Specific potentiation of photically evoked activity in the visual cortex

Summary

• 1. The visual cortex response to a contralateral flash is composed in the dark-adapted unanaesthetized cat of an initial (a) wave, with 14–22 msec latency, and a subsequent (post-primary) b complex, at 40–50 msec after the onset of the photic stimulus, consisting of a single or two main deflections followed by a fast (60–100 per sec) afterdischarge. The complex alterations of the cortical afterdischarge induced by variations in parameters of photic stimulation are not referrable to the optic tract oscillatory activity. The central (cortical) origin of the post-primary component is suggested, among other evidences, by its selective enhancement following a conditioning stimulation of the lateral geniculate body.

• 2. Steady light applied to the same retina as the testing flash electively potentiates the post-primary component evoked in the contralateral striate and parastriate cortex (increase in amplitude, shortening of latency and faster evolution), developing it to the pattern of a shock-evoked response, with positive spikes preceding the diphasic wave. The potentiation of the post-primary component by continuous illumination is additive with that induced by lateral geniculate or direct visual cortex stimulation. Inequalities between surface- and depth-recorded flash-evoked components become more marked under steady light, which induces a greater potentiation of the reversed post-primary components in the depth than at the surface. Steady light potentiation at the cortical level is associated with reduction of the first deflection and ensuing oscillations evoked at the level of the optic tract and lateral geniculate body.

• 3. The level of steady light potentiating effect and its mechanisms are discussed.