Sex Matters When You Ask the Right Question: What Affects Eye Movements in Face Comparison Tasks?

Knowing where people look in a face provides an objective insight onto the information entering the visual system and into the cognitive processes involved in face perception. Eye-tracking studies on face perception have mostly investigated observers’ viewing behavior when studying single faces. However, in day-to-day situations, humans also compare faces or match a person’s face to a photograph. During comparison, facial information remains visually accessible, freeing observers from time and encoding constraints [1]. Here, we recorded eye movements of human participants while they compared two faces presented simultaneously. We used (i) two different tasks (discrimination or categorization), and (ii) faces differing either in identity or in sex. In addition, we varied (iii) task difficulty, i.e. the similarity of the two faces in a pair. Eye movements to previously defined areas of interest (AOIs) on the faces were analyzed in terms of frequency, duration and the temporal pattern of fixations made. We found that the eyes were fixated most often in the discrimination tasks (37 of all fixations) but the nose in the categorization task (34.5), while the total number of fixations increased with task difficulty. Faces differing in sex were more difficult to discriminate than faces differing in identity (63 versus 76 correct responses), which was also reflected in more fixations to face pairs differing in sex (14.4 versus 11.8 fixations per trial). With increasing task difficulty, fixations to only some AOIs increased, in accordance with the literature (more to the eyes in the sex and more over all areas in the identity discrimination tasks; [2]). Unexpectedly, we found a striking effect of tasks on performance measures, as over 80 of participants could detect the more feminine of two faces (categorization task) even at the most similar level, but for the same face pairs their performance in a discrimination task was less than 30 correct. Another interesting finding is that observers mostly compared the inner halves of the two faces of a pair, instead of the corresponding features (e.g., the left eye of the left face with the left eye of the right face). This viewing behavior remained the same in a control experiment where participants’ head was not fixed. Quite surprisingly, female participants fixated significantly more often the eyes of the face stimuli than male participants, but only when the sex of the faces was a relevant feature in the task