Coherence in localized visual evoked potentials (VEPs) to spatial stimulus attributes (orientation and displacement)...

The purpose of this study was to compare visual evoked potentials (VEPs) of striate-peristriate and Cz to posterior parietal, especially IPL's supramarginal (Su) and angular (Ang, Ano, and Ant) leads in response to human face spatial attributes attend-task, to assess the concern of the former with basic features versus the latter's spatial attributes "rotated" rightwards, leftwards, or oblique (unusual angle of sight) or nodal one (eye)- and two-item, eye/mouth displacement. Monopolar cranially derived VEPs were recorded bihemispherically from six adults using 12 electrode arrays 1.5 to 2 cm apart. Subjects were instructed to fixate a marking on the monitor at a nodal eyebrow region, while attending the stimulus that was computer displayed for 200 or 300 ms, and followed by mask. Analysis was made on the first 240 ms of the records after stimulus onset; the test was compared to "full face," the hypothesized control. Localized VEP changes were observed task-bound and bihemispherically coherent in parietal leads, especially Ang, Ano, and Su. The early negativity of the "full face" control was replaced by an early positive wave with onset latencies of 70 +/- 15 ms, significantly shorter (p < 0.01) than the striate's 90 +/- 20 ms. The changes were observed for both rotation and displacement. The changes in the angular leads (Ano and Ang) are consistent with the encoding of spatial attributes of orientation and nodal face items displacement. Differentiation of direction and degree of rotation, "seen from above" versus "seen from below," largely encoded by the right Su. The composite double-item eye and displaced or obliquely rotated induced extended coherent activity in parietal leads. Oz, peristriate, and Cz showed no such changes. No attempt is made to imply source localization; we claim parietal localized and coherency of the VEP changes suggest high-order early processing of human face attributes. This occurs by a relatively fast pathway. The "triggers" for the changes are direction and degree of rotation or of displacement rather than actual geometry of the stimulus.