SPECIFICITY OF ERP TO CHANGE OF EMOTIONAL FACIAL EXPRESSION. Michael Wright Centre for Cognition and Neuroimaging, Brunel University, Uxbridge, UB8 3PH, U.K. Background: A change in emotional facial expression is known to elicit a prominent N170 component (Miyoshi, et al., 2004; Neuroreport, 15, ). There are many reports of rather small differences in N170 to different facial expressions (e.g. Batty & Taylor, 2003; Cognitive Brain Research, 17, ). The present study examines whether the ERP to expression change gives greater sensitivity and emotion specificity than does ERP to appearance of an emotional face. Method: Stimuli: The stimuli were grey-scale images of neutral and emotional faces from a standardised database (Lyons, et al. 1998). Each trial consisted of presentation of two facial expressions (of the same person) successively for 0.5 s without an interval (Fig.1). There were 10 individual faces in the database, giving 10 unique stimuli per expression change. These were presented in blocks, Results: Emotional expression-change produced a characteristic ERP with a prominent N170, but with a reduced P1 in comparison with face-onset stimuli (Fig.1). a)Comparison of angry and non-emotional (control) changes. The control was a pixel-matched magnification change (zoom). Significant enhancement of N170 and later potentials was found for angry versus control faces, in both angry-neutral and neutral-angry pairs, though in the latter condition, differences were larger. For angry- neutral versus zoom-neutral, significant differences were seen in the N170 of the neutral stimulus (Fig 3ab). b) Comparison of happy and fearful expression changes. Significant differences were found for N170-VPP and late potentials for neutral-happy versus neutral-fear but not for happy-neutral versus fear-neutral. A difference was also noted in the N170-VPP to the neutral stimulus in happy-neutral and fear-neutral conditions (Fig 3cd, Fig4). Fig.2. Group averaged ERP to angry-neutral, neutral-angry, zoom-neutral and neutral-zoom P4 (green) and FCz (blue). P1 is larger to first (face onset) stimulus than second (face change) stimulus. Face onset has a VPP, face change does not. Face change has a large p2/p3 complex that is larger to angry than to zoom. N170 peak latency (at P4) to all onset stimuli is similar ( =160ms). Latency to neutral-angry (165ms) is longer than to neutral-zoom (156ms) and latency to angry-neutral (174ms) is longer than to zoom-neutral (160ms). Neutral - angry Neutral - zoom Face 1 500msFace 2 500ms Fig.1 Fig. 4. grand average ERP to neutral-fear (green) and neutral- happy (red). Baseline corrected to second stimulus onset. Left: P4, bottom: Right: Fz. x- axis divisions 100ms. The ERP to the fearful and happy face differs from 150ms on. Conclusions: The sensitivity of ERP to facial expressions is greater for expression-change stimuli than for face-onset stimuli. Latency, waveform and amplitude differences around N170, P2, P3 are evident between expression-change and size- change images. Expression-change stimuli show selectivity for different emotions: differences were found between happy and fearful stimuli. There is some evidence that the ERP to a neutral face is affected by the preceding emotional face. Participants: 20 participants Design: In a given experiment, a comparison was made between two emotions, or between an emotion change and a control change. The second variable in each experiment was the order within each stimulus pair of emotional and neutral expressions. Recording and data analysis: ERP was recorded using a Neuroscan system (32 electrode, 10-20, Synamps amplifier, 0.1 – 100Hz, Scan 3.2 software). EEG files were epoched, baseline corrected to the pre-stimulus interval, artefact rejected (+-50μv all channels) and averaged. The grand mean and SD of the average ERP was computed. Difference waveforms were obtained for pairs of experimental conditions in each participant, enabling t-values to be computed for each electrode. Grand average ERP’s were baseline corrected to the 50ms preceding the relevant stimulus (first or second) prior to statistical comparison of different experimental conditions. angryneutral zoomneutral angryneutral zoom Fig 3a. Neutral- angry minus neutral-zoom. Fig 3 abcd. 2D scalp distribution showing statistical parametric maps (SPM’s) of differences between experimental conditions. The maps are based on paired-sample t-tests of differences in ERP amplitude averaged over successive 50ms intervals beginning with the first stimulus presentation. The second row corresponds to the second stimulus presentation and the third row begins with the offset of the second stimulus. The colour scale is adjusted so that non-significant differences (Bonferroni corrected for multiple comparisons) appear green. The sign of t (see colour scale) indicates the sign of the difference. The arrow shows the interval containing N170 to the first and second stimulus. Note that LPP differences between happy and fearful stimuli are long lasting. Fig 3b. Angry- neutral minus zoom-neutral. Fig 3c. Neutral-happy minus neutral- fear. Fig 3d. Fear- neutral minus happy-neutral Face on-> Change-> Off-> Face on-> Change-> Off-> Face on-> Change-> Off-> Face on-> Change-> Off-> "Coding Facial Expressions with Gabor Wavelets" Michael J. Lyons, Shigeru Akamatsu, Miyuki Kamachi, Jiro Gyoba Proceedings, Third IEEE International Conference on Automatic Face and Gesture Recognition, April , Nara Japan, IEEE Computer Society, pp