Are Faces Special?

modified from Carmel & Bentin (2002) and Gauthier et al. (1998)

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. This ERP was first reported by Bentin et al. (1996), although other researchers observed a related face-specific response back in the late 80's (Jeffreys, 1989). The N170 is most pronounced at the posterior temporal electrodes and is greater over the right hemisphere than the left.

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.

Faces and cars with high and low interstimulus perceptual variance (ISPV).

Thierry et al. (2007)

The actual stimuli used in Experiment 1 are shown below.

From Fig 4a (Thierry et al. 2007)

It turned out that interstimulus perceptual variability alone affected the size of the N170, with low ISPV stimuli (both faces and cars) producing a larger N170 than high ISPV stimuli.

Furthermore, when controlling for ISPV, the face-specific effect for the N170 went away. The face N170 peaked earlier in time than the car N170, but they did not differ in amplitude. However, a face-dominant effect for an earlier component, the P1, emerged (which was an unexpected finding, since it suggests that category-specificity can manifest itself at 100 msec, 70 msec before the N170 component).

Hmm, interesting. Why did this happen? The authors are cautious in their interpretation:

On the other hand, the P1, a peak generally regarded as an index of lower level perceptual processing, was surprisingly unaffected by differences in ISPV, but was sensitive to object category in both Experiments 1 and 2. This should not be interpreted as evidence for absolute category selectivity because only three object categories were tested here. Indeed, faces, cars and butterflies differ in terms of overall composition and various perceptual properties such as outline, contrast, subparts and complexity, which were not manipulated here. It is therefore possible that the P1 still reflects perceptual differences between these objects; that is, that apparent category selectivity in this component is an emergent property arising from low perceptual invariants.

Thierry G, Martin CD, Downing P, Pegna AJ (2007). Controlling for interstimulus perceptual variance abolishes N170 face selectivity. Nature Neuroscience. Published online: 04 March 2007.

Establishing when and how the human brain differentiates between object categories is key to understanding visual cognition. Event-related potential (ERP) investigations have led to the consensus that faces selectively elicit a negative wave peaking 170 ms after presentation, the 'N170'. In such experiments, however, faces are nearly always presented from a full front view, whereas other stimuli are more perceptually variable, leading to uncontrolled interstimulus perceptual variance (ISPV). Here, we compared ERPs elicited by faces, cars and butterflies while—for the first time—controlling ISPV (low or high). Surprisingly, the N170 was sensitive, not to object category, but to ISPV. In addition, we found category effects independent of ISPV 70 ms earlier than has been generally reported. These results demonstrate early ERP category effects in the visual domain, call into question the face selectivity of the N170 and establish ISPV as a critical factor to control in experiments relying on multitrial averaging.

Hypothesized neural generators of the face N170.

Bentin et al. (1996)

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see also Domain specificity vs. Connectionism


References

Bentin S, Allison T, Puce A, Perez E, McCarthy G (1996). Electrophysiological studies of face perception in humans. J Cog Neurosci. 8:551-565.

Gauthier I, Bukach C. (2007). Should we reject the expertise hypothesis? Cognition 103:322-30.

Gauthier I, Skudlarski P, Gore JC, Anderson AW. (2000). Expertise for cars and birds recruits brain areas involved in face recognition. Nat Neurosci. 3:191-7.

Gauthier I, Tarr MJ, Anderson AW, Skudlarski P, Gore JC. (1999). Activation of the middle fusiform 'face area' increases with expertise in recognizing novel objects. Nat Neurosci. 2:568-73.

Jeffreys DA. (1989). A face responsive potential recorded from the human scalp. Exp Brain Res. 78:193–202.

Kanwisher N, McDermott J, Chun MM. (1997). The fusiform face area: a module in human extrastriate cortex specialized for face perception. J Neurosci. 17:4302-11.

Kanwisher N, Yovel G. (2006). The fusiform face area: a cortical region specialized for the perception of faces. Philos Trans R Soc Lond B Biol Sci. 361:2109-28.

McKone E, Kanwisher N, Duchaine BC. (2007). Can generic expertise explain special processing for faces? Trends Cog Sci. 11:8-15.