Trial sequence of changed unit activity in auditory system of alert rat during conditioned response acquisition and extinction.

1. The activity of units in the auditory system of alert, freely moving rats was studies during the acquisition and extinction of a tone-signaled, appetitive classically conditioned response. Responses of neurons in inferior colliculus (N = 28), medial geniculate (N = 32), posterior nucleus of thalamus (N = 28), pretectal region (N = 19), and cortex (N = 100) were studies in 74 rats across 10-trial blocks. 2. During behavioral acquisition, neurons in posterior nucleus of thalamus were the first to show response increments to CS+ onset. They were followed by neurons in cortex, pretectal region, medial geniculate, inferior colliculus and by movement behavior. 3. Prestimulus background rates during acquisition showed significant decrements in cortical neurons. These background decrements began to be evidenced in the trial series before the response increases in posterior nucleus. These data strengthened the suggestion of a previous study that posterior nucleus responses could be dependent on tonic modulation from cortex. 4. Extinction appeared to be largely a reverse of acquisition. Cortex and behavior showed response decrements first in the trial series. They were followed by medial geniculate, pretectal region, posterior nucleus, and inferior colliculus neurons. 5. The hypothesis was advanced that the auditory lemniscal adjunct afferent system may play a primary role in the early phases of auditory conditioned-response acquisition.