Event-related potentials to unattended changes in facial expressions: detection of regularity violations or encoding of emotions?

Abstract

Visual mismatch negativity (vMMN), a component in event-related potentials (ERPs), can be elicited when rarely presented "deviant" facial expressions violate regularity formed by repeated "standard" faces. vMMN is observed as differential ERPs elicited between the deviant and standard faces. It is not clear, however, whether differential ERPs to rare emotional faces interspersed with repeated neutral ones reflect true vMMN (i.e., detection of regularity violation) or merely encoding of the emotional content in the faces. Furthermore, a face-sensitive N170 response, which reflects structural encoding of facial features, can be modulated by emotional expressions. Owing to its similar latency and scalp topography with vMMN, these two components are difficult to separate. We recorded ERPs to neutral, fearful, and happy faces in two different stimulus presentation conditions in adult humans. For the oddball condition group, frequently presented neutral expressions (p = 0.8) were rarely replaced by happy or fearful expressions (p = 0.1), whereas for the equiprobable condition group, fearful, happy, and neutral expressions were presented with equal probability (p = 0.33). Independent component analysis (ICA) revealed two prominent components in both stimulus conditions in the relevant latency range and scalp location. A component peaking at 130 ms post stimulus showed a difference in scalp topography between the oddball (bilateral) and the equiprobable (right-dominant) conditions. The other component, peaking at 170 ms post stimulus, showed no difference between the conditions. The bilateral component at the 130-ms latency in the oddball condition conforms to vMMN. Moreover, it was distinct from N170 which was modulated by the emotional expression only. The present results suggest that future studies on vMMN to facial expressions should take into account possible confounding effects caused by the differential processing of the emotional expressions as such.

Keywords: equiprobable condition; facial expressions; independent component analysis; oddball condition; visual mismatch negativity.

Figure 1

Grand-averaged ERPs in the oddball condition. Raw ERPs to happy and fearful deviant faces and to the neutral standard faces immediately preceding them. Stimulus onset at time 0.

Figure 2

Grand-averaged ERPs in the equiprobable condition. Raw ERPs to happy, fearful, and neutral faces. Stimulus onset at time 0.

Figure 3

Grand-averaged differential ERPs (emotional minus neutral face). Raw ERPs.

Figure 4

Back-projected 130-ms components for each participant. Differential response waveforms (emotional minus neutral) at P7 and P8 electrodes are drawn separately for fearful and happy faces and for oddball condition and equiprobable condition groups.

Figure 5

Back-projected 170-ms components for each participant. Differential response waveforms (emotional minus neutral) at P7 and P8 electrodes are drawn separately for fearful and happy faces and for oddball condition and equiprobable condition groups.

Figure 6

Scalp potential maps of the 130-ms component back-projected to the electrodes. Map for the equiprobable condition group on left and map for the oddball condition group on right.

Figure 7

Mean amplitude values (μV), confidence intervals, and scatterplots of the individual participants' values for each electrode in the equiprobable condition (EQ) and oddball condition (OB) for the 130-ms component (differential response; emotional minus neutral face). An asterisk (^*) indicates a significant difference (p < 0.05) between conditions at P7 and Pz electrodes.

Figure 8

Lateralization index for the 130-ms (A) and 170-ms (B) components separately for both conditions (oddball = OB and equiprobable = EQ). The values are calculated for the back-projected components' amplitudes from electrodes P7 (left) and P8 (right) as follows: (left – right)/(left + right). The bars represent the mean values in the group and the whiskers 95% confidence intervals. Individual participants' values are marked with filled circles.

Figure 9

Scalp potential maps of the 170-ms component back-projected to the electrodes. Map for the equiprobable condition group on left and map for the oddball condition group on right.

Figure 10

Mean amplitude values (μV) and confidence intervals, and scatterplots of the individual participants' values for each electrode in the equiprobable condition (EQ) and oddball condition (OB) for the 170-ms component (differential response; emotional minus neutral face). No significant differences between conditions were found.