Brain lateralization of holistic versus analytic processing of emotional facial expressions

- Whole- but not half-face expressions modulate right-hemisphere N170 and EPN.
- Expression (N170) and emotional (EPN) encoding involve holistic face processing.
- Separate analysis of visually salient smiles enhances left-hemisphere N170 activity.
- Modelled high visual saliency of smiling mouth area precedes left-hemisphere N170.
- Diagnostic smiles are discriminated earlier (P3) than diagnostic angry eyes (LPP).

This study investigated the neurocognitive mechanisms underlying the role of the eye and the mouth regions in the recognition of facial happiness, anger, and surprise. To this end, face stimuli were shown in three formats (whole face, upper half visible, and lower half visible) and behavioral categorization, computational modeling, and ERP (event-related potentials) measures were combined. N170 (150-180 ms post-stimulus; right hemisphere) and EPN (early posterior negativity; 200-300 ms; mainly, right hemisphere) were modulated by expression of whole faces, but not by separate halves. This suggests that expression encoding (N170) and emotional assessment (EPN) require holistic processing, mainly in the right hemisphere. In contrast, the mouth region of happy faces enhanced left temporo-occipital activity (150-180 ms), and also the LPC (late positive complex; centro-parietal) activity (350-450 ms) earlier than the angry eyes (450-600 ms) or other face regions. Relatedly, computational modeling revealed that the mouth region of happy faces was also visually salient by 150 ms following stimulus onset. This suggests that analytical or part-based processing of the salient smile occurs early (150-180 ms) and lateralized (left), and is subsequently used as a shortcut to identify the expression of happiness (350-450 ms). This would account for the happy face advantage in behavioral recognition tasks when the smile is visible.