Stimulus and behavioral factors contributing to the activation of monkey prefrontal neurons during gazing.

Many neurons in the inferior dorsolateral area of the monkey prefrontal cortex showed sustained increases in discharge rates during continuous gazing at a tiny light spot that had a reward significance. These increases might depend upon stimulus factors (light target), behavioral factors (gazing) or both. In this report, we tried to separate these factors and to test the extent to which each factor might contribute to the neuronal reaction. Monkeys were trained to exhibit two kinds of behavior: 1) maintained gazing at a light target and 2) "gazing" behavior without a clear target. We then examined neuronal behavior in these two kinds of gazing behavior. During "gazing at target," many prefrontal neurons showed tonic activation; thus the previous findings were confirmed. These neurons behaved in various ways in "gazing without target": 1) some of the neurons were activated to the same extent as in "gazing at target"; 2) many others also showed activation but with lower discharge rates; and 3) the rest of the neurons completely ceased activation. Such variation in discharge patterns may be interpreted as meaning that there is a continous and graded difference among individual neurons in the dependence of their gaze-related activation upon a visible target. Then it seems that the stimulus factors are involved in a graded manner in generation of the activation, and further that other factors, probably behavioral ones, also contribute in part to it.