Using multi-stimulus VEP source localization to obtain a retinotopic map of human primary visual cortex

Objective: The goal of this study was to acquire a detailed spatial and temporal map of primary visual cortex using a novel VEP stimulus and analysis technique. Methods: A multi-stimulus array spanning the central 18 degrees of the visual field was used where each of 60 checkerboard stimulus ‘patches’ was simultaneously modulated with an independent binary m-sequence (Sutter, 1992). VEPs corresponding to each patch were recorded from 3 subjects using a dense posterior electrode array. For each stimulus patch, single dipole source localization was conducted to determine the location, magnitude, and time-function of the underlying neural activation. To reduce ambiguity in the solution, a common time-function was assumed for stimulus patches at the same visual eccentricity (defining an annulus). The analysis was conducted independently for each annulus composed of 4–12 patches. Results: The loci of the dipole solutions followed a smooth retinotopic pattern across annuli consistent with the classical organization of primary visual cortex. Specifically, each dipole was found contralateral to the corresponding stimulus patch and field inversion was observed for all subjects. Conclusions: Using this technique, the most detailed spatial and temporal retinotopic map of primary visual cortex to date has been obtained.