Single Neurons in the Nucleus Accumbens Track Relative Reward
Abstract
A within subjects simultaneous contrast experiment evaluated nucleus accumbens (NAc) neural responses to a low (0.05 M) and high (0.5 M) concentration of sucrose in 6 rats. During continuous trials, rats were given repeated brief intraoral infusions of the low and then the high concentration of sucrose while electrophysiological activity of NAc neurons and oromotor behavior (EMG) were measured. Following the continuous phase of testing, the two concentrations were infused in an alternating manner. The results showed that rats reduced oromotor behavior when infused with the low concentration of sucrose when alternated with the high concentration (i.e., during alternating trials) relative to the infusion during the continuous low condition (negative contrast). Rats also increased oromotor behavior for the high concentration when presented during alternating relative to continuous trials (positive contrast). Of 137 NAc neurons, 35 exhibited brief inhibitions or excitations to tastant delivery during baseline testing that were correlated with oromotor output. Some NAc neuronal activity reflected negative or positive contrast effects while other neurons encoded alternating testing in general and still other neurons encoded sucrose concentration. These data demonstrate that neuronal activity in the NAc is altered in coincidence with the expression of contrast in consummatory behavior.