How the visual brain detects emotional changes in facial expressions: Evidence from driven and intrinsic brain oscillations.

The processing of facial expressions is often studied using static pictorial cues. Recent work, however, suggests that viewing changing expressions more robustly evokes physiological responses. Here, we examined the sensitivity of steady-state visual evoked potentials and intrinsic oscillatory brain activity to transient emotional changes in facial expressions. Twenty-two participants viewed sequences of grayscale faces periodically turned on and off at a rate of 17.5 Hz, to evoke flicker steady-state visual evoked potentials (ssVEPs) in visual cortex. Each sequence began with a neutral face (flickering for 2290 msec), immediately followed by a face from the same actor (also flickering for 2290 msec) with one of four expressions (happy, angry, fearful, or another neutral expression), followed by the initially presented neutral face (flickering for 1140 msec). The amplitude of the ssVEP and the power of intrinsic brain oscillations were analyzed, comparing the four expression-change conditions. We found a transient perturbation (reduction) of the ssVEP that was more pronounced after the neutral-to-angry change compared to the other conditions, at right posterior sensors. Induced alpha-band (8-13 Hz) power was reduced compared to baseline after each change. This reduction showed a central-occipital topography and was strongest in the subtlest and rarest neutral-to-neutral condition. Thus, the ssVEP indexed involvement of face-sensitive cortical areas in decoding affective expressions, whereas mid-occipital alpha power reduction reflected condition frequency rather than expression-specific processing, consistent with the role of alpha power changes in selective attention.