Single-unit and slow potential responses from rat frontal cortex during associative conditioning.

Single-unit and slow potential responses were recorded from the frontal cortex of unanesthetized, restrained rats trained to associate an auditory cue (tone) with rewarding medial forebrain bundle stimulation. Slow potential responses to the unpaired tone were minimal whereas large negative slow potential responses developed to the paired tone. Units were selected which had large-amplitude action potentials and positive first deflections, characteristics suggesting that the recordings were derived from pyramidal neurons. Responses of excitation, inhibition, and no change were observed during tone presentation. Forty-five percent of the units responded to the unpaired tone; with pairing, 90% of the units demonstrated significant responses. Furthermore, the magnitude of the excitatory responses was enhanced by pairing and a distribution pattern developed in which the overall response of the more superficial units was activation whereas deeper units were inhibited. The results suggest that the conditioning-related negative slow potential responses recorded from the surface of the rat frontal cortex reflect excitatory processes which are associated with an enhanced firing rate of neurons in the upper layers.