Eye-movement patterns reflect perceptual biases apparent in chimeric face processing

Studies of patients with focal brain lesions and neuroimaging indicate that face processing is predominantly based on right-hemisphere function. In addition, experiments with chimeric faces, where the left and the right sides of the face are different, have shown that observers tend to bias their responses toward the information on the left. Here, we monitored eye movements during a gender-identification task using blended face images for both whole and chimeric (half female, half male) faces (Burt and Perrett, 1997 Neuropsychologia 35 685 - 693). As expected, we found a left perceptual bias: subjects based their gender decision significantly more frequently on the left side of the chimeric faces. Analysis of the first saccade showed a significantly greater number of left fixations independent of perceptual bias, presumably reflecting the tendency to first inspect the side of the face better suited to face analysis (left side of face/right hemisphere). On top of this, though, there was a relationship between response and fixation pattern. On trials where participants showed a left perceptual bias, they produced significantly more and longer fixations on the left. However, for trials where participants showed a right perceptual bias, there was no reliable difference between the number or length of fixations on the left or the right. These results demonstrate that on a trial-by-trial basis subtle differences in the extent of left or right side scanning relate to the perceptual response of the participant, although an initial fixation bias occurs irrespective of response bias.