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Thursday, January 1, 2009

Are Faces Special?

There's been a glut of face-based science lately. There was the first American face transplant (the second if you count the ill-fated Travolta/Cage one...) Then an Atlanta group allegedly found that chimpanzees have a part of the brain specialized for recognizing the faces of their fellow chimps.

As I'll explain, this would be extremely important if true. This research is just the latest chapter in a long and contentious debate going back many years - a debate which, believe it or not, may hold the answer to Life, the Universe, and Everything! I'll get onto that later.

The human brain may, or may not, have regions which are "hard-wired" specifically for the visual processing of faces; the question of whether it does is generally known as the "Are Faces Special?" debate.(*) The majority of neuroscientists today would say that yes, they are, and that yes, we do have at least one such face area. I agree with this, but the debate's not yet ever - although as usual the reporting on this study glosses over such complexities.

On the face of it (ha), the evidence for specialized processing of faces in the human brain is very strong. We're very good at distinguishing and recognizing faces, despite the fact that they are all extremely similar - and when faces are shown upside-down, we are much worse at dealing with them. This suggests, to most people, that we have a specialized capacity for processing faces.

Following certain brain lesions, some patients lose their ability to recognize faces, a condition known as prosopagnosia. This most commonly follows damage to a part of the temporal lobe of the brain called the fusiform gyrus. Prosopagnosics may be able to identify and recognize other kinds of objects, but they just can't "get" faces. (Prosopagnosia can also be congenital i.e. present from birth). To a prosopagnosic, every face is as inscrutable as upside-down faces are to the rest of us.

Meanwhile, neuroimaging has consistently shown a small portion of the aforementioned fusiform gyrus on the right side, dubbed the "fusiform face area (FFA)", is more activated when people are looking at pictures of faces than when they are looking at inanimate objects, other body parts, or pictures of faces which have been scrambled up so as to no longer look like faces.

So all of the evidence seems to mesh together ("converge") splendidly: there's a face area in the human brain, located in the Fusiform Face Area, which is responsible for our unusually good face-processing abilities. Hurrah. However, there's a parade-raining alternative view, namely that faces are not special, we are only good at processing them because we have so much experience doing so, and the FFA's role lies in detecting the differences between similar objects about which we have learned to be highly familiar - faces being just one example.

The details of this debate are fairly bewildering. For what it's worth, the "expertise" account has always seemed rather contrived to me and expertise theorists seem to be on the defensive against the more confident and plausible "specialist" neuroscientists. But that's just my opinion. For an excellent overview of the neuroscience of faces see here; for a skeptical view of the Fusiform Face Area see this; for skepticism of the skepticism here.

Anyway, into this debate stepped Lisa Parr et. al. who used 18F-flurodeoxyglucose PET imaging to measure neural activity in five adult chimpanzees. Their goal was to see whether chimps possess a Fusiform Face Area or not. Chimps, unlike "lower" monkeys, are good at recognizing chimp (and human) faces. The five luckless chimps had to perform two tasks, one of which involved visually "matching" pictures of chimp faces in order to earn Kool-Aid. The other control task involved the same procedure but with matching non-face pictures ("Clip Art").
Subjects have been trained to control the movements of a cursor on the computer screen by manipulating the joystick ... At the beginning of a trial, a single image (the sample) appears on the computer screen on a black background. ... After this, the sample clears the screen and two comparison images appear on the monitor ... One of these comparisons matches the sample (the target), and the other (the foil) is a different image from the same category, either another face or another clip art object. ... Subjects must contact the image that matches the sample by contacting it with the joystick-controlled cursor.
Compared to the control Clip Art task, the chimp's brains were more active during the face task in a wide range of areas. The most "face-selective" area was the "Dorsal primary motor/medial parietal cortex (Left)" which has nothing to do with faces, vision, or any of that kind of stuff. The authors try to put a brave face on this (emphasis mine)...
The first [whole brain] analysis revealed numerous brain regions that showed greater metabolic activity during the face-matching task when compared directly to the object-matching ... [including] the posterior superior temporal sulcus (STS) and orbitofrontal cortex ... These regions comprise part of the distributed cortical network for face processing in humans. Notably absent from this analysis was activity in the fusiform gyrus, the primary region where face-selective activity is found in humans when the comparable analysis is used.
...but this is clearly a disappointing result. No chimp FFA found. Yet this is hardly surprising, considering that there were only five subjects in this experiment, and there's obviously a big difference between a human volunteer, and a chimpanzee who's spent years being experimented on, is locked in a cage, is highly trained on the computerized task, and who is doing the experiment for Kool-Aid.(**) The absence of evidence in this experiment is not evidence of absence.

All was not lost, however, because the authors then did a different analysis, looking for individual voxels (small parts of the brain) which were "face selective" or "object selective". (I suspect this was a post-hoc analysis, a naughty practice which I have warned about before, but here it's arguably OK). Short story - they found a lot of such voxels, most of them in areas which are nothing to do with faces or vision, but there were quite a few in the fusiform gyrus, which is where you might expect them to be based on human work (see above.) But honestly, neuroimaging data is noisy enough at the best of times and with just five subjects it's virtually impossible to draw any conclusions. A human PET study with n=5 would never get published; by the standards of chimp research n=5 is big because chimps are much harder to work with. Personally, I'm just glad I don't work with chimps.

What about Life, the Universe, and Everything? Well, the "Are Faces Special" debate is relevant to such cosmic questions (almost), because it's one aspect of the great "modularity" debate, which is basically psychologists and neuroscientists debating the existence of human nature. If humans are genetically programmed to have a part of the brain (a "module") specialized for processing faces, presumably this is because we evolved to do so. If so, then this makes it very plausible that other parts of our brains evolved for such specialist purposes, and this implies that our minds work in certain ways because of evolution (i.e Evolutionary Psychology) and that human nature is largely fixed.

On the other hand, if faces aren't special, maybe nothing is - maybe there is no such thing as human nature and all our behavior is learned by experience. That would imply that culture and history are much more important than biology in understanding human life, and that with sufficient political and social advancement anything is possible! Wow. That's the importance of neuroscience. At least, that's what neuroscientists will tell you when they want you to give them money or buy their books...

(*) If you're at a cognitive neuroscience conference and feel like a slap in the face, try opening a conversation with a pretty young grad student with the line "I don't know if faces are special, babe, but yours is".
(**) Although given the state of some undergrads, the differences might not always be in the human's favor.

ResearchBlogging.orgL PARR, E HECHT, S BARKS, T PREUSS, J VOTAW (2008). Face Processing in the Chimpanzee Brain Current Biology DOI: 10.1016/j.cub.2008.11.048

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