The race lightness illusion explored with pupillometry

Abstract

The pupil can react to imaginary light when people think that a stimulus is brighter than another although they are identical in brightness. In the present study, we similarly expected the participants to show dilations of the pupil that would be larger for Africans faces than Caucasians faces. Recent studies have also suggested that people tend to label black faces as darker and white faces as brighter. In this experiment, we presented face stimuli from Africans, Asians and Caucasian faces that were edited in adobe Photoshop so as to be isoluminant and re-sized to the same amount of pixels so as to fully control for the pixels brightness. We used an eye tracker and pupillometry to measure gaze and the pupillary changes when participants viewed face images. The analyses showed that several effects on pupillary change and some of these interacted with the between-subjects factors. Crucially, there was an effect of Faces Race which interacted with the sex of the participants, their ethnicity, and changed along as time elapsed from the onset. The key finding was that the pupil dilated more for the Caucasian faces than for the Asians and these in turn more than for the Africans faces. Data from the eye fixations analysis revealed that the center of the face was looked at the most and equally longer regardless of the race of the face or of the participants. The pupil dilated slowly and reached maximal dilation around 2 seconds after onset of each face. Finally, an analysis of the pixels luminance revealed a higher luminance for the Asians center of face compared to Caucasian and African faces, whereas the latter two sets of faces did not differ in their central regions pixels luminance. We concluded that the pupils dilate to the illusory brightness of faces of different ethnicity so as to adjust the amount of light entering the eye and match to much the expected luminance for each ethnicity. That is, by enlarging less the pupil when seeing the faces of Africans, these will look darker than when enlarging more the pupil for Caucasians face, which will consequently look brighter.