Functional properties of monkey caudate neurons. II. Visual and auditory responses.

1. Visual responses of caudate neurons were studied in monkeys trained to fixate on a small spot of light. A visual stimulus (another spot of light) was presented in various contexts of behavior using different behavioral paradigms. Visual receptive fields were usually large and centered on the contralateral hemifield. Among 217 neurons with visual responses, 184 were further classified into subtypes. 2. Visual responses in 64 neurons were not modulated by changing the paradigms (unconditional visual responses). In the other neurons, visual responses were dependent on the behavioral contexts in which the stimulus was presented. Three types of behavioral modulation were found. 3. A saccade-enhanced visual response (n = 37) was the one that was enhanced if the monkey made a saccade to the stimulus on its appearance. The enhancement was spatially selective: the response was depressed if the saccade was directed away from the stimulus. 4. Memory-contingent visual responses (n = 36) were present preferentially when the monkey remembered the location of the stimulus and a few seconds later made a saccade to the remembered location. Responses were greater when the location of the stimulus was randomized between trials. 5. Expectation-contingent visual responses (n = 46) were present preferentially when the stimulus came on while the monkey was not fixating another spot, and the stimulus was related directly to a reward. Unlike the other types, its receptive field included both contralateral and ipsilateral hemifields without a particular preference. 6. A small number of neurons (n = 16) showed a visual response that easily habituated. 7. Latencies of visual responses were usually between 100 and 200 ms. The latencies of the memory-contingent, expectation-contingent, and habituated visual responses tended to be longer than the others and tended to be more variable between trials. 8. Among auditory responsive neurons only a small proportion were related to the tasks. The response was greater to a contralateral sound. It was enhanced if the monkey used the sound as the cue for the future target location. 9. The results suggest that sensory responses of caudate neurons could be used to guide a subsequent sequence of learned behaviors by confirming predicted environmental states, renewing memory, or establishing a motor set.