Facial expression processing in developmental prosopagnosia
Individuals with developmental prosopagnosia experience lifelong deficits recognising facial identity, but whether their ability to process facial expression is also impaired is unclear. Addressing this issue is key for understanding the core deficit in developmental prosopagnosia, and for advancing knowledge about the mechanisms and development of normal face processing. In this thesis, I report two online studies on facial expression processing with large samples of prosopagnosics. In Study 1, I compared facial expression and facial identity perception in 124 prosopagnosics and 133 controls. I used three perceptual tasks including simultaneous matching, sequential matching, and sorting. I also measured inversion effects to examine whether prosopagnosics rely on typical face mechanisms. Prosopagnosics showed subtle deficits with facial expression, but they performed worse with facial identity. Prosopagnosics also showed reduced inversion effects for facial identity but normal inversion effects for facial expression, suggesting they use atypical mechanisms for facial identity but normal mechanisms for facial expression. In Study 2, I extended the findings of Study 1 by assessing facial expression recognition in 78 prosopagnosics and 138 controls. I used four labelling tasks that varied on whether the facial expressions were basic (e.g., happy) or complex (e.g., elated), and whether they were displayed via static (i.e., images) or dynamic (i.e., video clips) stimuli. Prosopagnosics showed subtle deficits with basic expressions but performed normally with complex expressions. Further, prosopagnosics did not show reduced inversion effects for both types of expressions, suggesting they use similar recognition mechanisms as controls. Critically, the subtle expression deficits that prosopagnosics showed in both studies can be accounted for by autism traits, suggesting that expression deficits are not a feature of prosopagnosia per se. I also provide estimates of the prevalence of deficits in facial expression perception (7.70%) and recognition (2.56% - 5.13%) in prosopagnosia, both of which suggest that facial expression processing is normal in the majority of prosopagnosics. Overall, my thesis demonstrates that facial expression processing is not impaired in developmental prosopagnosia, and suggests that facial expression and facial identity processing rely on separate mechanisms that dissociate in development.