Size-invariant facial expression categorization and associated gaze allocation within social interaction space.

As faces often appear under very different viewing conditions (eg brightness, viewing angle, or viewing distance), invariant facial information recognition is a key to our social interactions. Although we would clearly benefit from differentiating different facial expressions (eg angry vs happy) at a distance, there is surprisingly little research examining how expression categorization and associated gaze allocation are affected by viewing distance within the range of typical social space. In this study I systematically varied the size of faces displaying six basic facial expressions of emotion with varying intensities to mimic viewing distances ranging from arms length to 5 m, and employed a self-paced expression categorization task to measure participants' categorization performance and associated gaze patterns. Irrespective of the displayed expression and its intensity, the participants showed indistinguishable categorization accuracy and reaction time across the tested face sizes. Reducing face size decreased the number of fixations directed at the faces but increased individual fixation durations, and shifted gaze distribution from scanning all key internal facial features to fixating at mainly the central face region. The results suggest size-invariant facial expression categorization behaviour within social interaction distance which could be linked to a holistic gaze strategy for extracting expressive facial cues.