Classical conditioning of electrical self-stimulation of ventral tegmental area to brief visual stimuli in rats

Two groups of rats with bipolar electrodes implanted unilaterally into the left ventral tegmental area were trained to lever-press for the response contingent electrical stimulation. After preliminary lever-press training, two types of daily sessions were held on 10 consecutive days: type T+, current intensity set at the Threshold level and each response was accompanied by the visual signal (stimulus lights above the lever briefly went off): and type ST-, current set at the Sub Threshold level and no visual stimuli. On day 11, combination of the subthreshold current intensities and stimulus lights previously associated with the threshold stimulation (session type ST+) resulted in a significantly elevated response rates compared to the performance under the subthreshold current without visual stimuli (session type ST-). The extinction and recovery of this phenomenon are also demonstrated.