Response plasticity of single neurons in rabbit auditory association cortex during tone-signalled learning.

Single unit activity was monitored in rabbit auditory association cortex (AC) throughout the acquisition of classically conditioned, nictitating-membrane response. The CS was a tone burst at the characteristic frequency of each neuron. Rabbits which were pseudoconditioned or received conditioning trials but did not learn the response served as control groups. Significant alterations in CS-evoked firing rate were termed 'response plasticity'. Neurons in conditioned animals were more than twice as likely to show response plasticity during the 250 ms CS-US interval than neurons in control animals. Such differences were evident both in the early (0-60 ms) and late (60-250 ms) portions of the CS-US interval. Most early changes appeared at 21-40 ms after CS onset. Response plasticity was most commonly manifested as an increase or decrease in CS-evoked firing rate with little change in the response pattern (PST histogram shape). In some neurons, subcomponents of response patterns (early or late portions of the CS-US interval) were observed to change independently of each other. Spontaneous rate and UCS-evoked activity were not modified with learning. Early in training (transition trials), neural activity evoked by the tone CS in conditioned animals was not different from that in controls. Response plasticity was most pronounced after the CR was first learned (trained trials) and stabilized once the Cr was well established (overtrained trials). Recording sites of neurons showing conditioning-related response plasticity were co-extensive with those of cells that did not.