THE RELATION OF THALAMIC CELL RESPONSE TO PERIPHERAL STIMULI VARIED OVER AN INTENSIVE CONTINUUM

Abstract

Those neurons of the ventrobasal nuclear complex which are responsive to the position and rotation of the limbs at their joints were studied by the method of single unit analysis, in unanesthetized "de-afferented-head" monkeys. These cells, their linked peripheral afferents, and intervening second order neurons form a system which is an absolute detector of joint angle. Neural response has been measured in terms of impulses per unit time, treated as increment over the pre-stimulus discharge rate, and the validity of measurement established. Joint position has been varied by precise instrumental control, and measured along a ratio scale whose zero is the edge of the excitatory angle for each cell. The relation between thalamic cell discharge and joint position is adequately described by a power function of the form: R = KSn C, where R = response, K is a constant of proportionality, S = stimulus, and C is the constant for pre-stimulus activity. The exponent n averaged about 0.7 for the population of neurons studied. These findings have been compared with recent psycho-physical observations. The excitatory angles for thalamic cells are always single-ended and monotonic, with maximal discharge rates at the full extent of the exciting movement. The differences between these and the observations of others on the first order joint afferents have been interpreted as signs of the transforming action at the synaptic relays of an afferent system, and discussed from the point of view of their role in integrative action.