Interactions between two different visual stimuli in the receptive fields of inferior temporal neurons in macaques during matching behaviors.

Macaque monkeys were trained to determine whether shapes or colors of two visual stimuli were the same or different (matched/non-matched). Two stimuli were presented at different locations while the monkey fixated a small spot. In one paradigm, two timuli were presented simultaneously for 0.5 s (Sml-SO task). In the other paradigm, one of the stimuli was turned on 0.5 s before the onset of another stimulus, then the two stimuli were present for the following 0.5 s (Scc-SO task). The aim of the later task was to analyze the responses of TE neurons to a single presentation of each stimulus and the effects of successive onsets of two stimuli. Of 232 responsive neurons tested in both tasks, 143 showed a significant selectivity between paired stimuli (termed 'selective neurons'). During the Sml-SO task, some selective neurons showed a larger response to the different (non-matched) stimuli than to the double optimal stimuli (matched), even though one of the different stimuli was inhibitory. This effect was more prominent in neurons that showed a smaller response to the double presentations of the optimal stimulus than to the single presentation. Since another group of selective neurons showed smaller responses for the different stimuli, the average response amplitudes were similar between the identical and the different stimuli. During the Scc-SO task, when the optimal stimulus was turned on after the non-optimal stimulus (non-matched), the response to the second stimulus was mostly enhanced above the response level to the single presentation of the first stimulus. Since the responses to the second stimulus identical to the first stimulus tended to decrease, the difference in the responses between the matched and non-matched stimuli became significantly larger in the Scc-SO task. The reaction times of the monkeys were shorter during the Scc-SO task than the Sml-SO task. These changes in response amplitude between the different and the identical stimuli were not prominent in the non-selective neurons. These results suggest that non-linear interactions between two different stimuli play an important role in the discrimination of groups of visual stimuli, particularly in successive analysis.