Formation of associations in auditory cortex by slow changes of tonic firing

We review event-related slow firing changes in the auditory cortex and related brain structures. Two types of changes can be distinguished, namely increases and decreases of firing, lasting in the order of seconds. Triggering events can be auditory stimuli, reinforcers, and behavioral responses. Slow firing changes terminate with reinforcers and possibly with auditory stimuli and behavioral responses. A necessary condition for the emergence of slow firing changes seems to be that subjects have learnt that consecutive sensory or behavioral events are contingent on reinforcement. They disappear when the contingencies are no longer present. Slow firing changes in auditory cortex bear similarities with slow changes of neuronal activity that have been observed in subcortical parts of the auditory system and in other non-sensory brain structures. We propose that slow firing changes in auditory cortex provide a neuronal mechanism for anticipating, memorizing, and associating events that are related to hearing and of behavioral relevance. This may complement the representation of the timing and types of auditory and auditory-related events which may be provided by phasic responses in auditory cortex. The presence of slow firing changes indicates that many more auditory-related aspects of a behavioral procedure are reflected in the neuronal activity of auditory cortex than previously assumed.