Generation of the VESPA response to rapid contrast fluctuations is dominated by striate cortex: evidence from retinotopic mapping.

The VESPA (visual-evoked spread spectrum analysis) method derives an impulse response function of the visual system from scalp electroencephalographic (EEG) data using the controlled modulation of some feature of a visual stimulus. Recent research using VESPA responses to modulations of stimulus contrast has provided new insights into both early visual attention mechanisms and the specificity of visual-processing deficits in schizophrenia. To allow a fuller interpretation of these and future findings, it is necessary to further characterize the VESPA in terms of its underlying cortical generators. To that end, we here examine spatio-temporal variations in the components of the VESPA as a function of stimulus location. We found that the first two VESPA components (C1/P1) each have a posterior dorsal midline focus and reverse in polarity across the horizontal meridian, consistent with retinotopic projections to calcarine cortex (V1) for the stimulus locations tested. Furthermore, the focal scalp topography of the VESPA was strikingly constant across the entire C1-P1 timeframe (50-120 ms) for each stimulus location, with negligible global scalp activity visible at the zero-crossing dividing the two. This indicates a common focal source underpinning both components, which was further supported by a significant correlation between C1 and P1 amplitudes across subjects (r=0.54; p<0.05). These results, along with factors implicit in the method of derivation of the contrast-VESPA, lead us to conclude that these responses are dominated by activity from striate cortex. We discuss the implications of this finding for previous and future research using the VESPA.