Visual evoked potentials to small stimuli presented along a vertical meridian: Individual differences and dipole modelling

Abstract

The paper describes aspects of individual variability in Visual Evoked Potentials (VEPs) in terms of variability in cortical anatomy. VEPs were obtained from 6 subjects using small circular stimuli, adjusted in size for cortical magnification factor, and presented at various eccentricities along a vertical meridian. The eccentricities were 0°, ±3°, ±5°, ±10°, and −15°. The scalp topography of the first major component of the VEP, at 118 ms poststimulus onset, differed between upper and lower visual field stimulation with a polarity reversal at midline occipital/parietal sites. However, the degree of individual variability made the interpretation of the group averaged response difficult, especially for the 0° and −3° conditions. Using a three concentric sphere model of the head, equivalent dipole sources of the VEPs were estimated. These were interpreted in conjunction with magnetic resonance images of the brain. Results for individual subjects were consistent with the VEP generator lying within primary visual cortex. However, the cruciform model of primary visual cortex anatomy was inadequate as a predictor of these results. The advantages of interpreting individual rather than group averaged data are emphasised.