Visual neurones in the anterior optic tract of the privet hawk moth

SummaryThree classes of visual neurones were recorded in the right anterior optic tract (AOT) of the privet hawk moth. All classes are output cells from the optic lobe with ipsilateral visual fields (30° or more in diameter) within which they respond to small moving stimuli. The responses are altered in a variety of ways by gratings travelling over the rest of the binocular field, but surround movement on its own does not evoke a response.Units of the first class are excited by stimuli moving in a preferred direction (forwards or backwards) and inhibited by stimuli moving in the opposite direction. Surround gratings moving forwards across the right eye and backwards across the left eye (anticlockwise) suppress the response of units with forward preferred direction, but do not affect units with backward preferred direction. A surround grating moving clockwise inhibits the response of units with backward preferred direction and facilitates the response of units with forward preferred direction. A clockwise grating increases the velocity range of the units with forward preferred direction, making them sensitive to very slowly moving stimuli.Units of the second class are excited by stimuli moving horizontally through the receptive field centre; the response to backward motion is usually stronger than that to forward motion. Clockwise motion of a grating in the surround tends to enhance the response to forward motion and suppress that to backward motion, while anticlockwise surround motion has the opposite effect.Units of the third class have preferred directions which vary depending on the position of the stimulus. One type responds to small stimuli moving from the edge of the eye towards the centre and consequently to large receding stimuli. A second type (looming neurone) is excited by stimuli moving from the centre to the periphery and by large approaching stimuli. These complex receptive fields probably arise because the unit receives inputs from several movement detectors with different preferred directions and receptive fields. Furthermore, these input neurones have directionally selective surrounds which help increase the specificity of the looming neurone, by weakening its response to large stimuli travelling backwards or forwards across the retina.The Discussion is concerned with (1) the possible role of the AOT neurones in orienting to a moving target, and (2) the possibility that the directionally selective surrounds of the neurones are mediated by binocular movement detectors which project centrifugally to the optic lobe.