CHANGES IN THE DISTRIBUTED TEMPORAL RESPONSE PROPERTIES OF SI CORTICAL-NEURONS REFLECT IMPROVEMENTS IN PERFORMANCE ON A TEMPORALLY BASED TACTILE DISCRIMINATION TASK

1. Temporal response characteristics of neurons were sampled in fine spatial grain throughout the hand representations in cortical areas 3a and 3b in adult owl monkeys. These monkeys had been trained to detect small differences in tactile stimulus frequencies in the range of 20-30 Hz. Stimuli were presented to an invariant. restricted spot on a single digit. 2. The absolute numbers of cortical locations and the cortical area over which neurons showed entrained frequency-following responses to behaviorally important stimuli were significantly greater when stimulation was applied to the trained skin, as compared with stimulation on an adjacent control digit, or at corresponding skin sites in passively stimulated control animals. 3. Representational maps defined with sinusoidal stimuli were not identical to maps defined with just-visible tapping stimuli. Receptive-field/frequency-following response site mismatches were recorded in every trained monkey. Mismatches were less frequently recorded in the representations of control skin surfaces. 4. At cortical locations with entrained responses, neither the absolute firing rates of neurons nor the degree of the entrainment of the response were correlated with behavioral discrimination performance. 5. All area 3b cortical locations with entrained responses evoked by stimulation at trained or untrained skin sites were combined to create population peristimulus time and cycle histograms. In all cases, stimulation of the trained skin resulted in 1) larger-amplitude responses, 2) peak responses earlier in the stimulus cycle, and 3) temporally sharper responses, than did stimulation applied to control skin sites. 6. The sharpening of the response of cortical area 3b neurons relative to the period of the stimulus could be accounted for by a large subpopulation of neurons that had highly coherent responses. 7. Analysis of cycle histograms for area 3b neuron responses revealed that the decreased variance in the representation of each stimulus cycle could account for behaviorally measured frequency discrimination performance. A strong correlation between these temporal response distributions and the discriminative performances for stimuli applied at all studied skin surfaces was even stronger (r = 0.98) if only the rising phases of cycle histogram were considered in the analysis. 8. The responses of neurons in area 3a could not account for measured differences in frequency discrimination performance. 9. These representational changes did not occur in monkeys that were stimulated on the same schedule but were performing an auditory discrimination task during skin stimulation. 10. It is concluded that by behaviorally training adult owl monkeys to discriminate the temporal features of a tactile stimulus, distributed spatial and temporal response properties of cortical neurons are altered. Changes were consistent with earlier proposed models of cortical reorganization on the basis of the strengthening of afferent and intrinsic cortical excitatory synapses by behaviorally important, temporally coincident inputs. These results, as well as some results from preceding reports in this series, are discussed within the framework of these models.