Pattern-pulses and pattern-reversals evoke different cascades of cortical sources in the multifocal visual evoked potential

The multifocal visual evoked potential (mVEP), elicited by either pattern-pulses or pattern-reversals, provides an effective means to study visual processing and to identify retinal damage and visual field defects. It is often assumed that the first components of these VEPs, the C1 and N75, respectively, are generated in V1, based on source modeling and their polarity reversal between upper- and lower-field stimulus presentations. However, limitations in the spatial resolution of source modeling and the non-uniqueness of the polarity reversal heuristic leave this assumed V1 source uncertain. We recently demonstrated the utility of a novel method to resolve visual sources by correlating retinotopically varying VEP topographies with predictions from the Benson-2014 retinotopy atlas. Here, we apply this method to study the sources of both the pulse and reversal mVEP, presented at the same stimulus event rates of between 3-8 Hz per location (approximately 35 Hz overall event rate). This analysis suggested that although V1 dominated the generation of the pulse mVEP throughout its time course, the initial component of the reversal mVEP was instead dominated by extrastriate areas, with V1 dominance emerging later from approximately 110 ms onwards. Although the initial component of the reversal mVEP did exhibit the classic sign of a V1 source-polarity reversal across the horizontal meridian-this basic feature is also produced by extrastriate areas such as V2 and V3, and the strong lateralization of topographies near the vertical meridian predicted by a V1 source was not observed. These results suggest that the pulse and reversal mVEP evoke different cascades of generative visual areas when evoked at the event rate tested here.