Neuronal responses in the frontal cortico-basal ganglia system during delayed matching-to-sample task: ensemble recording in freely moving rats

Electrophysiological recording of single neuron activity has been conducted in rats to investigate the patterns of distributed neuronal responses in the frontal cortico-basal ganglia system that code information during a spatial-delayed matching-to-sample task (DMTSt). Rats were trained to press one of the two retractable levers presented randomly as a sample response. The first valid nose-poke after a delay resulted in the presentation of both levers. Pressing the same lever as the sample lever led to a water reward (match to sample), whereas pressing the lever opposite the sample lever resulted in a time-out (house light turned off). One hundred seventy-one neurons in the medial prefrontal cortex (mPFC), 51 in the dorsal striatum (STR), and 93 in the nucleus accumbens (NAc) were recorded during DMTSt. Different patterns of neuronal responses were observed during different behavioral episodes (sample, delay, and match phases) in all three recording areas. Space-related neuronal responses specific to the side of the lever pressed were more often found in the sample phase than in the match phase in all three areas studied. Neuronal responses specific to either correct or error trials were observed with similar percentages in the mPFC and the NAc, while the incidence of correct/error-coded activity in the STR was lower. Ensemble neuronal activity that coded sample versus match lever presses was observed in three out of five rats in sets of trials with similar speed and trajectory of lever press. The results reveal specific patterns of neural responses in the frontal cortico-basal ganglia system in rats during the DMTSt and suggest the existence of specific neuronal coding for different behavioral events associated with a learned short-term memory process.