ABSTRACT
Auditory interneurones originating in the metathoracic ganglion of females of the grasshopper Chorthippus biguttulus were investigated with respect to their processing of models of the male’s song. In these models two temporal configurations were varied: (i) the song pattern, consisting of ‘syllables’ and ‘pauses’, and (ii) the duration of short gaps within syllables.
A precise encoding of the song pattern is found only among receptors and ‘thoracic’ neurones (SN1, TNI), but not among ascending neurones. The only ascending neurone that reacts tonically at all intensities (AN6) encodes the song pattern much less precisely than do receptors. Other ascending neurones (AN3, AN11) encode the gross pattern of model songs, but only at low intensities.
One spontaneously active ‘local’ neurone (SN6) is tonically inhibited and encodes the pauses of a model song. A similar response, however, is not found in three ascending neurones (AN13, AN14, AN15), which are merely inhibited.
Among ascending neurones, AN12 is the most reliably influenced by the syllable-pause structure of the songs. Its phasic burst marks the onset of every syllable in a behaviourally attractive song. Its activity could account for the rising part of the corresponding behavioural response curve. However, no ascending neurone shows activity corresponding to the falling part of the behavioural response. Among local neurones, the phasic BSN1 neurones are most clearly influenced by varying syllable-pause combinations.
Gaps within the song syllables cause a complete inhibition of the activity of AN4. The response of AN4 to syllables with and without gaps is strikingly similar to the behavioural response and is maintained over the whole intensity range tested. Several local neurones, especially SN6, are strongly influenced by gaps within model songs - though only in certain intensity ranges.
In accordance with behavioural results, the pathways for information on song pattern and sound direction appear to be separated among ascending neurones. Among local interneurones, however, this separation does not appear to take place, since the most directional local neurone, BSN1, might also be suited for pattern-filtering tasks.
