Abstract
Neuroimaging studies have provided conflicting evidence with respect to the fusiform face area and its sensitivity to facial identity. Rotshtein et al. (2005) reported discrimination of individual faces within this region solely given their perception as different identities. Contrariwise, others have demonstrated sensitivity of the FFA even to small physical changes along morph continua of faces (Galaie-Dotan & Malach, 2006; Loffler et al., 2005). Although fMRI-adaptation was used across all studies, their incompatible results may result from differences in the stimulus material presented. While Rotshtein et al. (2005) presented famous faces, unfamiliar faces—for which categorical perception (CP) is less pronounced—were presented in the latter investigations. Here, we readdressed this question while presenting faces personally familiar to participants (N=13). Additionally, we ensured a fully balanced design in that both identities of a morph continuum were perceived with equal likelihood, thus nullifying the potential of novelty-driven effects (Rotshtein et al., 2005).
Behaviorally, we found CP of identity: stimulus-pairs located on opposite sides of the perceptual boundary (“between”) were more likely to be perceived as different, than those located on the same side (“within”). Regarding fMRI results, the magnitude of release from adaptation within the FFA was comparable for these conditions. This confirms the notion that the FFA is indeed sensitive to subtle physical changes between faces perceived as the same identity. A whole-brain analysis showed that various regions of the “core-system” (Haxby et al., 2000) adapted to face identity. Larger release from adaptation to “between” vs. “within” conditions was found within two face-selective regions not including the FFA (rSTG, left middle frontal gyrus).
Our results contradict the view that the FFA codes differences between familiar faces only if they are perceived as different identities. Furthermore, they highlight the importance of other face-selective regions for discriminating facial identity.