DYNAMICS OF SINGLE NEURON ACTIVITY IN MONKEY PRIMARY MOTOR CORTEX RELATED TO SENSORIMOTOR TRANSFORMATION

We investigated the dynamics of neuronal activity related to sensorimo tor transformation during single experimental trials of a given stimul us-response (S-R) association task. A monkey was trained to perform wr ist extension/flexion movements in the horizontal plane to align a poi nter with a visual target while single unit activity in the primary mo tor cortex (MI) was being recorded. The stimulus was a colored light-e mitting diode (LED) presented to either the left or right of a central reference point. The monkey had to point directly at the target (''co mpatible'' S-R mapping) or point to the opposite side of the target po sition (''incompatible'' S-R mapping), with the mapping rule specified by the color of the LED. Single neuron activities on the four correct trials (left/right stimulus x compatible/incompatible S-R mapping) we re compared to determine whether such activities were more related to stimulus encoding and representation, to response preparation and exec ution, or to the ''decision'' processes translating the stimulus repre sentation into a response representation. A novel mathematical techniq ue, called LOCUS ANALYSIS, has been developed to quantitatively analyz e and visualize the contribution of neuronal activity toward the senso ry, motor, or sensorimotor (i.e., decisional) aspects of the task. Our data show that as a trial evolves, neuronal activity in MI, at a popu lation level, is first correlated with the representation of the speci fic stimulus (the side of LED), then with the representation of the S- R mapping rule (the color of LED) as well as trial-specific S-R associ ation (the conjunction of stimulus side and stimulus color), and final ly with the representation of the behavioral response (extension or fl exion wrist movement). Immediately after the issuance of the movement command, the populational activity in MI remains correlated with the t rial-specific stimulus-response conjunctions, i.e., the context of the motor decision that the monkey has just made. Cells recorded successi vely in a single penetration tend to resemble each other in their patt ern of firing on the four correct trials, suggesting a modular organiz ation of neurons based on their functional role in the processing of t he S-R association task. Our results indicate that MI belongs to a dis tributed network such that its neuronal activity reflects the underlyi ng network dynamics that translate a stimulus representation into a re sponse representation via the activation and application of appropriat e S-R mapping rule.