An inhibitory process influencing visual responses in a fibre of the ventral nerve cord of locusts

Abstract A particular large fibre in each thoracic connective of locusts responds to visual stimuli and is amenable to prolonged experiment and analysis. It responds readily to movement, but luminance change in a stationary target can be equally effective. The optimal non-moving stimulus is strong dimming of a small target. Larger targets elicit responses with shorter latencies and lower thresholds but with a markedly smaller number of spikes (optimal area effect). Dimming a large target can block a response which would have occurred to the simultaneous or immediately previous dimming of a small target up to a time interval approximately compensating the difference in latencies and independently of the position of the two targets. A prolonged depression independent of location follows activation of the fibre. The degree of depression increases with both the amount of dimming and the area, even when increasing the area has resulted in a diminution of the first response. It is argued that the optimal area effect, the blocking effect, and post-excitatory depression are all expressions of a single inhibitory process. The receptive field encompasses approximately a hemisphere. No evidence was obtained for a segregation of functionally distinct areas. The organization of the inhibition is probably different from (a) inhibitory surround, (b) Limulus -type lateral inhibition, or (c) recurrent inhibition. Repeated dimming of a small light guide results in rapid habituation. However, dimming of a contiguous group of ommatidia is again effective. Reversing the direction of intensity change in the same location does not by itself reactivate the response. Habituation must be a process different from inhibition, because it is strictly localized while the latter is location-independent.