CATEGORICAL PERCEPTION OF SOMATESTHETIC STIMULI - PSYCHOPHYSICAL MEASUREMENTS CORRELATED WITH NEURONAL EVENTS IN PRIMATE MEDIAL PREMOTOR CORTEX

In this paper we describe a type of neuron of the medial premotor cort ex (MPC) that discharged differentially during a categorization task a nd reflected in their activity whether the speed of a tactile stimulus was low or high. The activity of these neurons was recorded in the MP C contralateral (right MPC, n = 88) and ipsilateral (left MPC, n = 103 ) to the stimulated hand of four monkeys performing this somesthetic t ask. Animals performed the task by pressing with the right hand one of two target switches to indicate whether the speed of probe movement a cross the skin of the left hand was low or high. Differential response s of MPC neurons occurred during the stimulus and reaction time period . We used an analysis based on signal detection theory to determine wh ether these differential responses were associated with the animal's d ecision. According to this analysis, 104 of the 191 neurons (right MPC , n = 48; left MPC, n = 56) coded the categorization of the stimulus s peeds (categorical neurons). In a light instruction task, we tested th e possibility that the categorical neurons (n = 71) were associated wi th the intention to press, or with the trajectory of the hand to one o f the two target switches used to indicate categorization. In this sit uation, each trial began as in the somesthetic categorization task, bu t one of the two target switches was illuminated beginning with the sk in indentation, continued during the delay period and turned off when the probe was lifted off from the skin. This condition instructed the animal which target switch was required to be pressed for reward. Very few neurons (14 of 71) maintained their differential responses observ ed in the categorization task. Some categorical neurons (n = 6) were a lso studied; the animal categorized the tactile stimulus speeds, but k new in advance whether the stimulus speed was low or high (categorizat ion + light instruction). This was made by illuminating one of the two target switches which was associated with the stimulus speed. The cat egorical response was considerably attenuated in this condition. Inter estingly, during the delay period, these neurons reflected in their ac tivity whether the stimulus was low or high. A number of the categoric al MPC neurons (n = 30) were studied when the same set of stimuli, use d in the categorization, were delivered passively. None of these neuro ns responded in this condition. These results suggest that the MPC, ap art from its well known role in motor behavior, is also involved in th e animal's decision during the execution of this learned somesthetic t ask.