Most neurons in macaque area MT (V5) respond vigorously to stimuli moving in a preferred direction, and are suppressed by motion in the opposite direction. The excitatory inputs come from specific groups of directionally selective neurons in lower-order areas, but the inhibitory signals are not so well understood. Some models (e.g. Simoncelli and Heeger, 1998, Vision Res) assume that these signals are pooled across the receptive field, but Qian et al. (1994, J Neurosci) suggested instead that inhibitory inputs interact with excitatory ones only within local regions of space. To explore the location and direction specificity of interactions between MT receptive field subregions, we stimulated small areas of the receptive field with Gabor patches drifting in the preferred direction. We presented these alone and in combination with stimuli drifting in non-preferred directions so that we could study inhibitory signals against background firing elevated by preferred stimuli. Non-preferred gratings suppressed responses strongly when they were presented in the same retinal location as the preferred grating. When the two gratings were separated, suppression was much reduced and was no larger than the suppression of spontaneous firing produced by a non-preferred stimulus presented alone. Our results show that non-preferred stimuli can only inhibit responses generated by excitatory stimuli from nearby regions of space; this suggests that direction-specific inhibition acts within spatially localized subregions of the receptive field. The results can be described by a model in which local excitation and inhibition are combined and rectified before a final stage of spatial pooling.
ASJC Scopus subject areas
- Sensory Systems