Event-related potentials to changes in facial expression in two-phase transitions

Abstract

The purpose of the study was to compare event-related potentials (ERPs) to different transitions between emotional and neutral facial expressions. The stimuli contained a single transition between two different images of the same face, giving a strong impression of changing expression though apparent motion whilst eliminating change in irrelevant stimulus variables such as image contrast or identity. Stimuli were calibrated for intensity, valence and perceived emotion category and only trials where the target emotion was correctly identified were included. In the first experiment, a magnification change (zoom) was a control condition. Transitions from neutral to angry expressions produced a more negative N1 with longer peak latency, and more positive P2 than did an increase in magnification. Critically, response to neutral following angry, relative to neutral following magnified, showed a generally more negative ERP with a delayed N1 peak and reduced P2 amplitude. In the second experiment, comparison of neutral-happy and neutral-frightened transitions showed significantly different ERPs to emotional expression change. Responses to the reversed direction of a transition (happy-neutral and frightened-neutral) were much reduced. Unlike the comparison of angry-neutral with magnified-neutral, there were minimal differences in the responses to neutral following happy and neutral following frightened. The results demonstrate in a young adult sample the directionality of responses to facial expression dynamics, and suggest a separation of neural mechanisms for detecting expression changes and magnification changes.

Fig 1. Comparison of ERP responses to onset of an angry face and transition from neutral to angry expression.

Butterfly and mean global field power (MGFP) plots are shown at the top of the figure for angry face onset at -500 ms (a) and neutral-angry transitions at 0 ms (c). The corresponding ERP scalp distributions are shown in the lower half of the figure, sampled at 25 ms intervals from onset of an angry face (b) or from the transition from neutral to angry (d).

Fig 2. Grand mean ERP waveforms for the response to the transition in a face pair for all four experimental conditions: black = neutral-angry, red = neutral-magnified, green = angry-neutral, blue = magnified-neutral.

Positive is upwards.

Fig 3. t-maps showing the scalp distribution of significant differences between experimental conditions.

The colour scales have been adjusted for each comparison so that t-values generating non-significant differences (FDR-corrected p > .05) are shown as green. Significant t-values are shown as yellow, orange and red for positive differences and cyan, light and dark blue for negative differences. a: neutral-angry minus neutral-magnified (t crit = ± 3.4); b: angry-neutral minus magnified-neutral (t crit = ± 2.7); c: neutral-angry minus angry-neutral (t crit = ± 2.9); d: neutral-magnified minus magnified-neutral (all n.s.).

Fig 4. Grand-average ERP waveforms transitions between neutral and happy or neutral and frightened facial expressions.

Blue trace = neutral-frightened, black trace = neutral-happy, red trace = frightened-neutral, green trace = happy-neutral. Positive upwards.

Fig 5. t-maps showing differences between experimental conditions.

The colour scales have been adjusted for each comparison so that t-values generating non-significant differences (FDR-corrected p > .05) are shown as green. Significant t-values are shown as yellow, orange and red for positive differences and cyan, light and dark blue for negative differences. a. neutral-frightened minus neutral-happy (t crit = ± 3.9); b. frightened-neutral minus happy-neutral, (all n.s); c. neutral-frightened minus frightened-neutral (t crit = ± 2.4); d. neutral-happy minus happy-neutral (t crit = ± 2.3).