GENERATORS OF VISUAL-EVOKED POTENTIALS INVESTIGATED BY DIPOLE TRACINGIN THE HUMAN OCCIPITAL CORTEX

Current source generators (dipoles) of the human visual evoked potenti als to pattern-onset stimuli were investigated with the dipole tracing method, using a realistic four-layer head model of scalp-skull-fluid- brain, which can equate the surface potential distributions on a scalp to one or two corresponding equivalent dipoles. Three healthy adult h uman subjects were used, and 29 electrodes were set on a scalp of each subject. Visual stimulus of a checkerboard pattern was presented for 250 ms in each of eight different visual fields (central and periphera l parts of each of four quadrant fields). The visual evoked potentials consisting of initial positive-late negative waves (CI and CII compon ents designated by Jeffreys and Axford(18)) were recorded mainly on th e occipital region contralateral to stimulated visual fields. The init ial positive wave (CI) of visual evoked potentials were divided into t wo components: early component of the CI (e-CI-an early small positive deflection with approximate peak latency of 70-90 ms) and late compon ent of the CI (1-Cl-a late large positive deflection with approximate peak latency of 100-120 ms). The dipole with a fit exceeding 98% dipol arity with our model at the shortest latencies was defined as an ''ear liest dipole'' of the evoked potentials, produced by the primary respo nses in the occipital cortex to an afferent volley from the lateral ge niculate body. These earliest dipoles, for eight different visual fiel d stimulations, were estimated at the approximate peak of the e-CI. Es timated dipoles were superimposed on a three-dimensional magnetic reso nance image of each subject's brain. Earliest dipoles for right upper and right lower quadrant-field stimulations were located at the left c alcarine cortices below and above the calcarine fissure, respectively; earliest dipoles for left upper and left lower quadrant-field stimula tions were located at the right calcarine cortices below and above the calcarine fissure, respectively. Furthermore, earliest dipoles for ce ntral and peripheral quadrant-held stimulations were located posterior ly and anteriorly in the calcarine cortex, respectively. The results f rom these non-invasive analyses of visual evoked potentials indicated topographic localization of the dipoles around the calcarine fissure b ased on the loci of the Visual fields. This was comparable to the reti notopy of the human occipital lobe based on clinicopathological studie s. (C) 1998 IBRO. Published by Elsevier Science Ltd.