VISUAL-EVOKED POTENTIALS TO SMALL STIMULI PRESENTED ALONG A VERTICAL MERIDIAN - INDIVIDUAL-DIFFERENCES AND DIPOLE MODELING

The paper describes aspects of individual variability in Visual Evoked Potentials (VEPs) in terms of variability in cortical anatomy. VEPs w ere obtained from 6 subjects using small circular stimuli, adjusted in size for cortical magnification factor, and presented at various ecce ntricities along a vertical meridian. The eccentricities were 0-degree , +/-3-degrees, +/-5-degrees, +/-10-degrees, -15-degrees. The scalp to pography of the first major component of the VEP, at 118 ms poststimul us onset, differed between upper and lower visual field stimulation wi th a polarity reversal at midline occipital/parietal sites. However, t he degree of individual variability made the interpretation of the gro up averaged response difficult, especially for the 0-degree and -3-deg rees conditions. Using a three concentric sphere model of the head, eq uivalent dipole sources of the VEPs were estimated. These were interpr eted in conjunction with magnetic resonance images of the brain. Resul ts for individual subjects were consistent with the VEP generator lyin g within primary visual cortex. However, the cruciform model of primar y visual cortex anatomy was inadequate as a predictor of these results . The advantages of interpreting individual rather than group averaged data are emphasised.