VISUAL-EVOKED MAGNETIC RESPONSES TO CENTRAL AND PERIPHERAL STIMULATION - SIMULTANEOUS VEP RECORDINGS

Visual evoked magnetic field (VEF) and visual evoked potentials (VEP) in response to pattern reversal left half-field central (0-2 degrees, 0-5 degrees) and to left half-field peripheral stimulation (2-15 degre es, 5-15 degrees) were simultaneously recorded in 10 normal subjects. The aim was to localize the origin of the largest wave around 100 ms. By comparing the magnetic evoked 100m wave and electric evoked P100 wa ve, we found that the response to the small central field (0-2 degrees ) stimulation was clearly evident in the VEP and not in the VEF, while the response to the peripheral field (2-15 degrees, 5-15 degrees) sti mulation was clearly present in both the VEP and VEF. Our findings sho w that in contrast to the already recognized predominantly macular ori gin of VEP activity, VEF activity has a peripheral rather than macular prevalence. Our VEF findings are related to the retinotopic organizat ion of the visual cortex; the 100m dipole to the central field (0-5 de grees) was localized more (1.04 +/- 0.84 cm) posterior than was the 10 0m dipole to peripheral stimulation (5-15 degrees). The localization o f 100m dipoles superimposed on magnetic resonance images (MRI) to cent ral stimuli showed interindividual variations that were in agreement w ith the known variability of the central field representation in the s triate cortex: on the convexity of the occipital pole, in the outer su rface of the occipital lobe, and around the calcarine fissure of the r ight hemisphere. In contrast, following peripheral stimulation, the 10 0m dipole was located along the medial surface of the hemisphere or in the calcarine fissure. Our results suggest that the main origin of th e largest wave around 100ms in response to pattern reversal stimuli is in the striate cortex.