Hemispheric specialization for holistic processing of faces in normal and prosopagnosic observers?

Holistic processing has been considered a property of the right hemisphere and associated with face perception whereas part-based processing has been considered a property of the left hemisphere and associated with word reading in Caucasian readers. This hemispheric profile may not hold for Chinese individuals given their greater reliance on holistic processing of faces and the bilateral hemispheric engagement for reading. To explore the hemispheric basis of holistic processing of faces and examine whether this holds equally for Caucasian and Chinese observers, we created composite stimuli, split down the vertical midline, by pairing one left half with the right half of another face of the same gender and race. We included both Chinese and Caucasian faces to account for possible other-race effects as well. Participants made same-different judgments about the left or the right halves of two sequentially presented composite faces, in two conditions: when the face halves were aligned or misaligned. For Chinese and Caucasian observers, a larger congruency effect for the aligned than misaligned faces was found (i.e., alignment by congruency interaction), suggesting strong holistic processing of left-right composite faces. However, there was neither effect of the visual field of the face half to be judged (i.e., no hemispheric modulation), nor of race of the participants. These findings reflect equal participation of both hemispheres in face perception and this held across both groups. This left-right composite face paradigm was further validated in individuals with congenital prosopagnosia whose performance in this task was impaired compared to the control observers. Taken together, holistic processing was demonstrated using left-right composite faces for the first time. The magnitude of holistic processing was equivalent for face halves presented in either visual field (processed in the corresponding hemisphere) and for all participants with normal face perception but not for the congenital prosopagnosia observers.