J. Zhang et al., DYNAMICS OF SINGLE NEURON ACTIVITY IN MONKEY PRIMARY MOTOR CORTEX RELATED TO SENSORIMOTOR TRANSFORMATION, The Journal of neuroscience, 17(6), 1997, pp. 2227-2246
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.