Responses to task-irrelevant visual features by primate prefrontal neurons

The primate brain is equipped with prefrontal circuits for interpreting vis ual information, but how these circuits deal with competing stimulus-respon se (S-R) associations remains unknown. Here we show different types of resp onses to task-irrelevant visual features in three functionally dissociated group., of primate prefrontal neurons. Two Japanese macaques participated i n a go/no-go task in which they had to discriminate either the color or the motion direction of a visual target to make a correct manual response. Pri or to the experiment, the monkeys had been trained extensively so that they acquired fixed associations between visual features and required responses (e.g., "green = go"; "downward motion = no-go"). In this design, the monke y was confronted with a visual target from which it had to extract relevant information (e.g., color in the color-discrimination condition) while igno ring irrelevant information (e.g., motion direction in the color-discrimina tion condition). We recorded from 436 task-related prefrontal neurons while the monkey performed the multidimensional go/ no-go task: 139 (32%) neuron s showed go/no-go discrimination based on color as well as motion direction ("integration cells"); 192 neurons (44%) showed go/no-go discrimination on ly based on color ("color-feature cells"); and 105 neurons (24%) showed go/ no-go discrimination only based on motion direction ("motion-feature cells" ). Overall, however, 162 neurons (37%) were influenced by irrelevant inform ation: 53 neurons (38%) among integration cells, 71 neurons (37%) among col or-feature cells, and 38 neurons (36%) among motion-feature cells. Across a ll types of neurons, the response to an irrelevant feature was positively c orrelated with the response to the same feature when it was relevant, indic ating that the influence from irrelevant information is a residual from S-R associations that are relevant in a different context. Temporal and anatom ical differences among integration, color-feature and motion-feature cells suggested a sequential mode of information processing in prefrontal cortex, with integration cells situated toward the output of the decision-making p rocess. In these cells, the response to irrelevant information appears as a congruency effect, with better go/no-go discrimination when both the relev ant and irrelevant feature are associated with the same response than when they are associated with different responses. This congruency effect could be the result of the combined input from color- and motion-feature cells. T hus these data suggest that irrelevant features lead to partial activation of neurons even toward the output of the decision-making process in primate prefrontal cortex.