CHRONOTOPOGRAPHICAL ANALYSIS OF THE HUMAN EVOKED POTENTIAL IN RELATION TO THE VISUAL FIELD (DATA FROM NORMAL INDIVIDUALS AND HEMIANOPIC PATIENTS)

On theoretical grounds, the exact location and orientation of the intracranial generators of evoked potentials cannot be evaluated from scalp recordings since “a given surface field can be generated by an infinite variety of source configurations.”’ However, interesting and valuable indications concerning the most probable sites of these sources may be obtained through a proper analysis of the spatial distribution of the various components of the evoked response resulting from well-defined experimental variables and taking into account the complexity of the geometry of the sensory cortex. It is well known that one of the main problems concerning the interpretation of the scalp-recorded evoked potentials and the understanding of their intracranial sources comes chiefly from their being made up of several components overlapping in both time and space. Scalp topography along with the use of experimental variables permitting the isolation of the overlapping components is one means of assessing, in normal humans, the origin of such brain events. Another way is to evaluate the effects of brain lesions on the various components of the response, in particular on their spatial distribution. Both approaches have been used in our laboratory for several years in order to shed some light on the underlying sources of the evoked response obtained for the visual modality.2-” The present paper deals with data obtained by means of spatiotemporal maps” concerning: (1 ) the modifications of the various components of the visual response obtained from “normal” individuals in relation to (a) the part of the visual field stimulated, and (b) the presence or absence of luminance and/or spatial contrast changes; (2) the changes of the pattern-reversal response as seen in patients with lateral homonymous hemianopia of various origins depending on the hemifield stimulated. Keeping in mind that the visual cortex presents the most complex geometry of all sensory projection areas, the analysis of the modifications of each component as a function of the region of the retina being stimulated will be discussed and interpreted in terms of a simple dipole sheet model of the visual cortex, similar to that proposed by several authors.’.’0,’’,’3-22

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