Relationship between visual evoked potentials and subjective differences between emotional expressions in "face diagrams"

The relationship between visual evoked potentials resulting from substitution of one image of a human "face diagram" for another and assessment of perceived differences between the emotional expressions of these faces were studied. Emotions were altered by changing the curvature of the mouth and/or the slope of the brows. Unlike the traditional approach, in which visual evoked potentials are recorded in response to presentation of a single stimulus bearing a face image, visual evoked potentials in the present study were recorded as the response to instantaneous substitution of a reference stimulus with a test stimulus, and thus represented the direct response to the difference between the stimuli. A characteristic of this approach was the use of a series of functionally associated test stimuli, in which there was a monotonic increase in the difference between the test and references images in terms of the variable characteristics of the stimuli. Analysis revealed differences in the amplitudes of the P120, N180, and P230 peaks in leads O1, O2, P3, P4, T5, and T6, which demonstrated high levels of correlation both with a points-scale assessment of perceived differences between the emotional expressions of faces and with the physical (configurative) differences between images of the same faces as defined by the differences in the orientation angles of lines determining mouth curvature and brow angle. Responses were seen to differences between stimuli for both types of change in pairs of images, both going from reference image to test image and from test image to reference image. Changes in the interpeak amplitude of the P120-N180 potential in the temporal areas of both hemispheres provided the earliest electrophysiological measure of perceived differences between the emotional expressions of human faces. This suggests application of the spherical model for the perception of emotions to specify emotional facial expressions in terms of the activity of line orientation detectors.