ABSTRACT
Behavioural studies in insects have shown that simulated rotations of the visual panorama in the yaw, roll and pitch planes lead to optokinetic responses which, under natural conditions, counteract involuntary rotations of the body and thus maintain a stable visual orientation. Electrophysiological investigations have found neurones that respond selectively to rotatory motion in these planes. This evidence, in conjunction with lesioning and electrical stimulation studies, has led to the hypothesis that these rotation-sensitive neurones control optokinetic responses (for a review, see Hausen and Egelhaaf, 1989). Two other types of visually induced behaviour, landing responses and the control of forward flight speed, rely on the detection of translatory rather than rotatory visual flow-fields. For example, it has been shown that flight speed is controlled in insects by adjusting the power output of the wings such that there is a constant preferred angular velocity of image flow from front to back (progressive motion) over both eyes (Apis mellifera:Esch et al. 1975; Drosophila melanogaster:David, 1982). Similarly, landing responses can be induced in flies by moving visual images progressively over both eyes (Eckert, 1984). In spite of these behavioural data there is little evidence, except for a few preliminary reports (Baader, 1988; Rowell, 1989), of neurones that monitor translatory flow-fields, i.e. cells that are maximally sensitive to motion in the same direction over both eyes and not sensitive (or less sensitive) to rotatory motion.
