Daily variation and appetitive conditioning-induced plasticity of auditory cortex receptive fields

Long‐term modification of cortical receptive field maps follows learning of sensory discriminations and conditioned associations. In the process of determining whether appetitive – as opposed to aversive – conditioning is effective in causing such plastic changes, it was discovered that multineuron receptive fields, when measured in rats under ketamine‐sedation, vary substantially over the course of a week, even in the absence of classical conditioning and electrode movement. Specifically, a simple correlation analysis showed that iso‐intensity frequency response curves of multiunit clusters and local field potentials recorded from auditory cortex are nonstationary over 7 days. Nevertheless, significant plastic changes in receptive fields, due to conditioned pairing of a pure tone and electrical stimulation of brain reward centres, are detectable above and beyond these spontaneous daily variations. This finding is based on a novel statistical plasticity criterion which compares receptive fields recorded for three days before and three days after conditioning. Based on a more traditional criterion (i.e. one day before and after conditioning), the prevalence of learning‐induced changes caused by appetitive conditioning appears to be comparable to that described in previous studies involving aversive conditioning.