SENSORY MODALITY SPECIFICITY OF NEURAL ACTIVITY RELATED TO MEMORY IN VISUAL-CORTEX

Previous studies have shown that when monkeys perform a delayed match- to-sample (DMS) task, some neurons in inferotemporal visual cortex are activated selectively during the delay period when the animal must re member particular visual stimuli. This selective delay activity may be involved in short-term memory. It does not depend on visual stimulati on: both auditory and tactile stimuli can trigger selective delay acti vity in inferotemporal cortex when animals expect to respond to visual stimuli in a DMS task. We have examined the overall modality specific ity of delay period activity using a variety of auditory/visual cross- modal and unimodal DMS tasks. The crossmodal DMS tasks involved making specific long-term memory associations between visual and auditory st imuli, whereas the unimodal DMS tasks were standard identity matching tasks. Delay activity existed in auditory/visual cross-modal DMS tasks whether the animal anticipated responding to visual or auditory stimu li. No evidence of selective delay period activation was seen in a pur ely auditory DMS task. Delay-selective cells were relatively common in one animal where they constituted up to 53% neurons tested with a giv en task. This was only the case for up to 9% of cells in a second anim al. In the first animal, a specific long-term memory representation fo r learned cross-modal associations was observed in delay activity, ind icating that this type of representation need not be purely visual. Fu rthermore, in this same animal, delay activity in one cross-modal task , an auditory-to-visual task, predicted correct and incorrect response s. These results suggest that neurons in inferotemporal cortex contrib ute to abstract memory representations that can be activated by input from other sensory modalities, but these representations are specific to visual behaviors.