interval of 10 s. (B) The response to the first replicate of this procedure, after a rest interval of 600 s in which there were no visual stimuli. Both curves show typical decremental responses, reading zero response around the 10th presentation. Previous work (e.g. Horn & Rowell, 1968 ; Rowell & Horn, 1968) shows that further stimulus presentations would maintain indefinitely a response level at or close to zero (mean level less than one spike/response), unless the animal’s arousal state changed and the response was dishabituated.

After the second group of stimuli the animal saw a moving stripe pattern continuously for 600 s, and immediately afterwards a second replicate of the small-field stimulus procedure was given. The response to this is shown in (C). The continuously moving stripes provide a very large number of light/dark transitions to the individual retinal cell (calculated to total 9600 over 600 s). If even a significant fraction of these stimuli had excited the labile synapses on the LGMD which mediate habituation in this system (O’Shea & Rowell, 1976), then their responsiveness would have been maintained at the minimal level it reached at the end of the second group of small-field stimuli. The third group would be expected to elicit zero response, as the synapses would be totally fatigued. Instead, (C) shows that significant, though not total, recovery of responsiveness has taken place during the 600 s of large-field stimulation. This shows that little excitation of the labile synapses occurred during this period.

(1) Response to small- and large-field stimuli

The differences in response to these two categories of stimulus have been extensively described (Palka, 1967a; Horn & Rowell, 1968; Rowell, 1971; O’Shea & Rowell, 1975a, 1976; Rowell & O’Shea, 1976a) and are summarized here. Rapid change (either positive or negative) in illuminance of a small area of the retina produces EPSPs, in the dendritic fan of the LGMD (subfield A of O’Shea & Williams, 1974), and action potentials, both in its axon and that of the descending contralateral movement detector (DCMD) neurone to which it is connected by a spike-transmitting electrical synapse (O’Shea & Rowell, 1975 b) (Fig 2). Movement of small contrasting objects in the visual field produces a succession of such small stimuli, and the response of the neurone is accordingly enhanced and prolonged by summation. The response decreases with repetition, the size of the PSPs wanes and the number of spikes falls eventually to nil (O’Shea & Rowell, 1976). By contrast, changes in illumination affecting simultaneously all of the retina, or large parts of it, produce few or no spikes and movement of stripe patterns which excite all or most of the retina typically cause only a weak response at the onset of the movement. The same visual effect and neurophysiological outcome is produced by voluntary or passive movement of the animal within a structured visual environment (Palka, 1967 b, 1969, 1972; Rowell & Horn, 1968; Rowell, 1971 a).

(2) Large-field stimuli do not decrease the response to subsequent small-field stimuli

After exposure to large-field moving stripes for periods of up to 0·5 h, the response of the LGMD to a test movement of a small contrasty target is relatively little diminished (Fig. 3), showing that the labile synapses on to the LGMD which mediate habituation in the system (O’Shea & Rowell, 1976) have been excited little by the moving stripes.

Fig. 3.

Prolonged exposure to large-field moving stripe patterns does not greatly diminish the response to a subsequent small-field stimulus. The DCMD axon, which transmits 1:1 spikes initiated in the LGMD, was recorded in the thoracic nerve cord. The visual field of the eye was restricted to 90°in the vertical plane and 180° in the horizontal (i.e. a quarter of a sphere). The small-field stimulus was a vertical black stripe, subtending 15°at the eye, moved once horizontally to and fro over 2· 5 s. The large-field stimulus consisted of an array of such stripes, the field including approximately six periods. It was moved to give a contrast frequency of 16 Hz. The animal was shown three groups of small-field stimuli each totalling 100 s, separated by two longer periods of 600 s each. The first long period was a rest interval, in which no visual stimuli were presented. In the second long period the animal was continuously exposed to the moving stripe field. This arrangement and its optical consequences are summarized in tabular form below the figure.

Fig. 3.

Prolonged exposure to large-field moving stripe patterns does not greatly diminish the response to a subsequent small-field stimulus. The DCMD axon, which transmits 1:1 spikes initiated in the LGMD, was recorded in the thoracic nerve cord. The visual field of the eye was restricted to 90°in the vertical plane and 180° in the horizontal (i.e. a quarter of a sphere). The small-field stimulus was a vertical black stripe, subtending 15°at the eye, moved once horizontally to and fro over 2· 5 s. The large-field stimulus consisted of an array of such stripes, the field including approximately six periods. It was moved to give a contrast frequency of 16 Hz. The animal was shown three groups of small-field stimuli each totalling 100 s, separated by two longer periods of 600 s each. The first long period was a rest interval, in which no visual stimuli were presented. In the second long period the animal was continuously exposed to the moving stripe field. This arrangement and its optical consequences are summarized in tabular form below the figure.

(3) Lateral inhibition

The above observations are consistent with the hypothesis that a lateral inhibition network between the retinotopic afferent pathways to the LGMD precedes the final synapse to the LGMD (see also Introduction). We tested this hypothesis by the following experiment (Fig. 4A). The animal saw in its visual field two horizontal bands of black and white vertical stripes, of period (λ) 15°. Each band subtended 22·5° vertically (i.e. the two together composed one quarter of the entire visual field). The separation between the two bands could be altered from 7·5° to 22·5°. When these bands were oscillated laterally at a speed corresponding to 7 transitions s− 1 at the individual receptor (i.e. contrast frequency 7 Hz), little or no spike activity was generated in the LGMD. Altering the separation of the bands had no effect on this result. When, however, a black square was added to the horizontal space between the two bands of stripes, and moved with them, the response was greatly increased. The size of the response increased with increasing separation of the stripe bands (Fig. 4A). This is to be expected from a classical lateral inhibitory network, where the inhibitory effect of activity elsewhere in the array is a function of its distance from the point under consideration.

Fig. 4.

Evidence for lateral inhibition in the input to LGMD.

(A) Inhibitory effect of moving stripes on the response to a moving target, and its dependence on spatial proximity. The animal saw the pattern shown on the left against an otherwise featureless visual field. The stripes extended laterally to cover the entire horizontal extent of the field. The whole pattern was moved to give a contrast frequency of 7 Hz in the striped region, which is slow enough to produce little or no postsynaptic inhibition in the LGMD (see Fig. 10). The separation of the two bands of stripes, and thus their proximity to the target, could be altered. The results (right) were obtained in a series in which the ISI was 3 min, and the smallest response was evoked first, and the largest last; this order ensures that response decrement due to repetition is not confused with the effect of spatial separation. For further explanation see text.

(B) Dependence of the excitatory component of the response to a moving stripe pattern on the area of the retina affected. The animal saw a pattern of stripes through a window, the diameter of which could be varied. At low contrast frequencies, the onset of stripe movement causes EPSPs and spikes in the LGMD (see also Fig. 11 and Table 2), but no IPSPs. The size of the excitatory response, here measured by spikes in the DCMD, is a function of the area of the retina affected by the moving stripe pattern. Inhibition reaches its maximum level with field sizes between 30 and 40° Data for two separate animals are shown as triangles and circles. Closed symbols are the values derived from the transient burst at the start of the movement; open symbols incorporate occasional spikes generated during the later part of the movement.

Fig. 4.

Evidence for lateral inhibition in the input to LGMD.

(A) Inhibitory effect of moving stripes on the response to a moving target, and its dependence on spatial proximity. The animal saw the pattern shown on the left against an otherwise featureless visual field. The stripes extended laterally to cover the entire horizontal extent of the field. The whole pattern was moved to give a contrast frequency of 7 Hz in the striped region, which is slow enough to produce little or no postsynaptic inhibition in the LGMD (see Fig. 10). The separation of the two bands of stripes, and thus their proximity to the target, could be altered. The results (right) were obtained in a series in which the ISI was 3 min, and the smallest response was evoked first, and the largest last; this order ensures that response decrement due to repetition is not confused with the effect of spatial separation. For further explanation see text.

(B) Dependence of the excitatory component of the response to a moving stripe pattern on the area of the retina affected. The animal saw a pattern of stripes through a window, the diameter of which could be varied. At low contrast frequencies, the onset of stripe movement causes EPSPs and spikes in the LGMD (see also Fig. 11 and Table 2), but no IPSPs. The size of the excitatory response, here measured by spikes in the DCMD, is a function of the area of the retina affected by the moving stripe pattern. Inhibition reaches its maximum level with field sizes between 30 and 40° Data for two separate animals are shown as triangles and circles. Closed symbols are the values derived from the transient burst at the start of the movement; open symbols incorporate occasional spikes generated during the later part of the movement.

If the stripes are stationary, and the small black square moved as before, there is no inhibitory effect. This result indicates that the inhibitory effect is derived from a phasic, not a tonic, signal as the stripes only exert their inhibitory effect when they move.

A similar result was obtained from an experiment in which the animals saw moving stripes (λ 20°) through a circular window of varying aperture. If the stripes moved slowly (CF = 6 Hz,) an excitatory response was seen in the LGMD at the onset of movement. This response increased as the diameter of the window decreased (Fig. 4B). Whole-field stimuli gave very little response. The first increase in response came when the window was reduced from 45° to 30° The response thereafter increased progressively down to a window diameter of 5° the smallest value tested. (At higher contrast frequencies other effects mask the response described here. This experiment is described in more detail in Section 6, iv.) This result shows that the responsiveness of an element of the retinal projection is inhibited by the activity of others up to 15– 22°away from it.

(4) Components of the response to large-field stimuli

Intracellular records from the fan dendrites of the LGMD show only very weak EPSPs during stimulation with slowly moving large-field stripe patterns. By contrast, the response to a transient large-field stimulus involves major and very different intracellular events. Such stimuli include large-field ON and OFF, or movement of large areas of stripes at high contrast frequency. Such stimuli evoke an initial compound EPSP, often leading to a spike and, almost simultaneously a large compound IPSP, which abruptly curtails the excitatory potentials (Fig. 5). The latency of both components decreases with increasing stimulus (either ΔI or area), from around 50– 60 ms (at threshold) to around 25 ms (with powerful, whole-field flash stimuli). The latency of the IPSP is about 41– 42 ms at a stimulus intensity at which the EPSP would have a latency of about 36 ms. The IPSP amplitude is a function of stimulus amplitude (see below) and area. The EPSPs elicited by large-field stimuli will be considered in more detail elsewhere ; the IPSP which follows them ensures that they do not give rise to more than one to very few spikes (two in Fig. 5). The rest of this paper concentrates on the IPSP. The recorded size of the IPSP varies with electrode placement, being smallest when the electrode is most distal in the fan. In normal recordings near the base of the fan, the maximal IPSPs are about 15 mV in amplitude. They will stop a spike train generated by a small moving target (see Fig. 2, O’Shea & Rowell, 1975 a), or by a whole-field stimulus. When they are experimentally abolished the excitatory component of the large-field response generates many spikes (Fig. 12 and Section 7 below).

Fig. 2.

Basic response of the LGMD to small-field stimuli, as recorded intracellularly in the LGMD fan (subfield A). A green light emitting diode subtending 14 ′ of arc at the eye was switched ON and then OFF after 400 ms. Upper trace, intracellular, LGMD ; lower trace, extracellular from ventral nerve cord showing DCMD spikes following 1:1 those of the LGMD. Superimposed on the extracellular recording is the voltage applied to the LED.

Fig. 2.

Basic response of the LGMD to small-field stimuli, as recorded intracellularly in the LGMD fan (subfield A). A green light emitting diode subtending 14 ′ of arc at the eye was switched ON and then OFF after 400 ms. Upper trace, intracellular, LGMD ; lower trace, extracellular from ventral nerve cord showing DCMD spikes following 1:1 those of the LGMD. Superimposed on the extracellular recording is the voltage applied to the LED.

Fig. 5.

Intracellular correlates of whole-field, instantaneous stimuli. Intracellular recording from the LGMD fan. The animal’s eye was placed at the centre of a hemispherical white diffuser. At the point marked by the double arrowhead the diffuser in front of the eye was illuminated by the discharge of an electronic flash tube. After 35 ms latency, a large compound EPSP rises suddenly from the baseline and gives rise to two spikes ; it is succeeded by a large compound IPSP, lasting some 300 ms, the onset of which abruptly terminates spiking. (The negative going deflexions on the trace were caused by the injection of hyperpolarizing current in an (unsuccessful) attempt to demonstrate a permeability change during the IPSP.)

Fig. 5.

Intracellular correlates of whole-field, instantaneous stimuli. Intracellular recording from the LGMD fan. The animal’s eye was placed at the centre of a hemispherical white diffuser. At the point marked by the double arrowhead the diffuser in front of the eye was illuminated by the discharge of an electronic flash tube. After 35 ms latency, a large compound EPSP rises suddenly from the baseline and gives rise to two spikes ; it is succeeded by a large compound IPSP, lasting some 300 ms, the onset of which abruptly terminates spiking. (The negative going deflexions on the trace were caused by the injection of hyperpolarizing current in an (unsuccessful) attempt to demonstrate a permeability change during the IPSP.)

Fig. 12.

Anatomical basis of the OFF IPSP.

(A) Diagrammatic frontal section of the optic lobe in the area of the second optic chiasma. M, Medulla; LO, lobula; POC, fibres running in posterior optic commissure. A recording microelectrode is placed intracellularly in the fen of the LGMD. Chiasma afferents leave the proximal face of the medulla and insert on the receiving face of the lobula. Additionally, the axons of some 500 other neurones, each with 8° × 12° dendritic fields in the proximal face of the medulla, run upwards across the chiasmatic projection and form a dorsal uncrossed bundle, which runs slightly anterior to the posterior optic commissure and ramifies within the dorsal lobe of the lobula. This area of neuropile also contains the dorsal dendritic field of the LGMD (subfield C of O’Shea & Williams, 1974). In some experiments, a lesion (×) was made in the dorsal surface of the optic lobe at the level of the chiasma, severing the dorsal uncrossed bundle, and usually some fibres of the posterior optic commissure as well.

(B) Recordings from LGMD fan prior to lesion. Top trace, LGMD fan, intracellularly; middle trace, ventral nerve cord, extracellularly, showing DCMD spikes; lower trace, photocell output – the 60 Hz ripple indicates that the fluorescent room lights are on. Whole field ON and OFF stimuli are generated by turning the room lights on and off; the stimulus amplitude is about 0·5 log10 unit. Both stimuli elicit EPSPs which generate one or a few spikes (see also Fig. 10), and subsequently large compound IPSPs which curtail spiking.

(C) Extracellular recording after lesion to dorsal uncrossed bundle, showing DCMD spikes. Before this record, the preparation was rested for 10 min in order to show a relatively unhabituated response. Compared with the control the OFF response consists of an unprecedented large burst (whole-field stimuli rarely elicit more than two or three spikes at most), whereas the ON response is normal.

(D) As in (C), but this time with an intracellular record from the LGMD as well. In this case, the necessity of holding the penetration does not allow so long a rest interval since the last stimulus, and the response is therefore less than in (C), though still much larger than the control. Note that the lesion has virtually abolished all the IPSPs from the OFF response, and instead a complex series of EPSPs are revealed, comparable in duration to the spike burst seen in the unfatigued preparation shown in (C). The ON response is normal, with the IPSP intact as in the control.

These experiments show that the OFF and ON IPSPS are mediated by independent pathways ; that the OFF IPSP is probably mediated by the uncrossed dorsal bundle; and that the IPSPs serve to limit the large response to whole-field stimuli which would otherwise occur.

Fig. 12.

Anatomical basis of the OFF IPSP.

(A) Diagrammatic frontal section of the optic lobe in the area of the second optic chiasma. M, Medulla; LO, lobula; POC, fibres running in posterior optic commissure. A recording microelectrode is placed intracellularly in the fen of the LGMD. Chiasma afferents leave the proximal face of the medulla and insert on the receiving face of the lobula. Additionally, the axons of some 500 other neurones, each with 8° × 12° dendritic fields in the proximal face of the medulla, run upwards across the chiasmatic projection and form a dorsal uncrossed bundle, which runs slightly anterior to the posterior optic commissure and ramifies within the dorsal lobe of the lobula. This area of neuropile also contains the dorsal dendritic field of the LGMD (subfield C of O’Shea & Williams, 1974). In some experiments, a lesion (×) was made in the dorsal surface of the optic lobe at the level of the chiasma, severing the dorsal uncrossed bundle, and usually some fibres of the posterior optic commissure as well.

(B) Recordings from LGMD fan prior to lesion. Top trace, LGMD fan, intracellularly; middle trace, ventral nerve cord, extracellularly, showing DCMD spikes; lower trace, photocell output – the 60 Hz ripple indicates that the fluorescent room lights are on. Whole field ON and OFF stimuli are generated by turning the room lights on and off; the stimulus amplitude is about 0·5 log10 unit. Both stimuli elicit EPSPs which generate one or a few spikes (see also Fig. 10), and subsequently large compound IPSPs which curtail spiking.

(C) Extracellular recording after lesion to dorsal uncrossed bundle, showing DCMD spikes. Before this record, the preparation was rested for 10 min in order to show a relatively unhabituated response. Compared with the control the OFF response consists of an unprecedented large burst (whole-field stimuli rarely elicit more than two or three spikes at most), whereas the ON response is normal.

(D) As in (C), but this time with an intracellular record from the LGMD as well. In this case, the necessity of holding the penetration does not allow so long a rest interval since the last stimulus, and the response is therefore less than in (C), though still much larger than the control. Note that the lesion has virtually abolished all the IPSPs from the OFF response, and instead a complex series of EPSPs are revealed, comparable in duration to the spike burst seen in the unfatigued preparation shown in (C). The ON response is normal, with the IPSP intact as in the control.

These experiments show that the OFF and ON IPSPS are mediated by independent pathways ; that the OFF IPSP is probably mediated by the uncrossed dorsal bundle; and that the IPSPs serve to limit the large response to whole-field stimuli which would otherwise occur.

(5) Dual nature of IPSPs

ON and OFF large-field stimuli both generate IPSPs, but these IPSPs differ from each other. The physiological differences between the ON and OFF IPSPs are as follows.

(i) Time course

Except at very low stimulus amplitudes (see below), the ON IPSP is usually larger than the OFF, and its individual component IPSPs are usually less apparent. Basically, however, the two potentials are very similar in appearance. The latencies of both IPSPs vary with stimulus intensity and do not differ from each other significantly. In the ON IPSP, however, the initial hyperpolarization is followed by a long period in which synaptic noise is reduced (Fig. 6A) and spiking can be inhibited (Fig. 6B). This second phase is probably due to an increase in the permeability of the postsynaptic membrane to an ion with an equilibrium potential close to the resting potential (e.g. Cl ), or it might indicate presynaptic inhibition of the afferents. This second phase is absent from the OFF IPSP.

Fig. 6.

(A) A slow-speed record to show the long ‘silent period’ which follows the ON IPSP with relatively high stimulus intensities, during which time synaptic noise is reduced. Four successive stimuli to the same preparation. The upper trace is an intracellular recording from the LGMD fan and the lower is the output of a photo cell. Note that the ON IPSP suppresses spike initiation in three of the four records (small arrowheads), but allows for the generation of one spike in the second record.

(B) The ‘silent period’ corresponds to a period of inhibition which persists after the end of the large IPSP. Three successive records from the same preparation. In all there is a spiking response to the movement of a small target (bottom trace is the movement analogue voltage). The to-and-fro movement of the target produces a large initial burst of spikes and a smaller burst as the target returns. In the second example a powerful flash was delivered to the eye. This not only elicits a large IPSP but also suppresses spiking during the second or return phase of the target movement. In the third example a weaker flash delivered rather earlier stops spiking for the duration of the IPSP only.

Fig. 6.

(A) A slow-speed record to show the long ‘silent period’ which follows the ON IPSP with relatively high stimulus intensities, during which time synaptic noise is reduced. Four successive stimuli to the same preparation. The upper trace is an intracellular recording from the LGMD fan and the lower is the output of a photo cell. Note that the ON IPSP suppresses spike initiation in three of the four records (small arrowheads), but allows for the generation of one spike in the second record.

(B) The ‘silent period’ corresponds to a period of inhibition which persists after the end of the large IPSP. Three successive records from the same preparation. In all there is a spiking response to the movement of a small target (bottom trace is the movement analogue voltage). The to-and-fro movement of the target produces a large initial burst of spikes and a smaller burst as the target returns. In the second example a powerful flash was delivered to the eye. This not only elicits a large IPSP but also suppresses spiking during the second or return phase of the target movement. In the third example a weaker flash delivered rather earlier stops spiking for the duration of the IPSP only.

(ii) Stimulus intensity /response amplitude

With increasing intensity (i.e. increasing ΔI, constant area) the OFF IPSP shows a rather sharp threshold, appearing abruptly at almost full amplitude and showing little change thereafter (Fig. 7 A, D). The ON IPSP is first seen at the higher stimulus intensities and rises in a graded manner with increasing stimulus amplitudes, eventually exceeding the OFF IPSP (Fig. 7B, D). Some gradation of response can be seen in the slow component of the ON IPSP. Thus for ΔI greater than about 0· 4 log10 units, the ON IPSP is larger, whereas at lower stimulus intensities the reverse is true. This is best seen by examination of the ratios of paired ON and OFF IPSPS (Fig. 7C).

Fig. 7.

Dependence of amplitude of IPSP on stimulus intensity (ΔI). The animal’s eye was placed at the centre of a translucent white hemispherical diffuser, illuminated by a tungsten filament light source. ON and OFF stimuli were produced by switching the supply to the source. IPSPs were recorded from the fan of the LGMD. (In the plots shown below, filled circles indicate points derived from 18 brightening or positive pulses stimuli, and open circles indicate points derived from 18 negative or dimming pulses. The former were given later in the experiment, when recording conditions were at their best, and for this reason tend to be clustered at the upper extreme of the range of variation in the first two plots.)

OFF IPSP. Mean values show a small decrease in IPSP amplitude at stimulus intensities less than about 0-5 log10 units, but this is largely due to an extension of the range of variation downwards. The regression line plotted through this subset of the data points shows a slight slope, but this is barely significant (r = 0· 28, n = 52, P = 0·05). At lower light intensities (see (D) below) the OFF IPSP shows a sudden but very variable threshold. Over the range shown here, low stimulus intensities merely increase the variance. Above about 0·5 log10 units, there is no change in amplitude; the × indicates the mean of readings for 0·76 log10 unit stimuli.

ON IPSP. The variance is again larger at low stimulus intensities, but here there is a clear linear relationship between intensity and amplitude, (r = 0·2, P < 0.·001). The absolute threshold is higher than for OFF IPSPS (see D below). Above about 0·5 log10 units, there is again no change in response amplitude.

(C) The ratio between ON and OFF IPSPs obtained from individual trials eliminates the variance brought about by changing recording conditions during’the experiment; this is indicated in this plot by the fact that the difference between the brightening and dimming pulse data (see above) is now gone. The plot indicates that the ON and OFF IPSPS are recorded at about equal amplitude for stimuli of about 0·4 log10 units ; at higher intensities, the ON response is uniformly larger, and at lower intensities the OFF response increasingly predominates.

(D) Specimen records to show the effects treated graphically above. All records from same preparation. The artifact (A) in all traces indicates the start of the OFF stimulus. With very low intensities of light, no IPSP can be detected, and spikes are produced at both ON and OFF. With higher intensities, the OFF IPSP appears and inhibits spiking. With still higher intensities, the ON IPSP appears and finally exceeds the OFF IPSP in amplitude in the bottom trace.

Fig. 7.

Dependence of amplitude of IPSP on stimulus intensity (ΔI). The animal’s eye was placed at the centre of a translucent white hemispherical diffuser, illuminated by a tungsten filament light source. ON and OFF stimuli were produced by switching the supply to the source. IPSPs were recorded from the fan of the LGMD. (In the plots shown below, filled circles indicate points derived from 18 brightening or positive pulses stimuli, and open circles indicate points derived from 18 negative or dimming pulses. The former were given later in the experiment, when recording conditions were at their best, and for this reason tend to be clustered at the upper extreme of the range of variation in the first two plots.)

OFF IPSP. Mean values show a small decrease in IPSP amplitude at stimulus intensities less than about 0-5 log10 units, but this is largely due to an extension of the range of variation downwards. The regression line plotted through this subset of the data points shows a slight slope, but this is barely significant (r = 0· 28, n = 52, P = 0·05). At lower light intensities (see (D) below) the OFF IPSP shows a sudden but very variable threshold. Over the range shown here, low stimulus intensities merely increase the variance. Above about 0·5 log10 units, there is no change in amplitude; the × indicates the mean of readings for 0·76 log10 unit stimuli.

ON IPSP. The variance is again larger at low stimulus intensities, but here there is a clear linear relationship between intensity and amplitude, (r = 0·2, P < 0.·001). The absolute threshold is higher than for OFF IPSPS (see D below). Above about 0·5 log10 units, there is again no change in response amplitude.

(C) The ratio between ON and OFF IPSPs obtained from individual trials eliminates the variance brought about by changing recording conditions during’the experiment; this is indicated in this plot by the fact that the difference between the brightening and dimming pulse data (see above) is now gone. The plot indicates that the ON and OFF IPSPS are recorded at about equal amplitude for stimuli of about 0·4 log10 units ; at higher intensities, the ON response is uniformly larger, and at lower intensities the OFF response increasingly predominates.

(D) Specimen records to show the effects treated graphically above. All records from same preparation. The artifact (A) in all traces indicates the start of the OFF stimulus. With very low intensities of light, no IPSP can be detected, and spikes are produced at both ON and OFF. With higher intensities, the OFF IPSP appears and inhibits spiking. With still higher intensities, the ON IPSP appears and finally exceeds the OFF IPSP in amplitude in the bottom trace.

The larger maximal size of the ON IPSP could be due to (a) current of different ion species, or differing proportions of them, and thus basically a function of differing equilibrium potential; (b) to different conductance of the activated subsynaptic membrane to any one ion; or (c) because the OFF synapse was electrically more remote from the recording electrode. The last explanation is a priori unlikely because the relative size of the potentials is not obviously a function of electrode position, and is excluded by the next result reported below.

(iii) Current required for reversal

Injection of hyperpolarizing current through the microelectrode diminishes and eventually reverses both ON and OFF IPSPS. The slope for the OFF IPSP is linear, while that for the ON shows an inflexion. The amount of current differs for the two IPSPs (Fig. 8), the OFF IPSP reverses first. Once again this difference could be due to underlying differences in equilibrium potential, conductance or in geometry; however, the latter explanation would demand the reverse geometry from that required by the preceding observation (ii), so can be discarded in favour of the other two factors, either or both of which could be operating.

Fig. 8.

Reversal of ON and OFF IPSPS by injection of hyperpolarizing current. Recording electrode was placed in the LGMD fan, and current injected via an active bridge circuit. Current flow was measured by a circuit placed in the ground line from the preparation.

(A) Plots showing that the OFF IPSP (filled circles) takes less current to reduce its amplitude to zero than the ON IPSP (open circles). Lines drawn by eye. Positive values (Le. reversed IPSPs) are not plotted because they are confused by the preceding EPSP, which is accentuated by hyperpolarization. For the same reason, the true values of the IPSPs may all be somewhat lower than reported in this paper.

(B) Specimen records from the experiment reported in (A) above. Intracellular records from the LGMD. The top trace shows a control, in which no current is injected into the cell; it shows the usual OFF and ON EPSP and IPSPs in response to a whole-field dimming stimulus of about 1·5 log10 unit lasting about 500 ms. In the remaining four traces, hyperpolarizing current is injected during the period indicated by the small arrows above the trace. Note progressive diminution and, in the case of the OFF response, eventual reversal of the IPSP. Post-inhibitory rebound causes the cell to spike when hyperpolarization is stopped.

Fig. 8.

Reversal of ON and OFF IPSPS by injection of hyperpolarizing current. Recording electrode was placed in the LGMD fan, and current injected via an active bridge circuit. Current flow was measured by a circuit placed in the ground line from the preparation.

(A) Plots showing that the OFF IPSP (filled circles) takes less current to reduce its amplitude to zero than the ON IPSP (open circles). Lines drawn by eye. Positive values (Le. reversed IPSPs) are not plotted because they are confused by the preceding EPSP, which is accentuated by hyperpolarization. For the same reason, the true values of the IPSPs may all be somewhat lower than reported in this paper.

(B) Specimen records from the experiment reported in (A) above. Intracellular records from the LGMD. The top trace shows a control, in which no current is injected into the cell; it shows the usual OFF and ON EPSP and IPSPs in response to a whole-field dimming stimulus of about 1·5 log10 unit lasting about 500 ms. In the remaining four traces, hyperpolarizing current is injected during the period indicated by the small arrows above the trace. Note progressive diminution and, in the case of the OFF response, eventual reversal of the IPSP. Post-inhibitory rebound causes the cell to spike when hyperpolarization is stopped.

(iv) Response to repetitive stimuli

When trains of alternate large-field ON and OFF stimuli are given at various repetition rates, it is found that the two IPSPs differ. At low rates (interstimulus interval (I.S.I.) 2 s) both are stable indefinitely. At higher rates, the ON response decreases and eventually disappears. It recovers rapidly after a few seconds rest. The OFF response resists decrement at higher repetition rates, and shows Significant though slight incrementation (Table 1); it is stable at an interstimulus interval of 200 ms (Fig. 9).

Fig. 9.

Change in IPSP amplitude during repetitive stimulation.

(A) Stimulation at increasing frequency with alternate ON and OFF whole-field stimuli, amplitude 1 log10 unit. Both PSPs are stable at interstimulus intervals of 2 s or larger. With increasing frequency, the ON IPSP wanes, and becomes merged in the irregular IPSPs that characterize the later phase of the OFF response at high frequencies (see B below). The OFF shows a slight but significant (see Table 1) potentiation at repetition rates greater than about 1/s.

(B) Specimen records from the experiment presented graphically in (A), representing the first and the sixteenth stimulus presentation. Note a slight increase in the size of the OFF IPSP (small arrow head) and a significant reduction in that of the ON IPSP (large arrowhead). The latter is virtually submerged in the later stages of the OFF IPSP, which becomes less synchronized with increasing repetition rate, producing the effect of oscillations in membrane potential.

(C) Intracellular records from a different preparation. The stimulus is a whole-field flash. The upper records show the 42nd– 48th stimuli at an interstimulus interval of 200 ms. The ON IPSP and both the EPSPs are rapidly fatigued at this frequency, and contribute little to the record; the OFF IPSP, however, can be seen to be stable. The bottom three traces show the 49th-5ist stimuli at an ISI of 3 s; there is almost immediate recovery of the EPSP, leading to spiking on the third stimulus.

Fig. 9.

Change in IPSP amplitude during repetitive stimulation.

(A) Stimulation at increasing frequency with alternate ON and OFF whole-field stimuli, amplitude 1 log10 unit. Both PSPs are stable at interstimulus intervals of 2 s or larger. With increasing frequency, the ON IPSP wanes, and becomes merged in the irregular IPSPs that characterize the later phase of the OFF response at high frequencies (see B below). The OFF shows a slight but significant (see Table 1) potentiation at repetition rates greater than about 1/s.

(B) Specimen records from the experiment presented graphically in (A), representing the first and the sixteenth stimulus presentation. Note a slight increase in the size of the OFF IPSP (small arrow head) and a significant reduction in that of the ON IPSP (large arrowhead). The latter is virtually submerged in the later stages of the OFF IPSP, which becomes less synchronized with increasing repetition rate, producing the effect of oscillations in membrane potential.

(C) Intracellular records from a different preparation. The stimulus is a whole-field flash. The upper records show the 42nd– 48th stimuli at an interstimulus interval of 200 ms. The ON IPSP and both the EPSPs are rapidly fatigued at this frequency, and contribute little to the record; the OFF IPSP, however, can be seen to be stable. The bottom three traces show the 49th-5ist stimuli at an ISI of 3 s; there is almost immediate recovery of the EPSP, leading to spiking on the third stimulus.

In total, these observations suggest that the ON and OFF IPSP derive from different synaptic connexions with the LGMD and also differ in their afferent pathways from the retina. These interpretations are confirmed by anatomical studies and ablations described below in Section 7.

(6) Response to complex stimuli

The response to complex stimuli is basically a summation of responses to the component ON and OFF stimuli which have been described above. We examined four different complex stimuli which could be readily reproduced and quantified.

(i) Flash

A brief flash from an electronic xenon discharge tube elicits almost simultaneously ON and OFF responses, which summate. The high intensity and wholefield stimulation make this a maximal stimulus for the system. It produces the shortest latencies, densest initial spike response and the largest hyperpolarization of any stimulus (an example is seen in Fig. 5).

(ii) Moving edge

The field can be progressively darkened or lightened by the movement of a shadow edge across it. Slow movement of this edge produces little response of any sort in the LGMD. Faster movement increases the numbers of EPSPs (and sometimes spikes) and generates small IPSPs; and rapid movement approximates to a whole-field ON or OFF stimulus, depending on velocity.

(iii) Moving stripes

The response to moving stripes is complex, involving not only the summation of large numbers of moving edge responses, but also increment and decrement (see Section 5, iv) due to repetitive stimulation. As would be expected from the above, IPSPs are elicited in proportion to both the speed of movement and the period of the stripe pattern - that is, to its contrast frequency (CF). The visual field of the eye is approximately 180°. When λ ≥ 180° (i.e. no more than one edge in the visual field), the stimulus at high speed of movement approximates to an alternation of whole-field ON and OFF stimuli. Under these conditions, the response is basically a series of IPSPs and EPSPs at the contrast frequency. When λ < 180° (i.e. more than one edge in the visual field at once), an increased contrast frequency increases the number of EPSPs and spikes to be elicited, but with higher contrast frequencies the summating IPSPs form an irregular but continuous envelope of hyperpolarization which suppresses most spikes as long as stripe movement is maintained. After such a stimulus there is a tendency for spikes to be produced by post-inhibitory rebound (Fig. 10).

Fig. 10.

Effect of increasing contrast frequency on the response to whole-field striped patterns. An array of vertical stripes is projected on to a hemispherical diffuser placed around the animal’s eye. The stripes are moved at various contrast frequencies (lower trace) and the response recorded intracellularly in the fan of the LGMD (upper trace). Note that only at the very slowest speed (CF = 3 Hz) is there an initial burst of spikes at the onset of motion. At higher frequencies (CF = 6 and 12 Hz), this excitation is no longer seen, even though there is as yet no IPSP generation. This suppression is ascribed to the lateral inhibition network. At a contrast frequency of 25/s, significant IPSPs are generated; all spikes are suppressed during the stripe movement, but there is some post-inhibitory rebound spiking. At a contrast frequency of approximately 80/s, the balance between excitation and inhibition has changed again, and although the IPSPs are larger than at a CF of 25, the EPSPs generate enough excitation to cause some spiking during movement, as well as on rebound. Arrows on all traces indicate 10 mV calibration pulses.

Fig. 10.

Effect of increasing contrast frequency on the response to whole-field striped patterns. An array of vertical stripes is projected on to a hemispherical diffuser placed around the animal’s eye. The stripes are moved at various contrast frequencies (lower trace) and the response recorded intracellularly in the fan of the LGMD (upper trace). Note that only at the very slowest speed (CF = 3 Hz) is there an initial burst of spikes at the onset of motion. At higher frequencies (CF = 6 and 12 Hz), this excitation is no longer seen, even though there is as yet no IPSP generation. This suppression is ascribed to the lateral inhibition network. At a contrast frequency of 25/s, significant IPSPs are generated; all spikes are suppressed during the stripe movement, but there is some post-inhibitory rebound spiking. At a contrast frequency of approximately 80/s, the balance between excitation and inhibition has changed again, and although the IPSPs are larger than at a CF of 25, the EPSPs generate enough excitation to cause some spiking during movement, as well as on rebound. Arrows on all traces indicate 10 mV calibration pulses.

(iv) Moving stripes over less than the whole-field

When the area of the moving stripes is progressively limited, by a window of varying diameter, a number of changes are seen relative to the whole-field response. These depend on either (a) the decreasing area of the retina stimulated, causing a decrease in the IPSPs generated and an increase (by removal of lateral inhibition) of the EPSPs, or (6) geometrical interaction between the window and the stripes. To emphasize the latter, it is necessary to choose a stripe pattern where λ is comparable to the diameter of the window. In the experiments reported here (Fig. 11), the diameter of the window was varied between whole-field and 5°, and the stripes had a periodicity of λ = 20°, with bright and dark stripes of equal width (see Fig. 4B). The interaction of stripe size with window diameter is clearly indicated in Fig. 11A and shows the generation of synchronous IPSPs. The effect of reduction of field size on lateral inhibition is apparent in Fig. 11B ; the small window effectively converts the stripe pattern into a small-field stimulus at high repetition rate, and the neurone responds accordingly. Large EPSPs are generated at the start of the movement but they decrement rapidly. At high contrast frequency, however, even with a 5° field, appreciable IPSPs are generated. The EPSP is very rapidly fatigued and spikes are greatly reduced (Table 2). This dependence of IPSP generation on the temporal frequency of the stimulus suggests temporal summation in the pathway, whereas the dependence on area indicates spatial summation.

Fig. 11.

Effect of moving stripe patterns over significantly less than the whole field. Note variation in vertical scale throughout these records; all calibration pulses marked with arrow heads are 10 mV. Equally spaced dark and light stripes, λ = 20°, were projected on to a diffuser surrounding the eye ; the area illuminated by stripes was controlled by an iris diaphragm in the focal plane of the projector (see Fig. 4B). For response to whole field stimuli, see Fig. 10. (A) Effect of decreasing field size on the inhibitory component Synchronous IPSPs at the contrast frequency (12/s) are conspicuous when the window = 10° in diameter as the 10° stripes then form whole-field ON and OFF stimuli. A similar geometrical effect is present with a window aperture of 5° but this area is too small to produce more than very small IPSPs in the record. With a 15°window, the synchronous component of the IPSPs is less marked, as there are no two boundaries in the stripe field at all times; the relatively large size of the stripe compared with window, however, still introduces a small periodic effect. At 30°the field alternates between two dark and one light stripe, and vice versa at contrast frequency; the expected periodic effect is seen, especially towards the end of the record. The dots below each record (except for the 10°field) indicate the expected arrival time of a synchronous IPSP at contrast frequency.

(B) Effect of decreasing field size on the excitatory component. This effect can be also seen in (A) above, but less clearly; in this record the lower contrast frequency (6 rather than 12/s) elicits weaker IPSPs, and the excitatory component is better seen. As the field size is decreased, larger and larger EPSPs are generated at the start of stripe movement due to the decrease in lateral inhibition. Weak IPSPs, synchronous with CF can also be seen at 30°and 5°, as expected from (A) above.

(C) At high-contrast frequencies, even a small-field stimulus can generate appreciable IPSPs, and simultaneously the excitatory synapses on to the LGMD are fatigued. The result is that spiking is markedly reduced, as shown in Table 2. The figure shows that a contrast frequency of 6/s over a 5°area gives very little inhibition, and a lot of spikes; at a contrast frequency of 25 / s, appreciable IPSPs are seen, and spiking is reduced.

These records taken together indicate that IPSP generation is favoured not only by increasing stimulus area and stimulus intensity, as shown in previous figures, but also by increasing frequency, which presumably suggests temporal summation in the afferent pathway to LGMD.

Fig. 11.

Effect of moving stripe patterns over significantly less than the whole field. Note variation in vertical scale throughout these records; all calibration pulses marked with arrow heads are 10 mV. Equally spaced dark and light stripes, λ = 20°, were projected on to a diffuser surrounding the eye ; the area illuminated by stripes was controlled by an iris diaphragm in the focal plane of the projector (see Fig. 4B). For response to whole field stimuli, see Fig. 10. (A) Effect of decreasing field size on the inhibitory component Synchronous IPSPs at the contrast frequency (12/s) are conspicuous when the window = 10° in diameter as the 10° stripes then form whole-field ON and OFF stimuli. A similar geometrical effect is present with a window aperture of 5° but this area is too small to produce more than very small IPSPs in the record. With a 15°window, the synchronous component of the IPSPs is less marked, as there are no two boundaries in the stripe field at all times; the relatively large size of the stripe compared with window, however, still introduces a small periodic effect. At 30°the field alternates between two dark and one light stripe, and vice versa at contrast frequency; the expected periodic effect is seen, especially towards the end of the record. The dots below each record (except for the 10°field) indicate the expected arrival time of a synchronous IPSP at contrast frequency.

(B) Effect of decreasing field size on the excitatory component. This effect can be also seen in (A) above, but less clearly; in this record the lower contrast frequency (6 rather than 12/s) elicits weaker IPSPs, and the excitatory component is better seen. As the field size is decreased, larger and larger EPSPs are generated at the start of stripe movement due to the decrease in lateral inhibition. Weak IPSPs, synchronous with CF can also be seen at 30°and 5°, as expected from (A) above.

(C) At high-contrast frequencies, even a small-field stimulus can generate appreciable IPSPs, and simultaneously the excitatory synapses on to the LGMD are fatigued. The result is that spiking is markedly reduced, as shown in Table 2. The figure shows that a contrast frequency of 6/s over a 5°area gives very little inhibition, and a lot of spikes; at a contrast frequency of 25 / s, appreciable IPSPs are seen, and spiking is reduced.

These records taken together indicate that IPSP generation is favoured not only by increasing stimulus area and stimulus intensity, as shown in previous figures, but also by increasing frequency, which presumably suggests temporal summation in the afferent pathway to LGMD.

(7) Anatomical basis of the IPSPs

The distal portion of the LGMD consists of three separate dendritic subfields, A, B and C (see Fig. 14), which connect to the axon with C being most proximal (closest to the zone of spike initiation, see O’Shea, 1975; O’Shea & Rowell, 1976) and subfield A most distal (O’Shea & Williams, 1974). Spike initiation in the visual, mode takes place where the axon thickens, a considerable distance (∼ 150μm) proximal to the junction of subfield C. Most of our intracellular recordings are made from the basal (proximal) portion of subfield A which forms a fan curving around the outer face of the lobula, and which receives excitatory afferents from the second optic chiasma (see fig. 8 of paper II of the series, O’Shea & Rowell, 1976). In this area the dendritic branches are thick and present a relatively easy target. IPSPs recorded there are of large amplitude (15 mV) and can be reversed by the injection of hyperpolarizing current (see Section 5, iii); we have been unable, however, to demonstrate an increase of permeability during the IPSP by the injection of constant current pulses during the IPSP, which implies that the active sites are some distance from the electrode (or that the synapses are not chemical, which we think improbable). The synapses are unlikely to be more distal than the electrode because placements in the outer branches of the fan dendrites record a reduced amplitude IPSP. One would expect an inhibitory input to be close to the site of spike initiation (i.e. proximally), and we therefore adopted the hypothesis that the ON and OFF IPSPS were generated by synapses on the most proximal subfield, subfield C, or possibly on both C and B. Recordings from electrode placements near the junction of subfield C show large IPSPs and reduced EPSP amplitude, which is consistent with this hypothesis. The results of lesion experiments, reported below, provide direct evidence that synapses responsible for the OFF IPSP are indeed located on subfield C.

Fig. 14.

Schematic diagram of the neural circuitry thought to be operating to discriminate against large-field stimuli in the response of the LGMD neurone. The rationale for the retinotopic projection to the LGMD was given in a preceding paper (O’Shea & Rowell, 1976), and this diagram follows on from fig. 7 of that paper. Conventions : filled circles, excitatory chemical synapses; with oblique cross-hatching, decrementing synapse; open triangles, chemical inhibitory synapses; heavy continuous lines, ON/OFF units; heavy broken lines, ON units; light continuous lines, OFF units.

(A– C) Dendritic subfields of the LGMD after O’Shea & Williams (1974) ; asterisks, identified neurones. ON and OFF units converge in the medulla to give phasic ON/OFF afferents running in the second optic chiasma. These contact subfield A of the LGMD via decrementing synapses. The evidence for the remaining details is given in this paper. The lateral inhibitory network (LIN) is located between the ON/OFF afferents ; it is probably located in the medulla but could be in the extreme distal face of the lobula. The neurones of the dorsal uncrossed bundle (DUB) collect from phasic OFF units in the proximal face of the medulla, run to the dorsal lobe of the lobula, and contact subfield C of the LGMD. A similar system must derive from medulla ON units and run to the LGMD, but has not been identified morphologically. If it runs in the chiasma, as seems likely, it may form synapses on subfield B of the LGMD, which is located in the approximate area. Both the ON and OFF inhibitory channels form feedforward loops around the lateral inhibition network. The incremental and decremental properties shown by their PSPs probably derive from the synapses they make on the LGMD.

Fig. 14.

Schematic diagram of the neural circuitry thought to be operating to discriminate against large-field stimuli in the response of the LGMD neurone. The rationale for the retinotopic projection to the LGMD was given in a preceding paper (O’Shea & Rowell, 1976), and this diagram follows on from fig. 7 of that paper. Conventions : filled circles, excitatory chemical synapses; with oblique cross-hatching, decrementing synapse; open triangles, chemical inhibitory synapses; heavy continuous lines, ON/OFF units; heavy broken lines, ON units; light continuous lines, OFF units.

(A– C) Dendritic subfields of the LGMD after O’Shea & Williams (1974) ; asterisks, identified neurones. ON and OFF units converge in the medulla to give phasic ON/OFF afferents running in the second optic chiasma. These contact subfield A of the LGMD via decrementing synapses. The evidence for the remaining details is given in this paper. The lateral inhibitory network (LIN) is located between the ON/OFF afferents ; it is probably located in the medulla but could be in the extreme distal face of the lobula. The neurones of the dorsal uncrossed bundle (DUB) collect from phasic OFF units in the proximal face of the medulla, run to the dorsal lobe of the lobula, and contact subfield C of the LGMD. A similar system must derive from medulla ON units and run to the LGMD, but has not been identified morphologically. If it runs in the chiasma, as seems likely, it may form synapses on subfield B of the LGMD, which is located in the approximate area. Both the ON and OFF inhibitory channels form feedforward loops around the lateral inhibition network. The incremental and decremental properties shown by their PSPs probably derive from the synapses they make on the LGMD.

Subfield C branches in the dorsal lobe of the lobula, where its extent is larger than that described in our earlier account (Rowell & Bacher, unpubl.). A number of fibre tracts from various parts of the optic lobe run dorsally above the second chiasma; they do not show the lateral reversal of the chiasma projections and are therefore called ‘uncrossed bundles’. One of these arises from the proximal face of the medulla, runs dorsally and then proximally to the dorsal lobe of the lobula, and ramifies in the neuropile there. This dorsal uncrossed bundle (DUB) was selected as a candidate for the inhibitory input to the LGMD.

Lesions which cut the DUB between medulla and lobula produced a wholly atypical response to whole-field stimulation (Fig. 12). OFF stimuli elicited a large burst of spikes in the LGMD, as opposed to the 2– 3 which are the normal maximum. The ON response, however, was unchanged. Intracellular recordings from these animals showed that the OFF IPSP was largely abolished, whereas the ON IPSP was normal.

We attempted unsuccessfully to find a lesion which would selectively abolish the ON IPSP. We could not apply this technique to the chiasma, as lesions there abolish all excitatory visual input to the LGMD, and this in turn makes it impossible to dentify the neurone. We therefore conclude tentatively that the ON inhibitory pathway runs in the chiasma.

(8) Morphology of the neurones composing the dorsal uncrossed bundle

The DUB is composed of approximately 500 similar neurones which have a dendritic arborization superficially on the proximal face of the medulla (Fig. 13). There is a differentiation of this dendritic field into two subfields, the smaller of the two projects slightly deeper into the medulla than the other. The dendritic field of each fibre is a vertical ellipsoid which covers approximately 8 × 12° of the visual field, assuming a linear representation of space on the medulla surface. They are stacked Together such that the whole visual field appears to be covered with virtually no overlap. The fibres running in the DUB appear to be of one type. The axons have a diameter of about 2 μ m as they run between medulla and lobula and enter the dorsal lobe of the lobula just anterior to the posterior optic bundle, which runs above the lobula at this point. Silver impregnation (Golgi-Colonnier) shows that each fibre has extensive ramification of its terminals within the dorsal lobe. This region also contains the dendrites of subfield C of the LGMD, with which we assume they make synaptic connexion.

Fig. 13.

Morphology of the DUB system in the genus Schùtocerca.

(A) Reconstruction of the proximal portion of the right optic lobe in Schùtocerca gregaria (from a Bielschowsky reduced silver preparation). The region has been desheathed distally from the optic peduncle (o.p.; n.s., neural sheath); the cell bodies have been omitted and the relevant neuropiles have been left in. Note the lobula (Lo.) with its outer (o.l.), inner (i.l.) and dorsal (d.l.) lobes ; and the medulla (Me.), which is represented only by the bowl-like form of its proximal face (p.f.). A point of reference is the LGMD (see O’Shea & Williams; 1974) with its three dendritic subfields (A, B and C). Subfield C arborizes in the dorsal lobe and it is in this region that the DUB runs. Note the origins of this bundle from the whole proximal surface of the medulla.

(B, C) Details of the constituent neurones of the DUB have been reconstructed from Golgi preparations. Their relationships to the gross anatomy are indicated by the two rectangles in (A).

(B) Detail of the medullary (distal) dendritic fields of two DUB neurones, one of which shows the division into a small (s.) and a large (l.) subfield, the position of the cell body (c.b.) and the axon (ax.) projecting to the dorsal lobe. S. gregaria, Golgi-Colonnier.

(C) Detail of the terminations of several DUB neurones in the dorsal lobe of the lobula. S. gregaria americana, Golgi-Colonnier.

Fig. 13.

Morphology of the DUB system in the genus Schùtocerca.

(A) Reconstruction of the proximal portion of the right optic lobe in Schùtocerca gregaria (from a Bielschowsky reduced silver preparation). The region has been desheathed distally from the optic peduncle (o.p.; n.s., neural sheath); the cell bodies have been omitted and the relevant neuropiles have been left in. Note the lobula (Lo.) with its outer (o.l.), inner (i.l.) and dorsal (d.l.) lobes ; and the medulla (Me.), which is represented only by the bowl-like form of its proximal face (p.f.). A point of reference is the LGMD (see O’Shea & Williams; 1974) with its three dendritic subfields (A, B and C). Subfield C arborizes in the dorsal lobe and it is in this region that the DUB runs. Note the origins of this bundle from the whole proximal surface of the medulla.

(B, C) Details of the constituent neurones of the DUB have been reconstructed from Golgi preparations. Their relationships to the gross anatomy are indicated by the two rectangles in (A).

(B) Detail of the medullary (distal) dendritic fields of two DUB neurones, one of which shows the division into a small (s.) and a large (l.) subfield, the position of the cell body (c.b.) and the axon (ax.) projecting to the dorsal lobe. S. gregaria, Golgi-Colonnier.

(C) Detail of the terminations of several DUB neurones in the dorsal lobe of the lobula. S. gregaria americana, Golgi-Colonnier.

(1) Lateral inhibition

The results presented in this paper support the hypothesis that the synapses between the retinotopic projection of ON/OFF excitatory afferents and the LGMD fan are preceded by a phasic lateral inhibition network between the different afferent channels. There are two biological consequences of this circuitry. First, it provides discrimination against large-field stimuli, and secondly, it acts before the labile synapses and avoids the possibility that they will be appreciably fatigued by ‘unsuitable’ (i.e. whole-field) stimulation. This latter aspect, termed ‘protection from habituation’ by Krasne & Bryan (1973), was examined at more length in an earlier publication (O’Shea & Rowell, 1975 a).

What can be said of the nature of the lateral inhibition circuit, and where is it located? The results show it to be phasic in nature. As it works equally well for ON and OFF stimuli, it must be fed either by phasic ON/OFF units or by separate ON and OFF phasic units. This would seem to eliminate the possibility that inter-cartridge inhibition in the lamina is reponsible (Laughlin, 1974), and although spiking neurones of more complex properties than the large monopolar cells (LMCs) have been recorded in the lamina (Arnett, 1972; Mimura, 1974), they have not been shown to be centripetal and the lamina therefore is an unlikely site for the phasic lateral inhibition described here. The hypothesis that lateral inhibition is fed by phasic ON/OFF units, which is to be preferred on the basis of economy in the number of connexions required, would most likely locate it between the chiasma afferents to the LGMD (models of the input circuitry to the LGMD fan from the chiasma are given by O’Shea & Rowell, 1976). There appear to be no synaptic structures on the second chiasma itself, so the lateral interaction between these afferents must take place either in the proximal medulla or the distal lobula, before the synapses on to the LGMD. The other and more complex hypothesis (i.e. separate ON and OFF inputs) would almost certainly locate the lateral inhibitory connexions in the medulla, where the existence of DUB shows that a retinotopic projection of OFF units, suitable for this purpose, is available near the proximal face. On balance we favour the hypothesis that the lateral inhibitory circuit is situated between ON/OFF chiasma afferents and is probably mediated by amacrine neurones (Strausfeld, 1976; see especially his plates 7.12D and 7.12E) in the most proximal layers of the medulla.

(2) Feed-forward inhibition

The OFF IPSPS in the LGMD derive from the neurones of the DUB. These units have 8× 12° dendritic fields in the proximal layers of the medulla, where they must receive input from a similar portion of a retinotopic projection of OFF cells. As general properties are very similar for both ON and OFF IPSPS, we assume a similar anatomical substrate exists for the ON IPSPS. Both require either spatial or temporal summation of their input in order to function and this explains the equivalence of area or intensity of visual stimulation in producing inhibition. The DUB neurones are clearly derived from the retinal projection before the latter give rise to ON/OFF units which project to the DCMD. They thus form a feed-forward inhibitory loop around the lateral inhibition circuit, as shown in Fig. 14. It will be noticed that this arrangement agrees with the hypothetical scheme (Fig. 1D) developed in the Introduction as a way in which lateral inhibition can be supplemented by feed-forward inhibition to provide protection against transient whole-field stimuli.

We assume that the IPSPs generated in the LGMD by the two feed-forward loops are normal and chemically mediated. They can be diminished and reversed by injected current, and seem conventional in all ways. The hyperpolarization they generate at peak (at least 15 mV below resting potential) presumably indicates a K+ conductance.

(3) Equivalence of locust and cricket neurones

In the course of this work we have repeated and substantiated virtually all the experiments reported by Palka in his work on inhibition in this system (1967a, b, 1969, 1972). His first two papers use the DCMD and the same animal as we do. The two later papers, however, use crickets and the DIMD neurone. The correspondence between the two sets of results is therefore of interest.

(4) Analogies and comparisons with the vertebrate visual system

The neural mechanisms which determine the complex responses of the LGMD may underlie similar responses in other systems. This is because combining many complex response characteristics may set rather specific demands and limits on the possible underlying mechanisms and there may therefore be relatively few neural solutions to complex integrative problems. For example, it is essential in the MD system to protect decrement-prone neurones from the effects of repeated large-field stimulation. The most likely means of achieving this (according to parsimony) is to combine the mechanism of protection with the one which determines the optimal excitatory field size. We showed that this is the dual role of lateral inhibition in the MD system (ibid;O’Shea & Rowell, 1976,a), and suggest that preservation of sensitivity in movement detecting neurones is perhaps a general and hitherto little considered role for lateral inhibition. It is noteworthy, therefore, that a protective role for lateral inhibition has been demonstrated recently in a study on retinal ganglion cells in the cat (Barlow & Levick, 1976). These authors suggest that lateral inhibition may be the nervous system’s tool for controlling decrement in responses of retinal ganglion cells to maintained stimuli.

We have suggested that phasic lateral inhibition in combination with phasic feedforward inhibition underlies the preference in the MD system for small-field stimuli and insensitivity to large-field transients. This phasic inhibitory system is preceded in the locust by a tonic lateral inhibitory network (Rowell & O’Shea, 1976b). We do not know how or where the inhibitory tonic-to-phasic transformation is made but suggest that such a transformation is essential in preventing activation of movement detectors during rapid displacements of the visual field caused by the animal’s own movements.

A very similar organization exists in the vertebrate retina. All retinal ganglion cells are subject to tonic lateral inhibition, and some have a second, change-sensitive, inhibitory surround. Feed-forward inhibition is restricted to the change or movement sensitive retinal ganglion cells and is derived, via amacrine cells, from change in the surround of the ganglion cell’s receptive field (Werblin, 1972). Werblin (1973) has suggested that phasic inhibition provides for detection of movement of small objects while avoiding the overwhelming effect of the vast changes in contrast that result from blinking or eye movements. The parallel with the locust is striking! A major difference in organization does, however, exist.

The LGMD’s receptive field does not have the centre/surround organization of the retinal ganglion cell. Any part of the LGMD’s receptive field can be inhibitory, and change in any small area can be excitatory. Phasic inhibition in the vertebrate retina is fed laterally from bipolar cells and forward to the change-sensitive ganglion cells. In the vertebrates therefore, lateral and feed-forward phasic inhibition occurs at the same time and via the same intemeurones (the amacrine cells of the inner plexiform layer). In the locust we have shown that the need to protect labile sites from decrement means that feed-forward inhibition alone cannot determine the insensitivity of the LGMD to large area stimuli. Phasic lateral and feed-forward inhibition are therefore separated and have different functions in the MD system. It may be that inputs to neurones in vertebrates with characteristics similar to the LGMD are organized in the same way (i.e. arrays of On/Off ganglion cells which are sensitive to change and with phasic lateral inhibitory interactions at a pre-decremental site for protection from habituation and a separate feed-forward inhibition for protection against transient excitation). This organization could be the basis in vertebrates and insects for protecting labile sites and for suppressing responses during visual displacements of the kind which occur in saccadic movements.

This work was supported by grant NS 09404 from the U.S.N.I.H., The British Science Research Council and a travel grant to J.L.D.W. from the Max Planck Gesellschaft. C. H. F. R. was a research Professor of the Miller Institute for Basic Research in the Sciences for one year of the study. Ms Bea Bacher and Fr. H. Bamberg provided histological and photographic assistance. Dr K. G. Pearson, Professor Adrian Horridge and Ms Barbara Shotland critically reviewed the MS. We thank all these persons and institutions for their valued help.

Arnett
,
D. W.
(
1972
).
Spatial and temporal integration properties of units in the first optic ganglion of Dipterans
.
J. Neurophysiol
.
35
,
429
44
.
Barlow
,
H. B.
&
Levick
,
W. R.
(
1976
).
Threshold setting by the surround of cat retinal ganglion cells
.
J. Physiol., Lond
.
359
,
737
57
.
Horn
,
G.
&
Rowell
,
C. H. F.
(
1968
).
Medium- and long-term changes in the behaviour of visual neurones in the tritocerebrum of locusts
.
J. exp. Biol
.
49
,
143
69
.
Krasne
,
F. B.
&
Bryan
,
J. S.
(
1973
).
Habituation: regulation via presynaptic inhibition
.
Science, N.Y
.
183
,
583
4
.
Laughlin
,
S. B.
(
1974
).
Neural integration in the first optic neuropile of dragonflies. III. The transfer of angular information
.
J. comp. Physiol
.
93
,
377
96
.
Mimura
,
K.
(
1974
).
Analysis of visual information in lamina neurones of the fly
.
J. comp. Physiol
.
88
,
335
72
.
O’shea
,
M.
(
1975
).
Two sites of axonal spike initiation in a bimodal interneuron
.
Brain Res
.
96
,
93
8
.
O’shea
,
M.
&
Williams
,
J. L. D.
(
1974
).
The anatomy and output connection of a locust visual inter neurone; the lobula giant movement detector (LGMD) neurone
.
J. comp. Physiol
.
91
,
257
66
.
O’shba
,
M.
&
Rowell
,
C. H. F.
(
1975a
).
Protection from habituation by lateral inhibition
.
Nature, Lond
.
254
,
53
55
.
O’shea
,
M.
&
Rowell
,
C. H. F.
(
1975b
).
A spike-transmitting electrical synapse between visual interneurons in the locust movement detector system
.
J. comp. Physiol
.
97
,
143
58
.
O’shea
,
M.
&
Rowell
,
C. H. F.
(
1976
).
The neuronal basis of a sensory analyser, the acridid movement detector system. II. Response decrement, convergence, and the nature of the excitatory afferents to the fen-like dendrites of the LGMD
.
J. exp. Biol
.
65
,
289
308
.
Palka
,
J.
(
1967a
).
An inhibitory process influencing visual responses in a fibre of the ventral nerve cord of locusts
.
J. Insect Physiol
13
,
235
48
.
Palka
,
J.
(
1967b
).
Head movement inhibits locust visual unit’s response to target movement
.
Am. Zool
.
7
,
728
.
Palka
,
J.
(
1969
).
Discrimination between movements of eye and object by visual interneurones of crickets
.
J. exp. Biol
.
50
,
733
32
.
Palka
,
J.
(
1972
).
Moving movement detectors
.
Am. Zool
.
12
,
497
505
.
Rowell
,
C. H. F.
&
Horn
,
G.
(
1968
).
Dishabituation and arousal in the response of single nerve cells in an insect brain
.
J. exp. Biol
.
49
,
171
83
.
Rowell
,
C. H. F.
(
1971
).
The orthopteran descending movement detector (DMD) neurones: a characterisation and review
.
Z. vergl. Physiol
.
73
,
167
94
.
Rowell
,
C. H. F.
&
O’shea
,
M.
(
1976a
).
The neuronal basis of a sensory analyser, the acridid movement detector system. I. Effects of simple incremental and decremental stimuli in light and dark adapted animals
.
J. exp. Biol
65
,
273
88
.
Rowell
,
C. H. F.
&
O’shea
,
M.
(
1976b
).
The neuronal basis of a sensory analyser, the acridid movement detector system. HI. Control of response amplitude by tonic lateral inhibition
.
J. exp. Biol
.
65
,
617
35
.
Strausfeld
,
N. J.
(
1976
).
Allot of an Intect Brain
.
Springer-Verlag
.
Werblin
,
F. S.
(
1972
).
Lateral interactions at inner plexiform layer of vertebrate retina: antagonistic responses to change
.
Science, N. Y
.
175
,
1008
10
.
Werblin
,
F. S.
(
1973
).
The control of sensitivity in the retina
.
Scient. Am
.
228
,
70
9
.