Activation state of the cortex could be changed by reinforcement of low-amplitude visual evoked potentials in rabbits

Visually evoked potentials (VEP) were recorded chronically from occipital cortex in awake rabbits. The VEP typically consisted of a triphasic response (positive deflection P60--negative deflection N205--positive deflection P450) that was followed by a late negative shift starting about 750 ms after the eliciting flash. After computation of a discriminating amplitude value (200 or 250 microV) the VEP were divided for comparison in high- and low-amplitude groups. Significant differences between these groups existed in amplitudes and latencies of the early VEP components. High-amplitude VEP were followed by larger late negative shifts and had significantly (P less than 0.01) earlier second positive deflections. In these high-amplitude cases, the EEG-baseline was more negative than in the low-amplitude VEP. In addition, we found a more synchronized background EEG in the low-amplitude VEP group. We conclude that different VEP amplitudes depended on different activation states of the cortex which could be changed by reinforcement.