Tuesday, May 29, 2007

C'mon, Aren't Faces REALLY Special?

A recently published paper in Nature Neuroscience by Thierry and colleagues (2007) claimed that faces really aren't that special after all, particularly where the N170 component is concerned. The Neurocritic covered this article (perhaps a little too UNcritically) back in March: Are Faces Special? Fellow bloggers at The Phineas Gage Fan Club and Cognitive Daily also posted about the paper. What did it claim to show? Let's review some material from a previous post.
An ever-controversial topic in the field of high-level vision and object recognition is the question of whether faces have a privileged status relative to other objects, processed by a special modular region of ventral temporal cortex called the fusiform face area (Kanwisher et al., 1997; Kanwisher & Yove, 2006; McKone et al., 2007), or whether faces are just one example of a stimulus class that requires substantial expertise in order to distinguish between similar exemplars (Gauthier et al., 1999, 2000; Gauthier & Bukach, 2007). A new article in Nature Neuroscience tackles this issue and comes up with a surprising answer.

Thierry and colleagues (2007) recorded event-related potentials (ERPs), which are synchronized brain waves time-locked to the occurrence of particular stimuli or events. In particular, the N170 component is thought to be a highly specific ERP response to faces (as opposed to other objects) that shows a peak at 170 msec after stimulus presentation (Bentin et al, 1996).

Bentin et al. (1996)
In the new NN paper, Thierry et al. argue that previous studies of the N170 component did not adequately control for variability across stimulus classes, i.e., face stimuli have been much more similar to each other than the non-face stimuli.
The bottom line was that if interstimulus perceptual variance was adequately controlled across stimulus classes, the specificity of N170 for faces went away. [But an even earlier component, P1, popped up to discriminate between faces and cars. This wasn't adequately explained.]

But wait! There are problems with this paper -- as outlined by Bruno Rossion from the Face Categorisation Lab in Belgium -- not the least of which is that Thierry et al. didn't adequately control for interstimulus variance in their own study! His reply (and perhaps others) will appear in a future issue of Nature Neuroscience. Watch his web site (and this space) for an update as it occurs. As for now, you can read Bruno's comments here.

Reference

Thierry G, Martin CD, Downing P, Pegna AJ (2007). Controlling for interstimulus perceptual variance abolishes N170 face selectivity. Nature Neuroscience 10:505-511.

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