Focal lesions of visual cortex--effects on visual evoked potentials in rats

Focal lesions were placed in the visual cortex of Long-Evans hooded rats, immediately below skull screw recording electrodes. Lesions were produced by heat, and extended an average depth of about 0.9 mm below the cortical surface. Evoked potentials recorded from the electrode overlying the cortical lesion were compared with simultaneously recorded potentials from a contralateral homotopic site. The effects of the lesion were selective. Flash-evoked potential peaks P1, P2, and N2 were depressed by the lesion, and peaks N1 and P3 were augmented; peak N3 was unaffected. Pattern reversal evoked potential peak N3 was depressed by the lesion, and peaks N1 and P2 were made more distinct. The results emphasized that different peaks have different generators, and suggest in particular that flash-evoked potential peaks P1 and N2, and peak N3 of the pattern reversal-evoked potential require the superficial layers of the cortex.