The neuronal basis of a sensory analyser, the acridid movement detector system. IV. The preference for small field stimuli.

1. The lobula giant movement detector neurone (LGMD) and its postsvnaptic cell, the descending contralateral movement detector neurone (DCMD), show a preference for smallrather than large-area stimuli within their 180 receptive field. Intracellular recordings from LGMD show that two different neural mechanisms are responsible. 2. Large-field moving stripes cause little decrement in the labile afferent synapses to LGMD. The response to a small moving target is progressively inhibited by large-field moving stripes when these are closer than ca. 20; and moving stripes produce progressively more excitatory response when their area is limited to 40 or less. These observations indicate a phasic lateral inhibitory network between the afferent excitatory pathways to subfield A of LGMD. The responsible units have not been identified, but are probably in the proximal medulla or distal lobula. This network reduces input to LGMD during sustained large-field stimulation and, in addition, protects the decrement-prone small-field neurones from the potentially fatiguing effects of whole-field movements. 3. Large-field stimuli also produce long-lasting compound IPSPs in LGMD, which limit or prevent spiking. Separate and different IPSPs are generated by ON and OFF stimuli. The neurones mediating OFF IPSPS have been identified anatomically and their function confirmed by selective lesion. Their input dendrites cover approximately 8° x 12 of the proximal face of the medulla. The axons form a dorsal uncrossed bundle (DUB) above the second chiasma, and terminate on the dorsal lobe of the lobula, apparently synapsing on dendritic subfield C of LGMD. 4. The ON and OFF inhibitory inputs form' feed-forward' inhibitory loops around the ON/OFF excitatory afferents. 'Feed-forward' inhibition is necessary in addition to lateral inhibition to prevent a large excitatory response to whole-field transients. Lateral inhibition is necessary in addition to feedforward inhibition to protect the small-field excitatory afferents from depression. Thi9 combination may be widespread in sensory systems and could provide a basis for saccadic suppression.