The flash-evoked afterdischarge

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(1) The flash-evoked afterdischarge has been analysed under various functional states along the visual pathway: optic tract (OT), lateral geniculate body (LG) and cortical areas 17 and 18 (VC). Experiments were mainly conducted in the unanaesthetized cat (encéphale isolé, cerveau isoléor immobilized with Flaxedil), but findings from anaesthetized preparations were also recorded in order to correlate data obtained in various experimental conditions. The use or non-use of anaesthetics and the different locations of the recording LG and VC electrodes may account for divergences in literature concerning the correlations between cortical and subcortical flash-evoked events, and the nature of the VC photic afterdischarge. (2) A differentiation between the early wave and the subsequent (post-primary) complex was attempted in the unanaesthetized cat. Similar dissociations could be found in experiments performed on vigil submammals and night-active monkeys. By changing the parameters of test flash stimulus and during background illumination parallel alterations of the initial ( a) VC wave and of the OT and LG responses occurred. This indicates the extracortical (retinal) origin of the early wave of the VC. On the other hand, central specific origin and cortical elaboration of the b-complex (after-discharge) are suggested by many experimental findings: (i) alterations induced by variation in parameters of the test flash and by steady retinal illumination, opposite in sign to those undergone by the early VC deflection and the OT and LG responses; enhancement of the b-complex by decreasing the flash intensity is though to result from diminution of inhibitory mechanisms set in motion by strong photic stimulation; (ii) potentiation by preceding LG or direct VC stimulation, without affecting the early wave; (iii) close time-relation between the b-complex recorded from the surface and unit discharges in the depth of the VC under the potentiating influence of steady light; (iv) simulation of naturally desynchronized photically elicited impulses by trains of high-frequency pulses applied to the central afferent pathway and inducing a multiple response in the VC, the late components of which resemble in reactivity the flash-evoked afterdischarge; (v) the pattern of the b-complex which, under the selective potentiation induced by steady light, strikingly resembles that of a centrally evoked response with fast spikes preceding the biphasic slow sequence; in a few, privileged experiments, close temporal relation could be found between such a b-wave and grouped unit discharges recorded from the lateral part of the LG; even in such a case, the most part of the b-complex seemed to be elaborated in the VC since decortication left unaltered the fast positive spikes while inducing disappearance of the subsequent slow sequence. (3) The mechanisms of the steady-light potentiating effect mediated by the specific pathway and mainly exerted at the VC level are discussed. (4) Some correlations between the photically evoked afterdischarge and perceptual integrative processes of the VC are suggested.