A QUANTITATIVE STUDY OF NEURONAL DISCHARGE IN AREA-5, AREA-2, AND AREA-4 OF THE MONKEY DURING FAST ARM MOVEMENTS

1. The properties of parietal neurons were studied in four adult rhesus monkeys during fast arm movements. The animals were trained to perform flexion or extension of the forearm about the elbow in response to specific auditory cues. Single neuron activity was recorded in 272 area 5 neurons, 81 neurons of the somatosensory cortex, and 92 neurons of the motor cortex. 2. In area 5, 42% of neuronal changes occurred before movement onset (early changes) and 58% after (late changes), with 21% before the earliest electromyogram. The range of modification in activity took place between 260 ms before movement onset and 180 ms after. Complex receptive fields were found in area 5 with a greater proportion among the late neurons (72%) than among the early neurons (32%). 3. Different patterns of activity were observed in neurons recorded in both movement directions. Reciprocal neurons represented 52% of the motor cortex neurons and 41% of the neurons in the somatosensory cortex but only 14% of the area 5 neurons. Of the remainder area 5 neurons, 46% were direction-sensitive neurons and 39% coactivated neurons. This suggests a more complex encoding of movement direction in area 5 than in area 2 or 4. 4. Temporal characteristics of the neuronal bursts were quantitatively analyzed in areas 5, 2, and 4. Neuronal burst duration was longer in area 5 than in the other areas. Above all, a variability of burst parameters, which did not depend on variable movement execution, was noticed in area 5. Therefore neuronal activity in this cortical area cannot be simply explained by a convergence of sensory and motor inputs but may depend on the behavioral context in which the movement is performed. 5. A correlation between neuronal burst duration and movement duration was found in 41% of area 2 neurons. In area 5, this correlation was observed in 20% of the late neurons and in 14% of the early neurons. A correlation between neuronal discharge frequency and movement velocity was found in 34% of area 2 neurons and 24% of area 4 neurons. About 16% of both late and early neurons in area 5 showed such a correlation. These neurons received polyarticular input, and it is suggested that they may be involved in the kinematic encoding of polyarticular movements. 6. A topographic and functional organization of area 5 was noticed. In anterior area 5, 83% of the neurons had receptive fields and most of the reciprocal neurons and those exhibiting a correlation with movement parameters were found there. On the other hand, only 35% of the neurons received peripheral inputs in posterior area 5, but this region contained most of the early neurons (69%). 7. Statistical analysis showed that there was movement related-activity in 69% of the area 2 neurons but in only 39% of the area 5 late neurons. Among the area 5 early neurons, 21% showed movement-related activity and 23% stimulus-related activity. The proportion of movement-related neurons was comparatively higher in the motor cortex (58%), but the proportion of stimulus-related neurons was lower (13%). 8. Our results show the existence of various populations of neurons in area 5. Every step in the information processing between stimulus analysis and the performance of the movement is represented in the discharge of area 5 neurons. The heterogeneity of neuronal activity in this cortical area may be the basis of its integrative function in the control of arm movement.