Contrast negation differentiates face processing pathways

A recent study published by Pam Pallett and Ming Meng in the Journal of Vision demonstrates that there are shared neural mechanisms during perceptual encoding, and at least partially separate neural mechanisms during recognition for facial identity versus facial expression. 

 Bruce and Young (1986) proposed a model for face processing that begins with structural encoding, followed by a split into two processing streams: one for the dynamic aspects of the face (e.g., facial expressions of emotion) and the other for the invariant aspects of the face (e.g., gender, identity). Yet how this is accomplished remains unclear. Pallett and Meng took a psychophysical approach using contrast negation to test the Bruce and Young model. Previous research suggests that contrast negation impairs processing of invariant features (e.g., gender) but not dynamic features (e.g., expression). In Pallett and Meng’s first experiment, participants discriminated differences in gender and facial expressions of emotion in upright, inverted, and contrast-negated faces. Results revealed a profound impairment for contrast-negated gender discrimination, whereas expression discrimination remained relatively robust to contrast negation. To test whether this differential effect occurs during perceptual encoding, Pallett and Meng conducted three additional experiments in which they measured aftereffects following upright, inverted, or contrast-negated face adaptation for the same discrimination task as in the first experiment. Results showed a mild impairment with contrast negation during perceptual encoding for both gender and expression, followed by a marked gender-specific deficit during contrast-negated face discrimination. Taken together, these results suggest that there are shared neural mechanisms during perceptual encoding, and at least partially separate neural mechanisms during recognition and decision making for dynamic and invariant facial-feature processing.