N170 face controversy continues

A while back I blogged a paper by Thierry et al (2007 – see also the Neurocritic’s post). Some controversy is brewing about the paper now, so I thought I’d offer an update.

To recap: the Thierry et al (2007a) paper is interesting because it challenges the notion that a specific component of the EEG waveform called the N170 (since it is negative, and occurs at 170 ms) is specific to faces. Thierry et al found that the N170 disappeared when they controlled for inter-stimulus perceptual variance (ISPV), that is, the fact that faces tend to be presented in portraits, while other stimuli are often shown at various angles and sizes.

As the comments on the Neurocritic’s post suggest, some investigators were not entirely convinced by Thierry et al’s (2007a) demonstration. Now one of the critical comments has made its way into the latest issue of Nature Neuroscience, along with a reply from Thierry et al (2007b).

Bentin et al (2007) point to previous research that shows how controlling for ISPV does not in fact explain the N170 specificity to faces. They’ve packed a bit too much information into this figure than necessary, but it’s worth a look:

The black line indicates the mean pixel-wise correlations within each stimulus group, that is, which is an indicator of how much ISPV there was. The grey bars show the N170 amplitude. If Thierry et al (2007) is correct in their argument that the N170 reflects ISPV rather than face specificity, you would expect the black line (indicating ISPV) and the grey bars (indicating N170 amplitude) to match up reasonably well. This isn’t really happening. To make this point even clearer, they show in the supplements that Thierry et al’s (2007) own data seems to indicate the same thing. Bentin et al (2007) also take Thierry et al (2007a) to task for failing to note how their conceptualisation of the N170 contradicts a bulk of well-known effects in previous literature.

So how could Thierry et al (2007a) get such contradictory results? Bentin et al (2007) suggest that one reason may be Thierry et al’s (2007) choice of EEG recording sites, which differ from those generally used by other investigators.

In their reply, Thierry et al (2007b) argue that, among other things, the recording site explanation doesn’t hold, since the same pattern of results appeared across all electrodes. They also contend Bentin et al’s (2007) notion that there is a generally agreed standard for electrode selection.

As for the lack of similarity between the pixel-wise correlations and the N170, this is explained (if you can call it that) by arguing that pixel-wise correlations may not be a perfect measure of ISPV – individual pixels may have very different effects depending on their location, which is not taken into account with such an analysis.

Confused yet? I don’t think there are any clear answers at this point. At the most basic level, the findings of Thierry et al (2007a) contradict previous findings, something that they appear to have failed to mention themselves. There is also some disagreement over what type of analysis is appropriate for this kind of research. Personally, I would like to see an independent replication of the results before I make any further attempts to understand what’s going on.

References
Bentin, S., Taylor, M.J., Rousselet, G.A., Itier, R.J., Caldara, R., Schyns, P.G., Jacques, C., and Rossion, B. (2007). Controlling interstimulus perceptual variance does not abolish N170 face sensitivity. Nature Neuroscience, 10, 801-802.

Thierry, G., Martin, C.D., Downing, P., & Pegna, A.J. (2007a). Controlling for Interstimulus Perceptual Variance Abolishes N170 Face Selectivity. Nature Neuroscience, 10, 505-511.

Thierry, G., Martin, C.D., Downing, P., & Pegna, A.J. (2007b). Is the N170 sensitive to the human face or to several intertwined perceptual and conceptual factors? Nature Neuroscience, 10, 802-803.