Human brain responsivity to different intensities of masked fearful eye whites: an ERP study

Previous studies have shown differential event-related potentials (ERPs) to intensities of fearful facial expressions. There are indications that the eyes may be particularly relevant for the recognition of fearful expressions, even the amount of white sclera exposed above and on sides of the dark pupil could activate the amygdala response. To investigate whether the ERP differences between intensities of fearful expressions are driven by the differential salience of the eyes in the fearful faces, ERPs were measured within a backward masking paradigm, where observers were asked to do a gender-decision task with male and female neutral faces. The emotional stimuli used were low-intensity (50%), prototypical (100%), and caricatured (150%) fearful eye whites that were derived from corresponding intensities of fearful faces respectively. Three groups of white squares that have the same pixels as the eye whites were created as control conditions. Analysis of the ERP data showed a linear increase in amplitudes of the parietal-occipital P120 by three intensities of fearful eye whites. These ERP effects were proved sensitive to intensities of negative emotions but not to the simple physical features as the same patterns of differences were not observed on white squares. Larger parietal-occipital P250 amplitudes were observed for caricatured 150% than low-intensity 50% fearful eye whites. It might reflect the subcortical pathway of emotion-specific, fearful processing. The results demonstrate that the human brain is sensitive to intensities of fear, even if just shown intensities of fearful eye white in the absence of awareness.