A Directionally Sensitive Motion Detecting Neurone in the Brain of A Moth

Abstract

ABSTRACT In the moth, Manduca sexta, a pair of neurones, one on each side of the brain, were characterized morphologically and physiologically as descending interneurones, selective for horizontal motion over a large area of the moth’s visual field. Their cell bodies and dendritic processes are located in the protocerebrum of the brain. Their axons, 12–15μm in diameter, project down the ipsilateral connective, branching profusely on the ipsilateral side of the suboesophageal, prothoracic and pterothoracic ganglia. Each neurone responds to movement over either retina. Their preferred directions are from front to back across the ipsilateral eye and back to front over the contralateral one. Movement in the opposite direction supresses their usual ‘resting’ discharge. The neurones are particularly sensitive to movements within the frontal, ventral visual field. Each neurone responds repeatedly, for up to 5 h, to a stimulus oscillating back and forth across the retinae. The response is not diminished during concurrent wing flapping. An increase in the velocity of stimulus movement produces a proportional increase in firing frequency. For stripes of 2·5 cm wavelength and subtending 32° at the eye, the maximum response occurs at a velocity of 3cm/s which gives a contrast frequency of 1-2 Hz. The latency of the neurone’s response, measured from its axon as it enters the pterothoracic ganglion, depends on at least two factors: light intensity and the speed of stimulus movement. The neurone gives a directional response to stripes of period 6·4° in bright light. The response falls to 16· in dim light. At night, in dim light, the latency of response is much reduced and the threshold light intensity, necessary for a directional response, decreases by two orders of magnitude.