Investigating representations of facial identity in human ventral visual cortex with transcranial magnetic stimulation
The occipital face area (OFA) is face-selective. This enhanced activation to faces could reflect either generic face and shape-related processing or high-level conceptual processing of identity. Here authors examined these two possibilities using a state-dependent transcranial magnetic stimulation (TMS) paradigm. The lateral occipital (LO) cortex which is activated non-selectively by various types of objects served as a control site. They localized OFA and LO on a per-participant basis using functional MRI. They then examined whether TMS applied to either of these regions affected the ability of participants to decide whether two successively presented and physically different face images were of the same famous person or different famous people. TMS was applied during the delay between first and second face presentations to investigate whether neuronal populations in these regions played a causal role in mediating the behavioral effects of identity repetition. Behaviorally they found a robust identity repetition effect, with shorter reaction times (RTs) when identity was repeated, regardless of the fact that the pictures were physically different. Surprisingly, TMS applied over LO (but not OFA) modulated overall RTs, compared to the No-TMS condition. But critically, authors found no effects of TMS to either area that were modulated by identity repetition. Thus, they found no evidence to suggest that OFA or LO contain neuronal representations selective for the identity of famous faces which play a causal role in identity processing. Instead, these brain regions may be involved in the processing of more generic features of their preferred stimulus categories.
Stimuli examples and pixel-wise differences. (A) Examples of repeated identity trials stimuli (blue frame) and different identity trials stimuli (red frame). The first presented face image appears on the left, the second on the right. Repeated identity trials depicting Michelle Obama (top) and Tom Hanks (bottom). Different identity trials depicting Al Pacino, Dustin Hoffman, Britney Spears, and Scarlett Johansson are shown. See the list of individuals in Table 1; please note that these are examples for illustrative purposes and the exact images used in the experiment are available on request from the authors. (B) Pixel-wise differences for repeated identity trials and different identity trials (see further details in Materials and Methods). No significant difference in low-level image differences was found between the trial types. (C) Examples from the post hoc stimuli analysis: all stimuli were cropped to restrict the picture to the face only and normalized to a common size (see further details in Materials and Methods). (D) Pixel-wise differences as in (B) for the normalized images (described in C).
Reference: Gilaie-Dotan S, Silvanto J, Schwarzkopf DS and Rees G (2010) Investigating representations of facial identity in human ventral visual cortex with transcranial magnetic stimulation. Front. Hum. Neurosci.4:50. doi: 10.3389/fnhum.2010.00050