Activity and excitability to electrical current of cortical auditory receptive neurons of awake cats as affected by stimulus association

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Abstract
Unit activity and excitability were studied at the mid-lateral and suprasylvian cortex of naive, blink-conditioned and randomization cats. The latter received the same CS [conditioned stimulus] and US [unconditioned stimulus] as did the conditioned animals, but in random temporal order and with random inter-trial intervals with a mean comparable to that used for conditioning. The randomization group failed to develop a blink CR [conditioned response]. In the conditioned group spontaneous and evoked unit discharges and unit excitability to extracellularly injected currents were increased above levels in naive animals. The degree of enhancement of evoked activity and excitability was greatest in units that responded to the CS. This could have supported the discrimination of the CS by these animals. In the randomization group spontaneous and evoked unit activity and excitability of units to extracellularly injected currents were increased above levels in naive animals, but evoked activity and unit excitability were not increased to the degree found in the CS-responsive units in conditioned animals. Randomization is not a neutral control for conditioning. Transient changes in rates of spontaneous unit discharge accompanied serial presentation of auditory stimuli in all animals. The mechanism mediating enhanced excitability to extracellular current injection in the CS-responsive units in the blink-conditioned cats may be pre- or intra-synaptic as opposed to postsynaptic, since no differences in excitability between these and other cells were found with intracellular current injection.