Augmenting responses evoked in area 17 of the cat by intracortical axon collaterals of cortico‐geniculate cells.

Evoked potentials were recorded in the visual cortex of the cat after electrical stimulation of the lateral geniculate nucleus (l.g.n.). The primary response, mediated by geniculo‐cortical fibres, was depressed at stimulation frequencies above 7 Hz and replaced by a late potential, the incremental response, which gradually increased in amplitude with successive stimuli. The incremental response was a negative‐positive potential in the depth of the cortex with the negative component having maximal amplitude in layer 4. The response reversed polarity in layer 1 to become a positive‐negative potential at the surface. The latency of the negative component of the incremental response was about 3.5‐4 ms in layer 4, compared to about 1.5 and 2.5 ms for the mono‐ and disynaptic components of the primary response. The incremental response could only be evoked from the l.g.n. and the optic radiation, not from the optic tract, superior colliculus or other surrounding structures. Within the l.g.n., the effect was only evoked from stimulation sites in approximate retinotopic register with the recording site in the cortex. Low threshold points were found in the A laminae, completely overlapping with the low threshold points for the primary response. Thresholds increased steeply when the stimulation electrode was lowered into the C laminae. The incremental response could still be evoked ten days after the destruction of all cells in the l.g.n. complex by kainic acid. It is concluded that the described incremental response is identical to the augmenting response of Dempsey & Morison (1943) and is mediated by intracortical axon collaterals of antidromically activated cortico‐geniculate neurones.