The influence of stimulus orientation on the vertex positive scalp potential evoked by faces.

The scalp-recorded "vertex-positive peak" (VPP) evoked by images of faces in humans has previously been shown to be delayed when an originally upright stimulus is inverted or rotated by 90 degrees (Jeffreys 1989a). This paper describes a study of the effects on this scalp potential of smaller face orientation changes (15 degrees increments). The results showed that, under normal viewing conditions of clearly defined facial images, the VPP latency, which was minimal for face orientations within 15 degrees of the vertical (0 degree), increased almost linearly for incremental rotations from 15 up to 90 degrees, but was relatively unchanged or decreased slightly for further rotations from 90 up to 180 degrees. Similar results were observed for clockwise and anticlockwise rotations, and for different facial representations. These stimulus orientation changes did not change the latency of simultaneously recorded, pattern-specific potentials recorded from occipital scalp locations; nor did they greatly affect the VPP amplitude. By contrast, rotations of "Mooney figure" stimuli away from the vertical produced concurrent reductions in both the perception of a face and the amplitude of the evoked VPP. Experiments in which the orientation of both the stimulus face and the subject's head were varied further showed that minimal latency VPP responses were evoked for parallel stimulus and viewing orientations. The speed of response is thus determined by the orientation of the subtended retinal image.