"[The nerve cells will] respond more to some faces, less to others, but they will fire some response to almost every face."
Furthermore, most of the remaining 3 percent of the cells reacted to a type of facial image not included in the original experiment, such as the back of a head or a head titled upward.
The discovery casts light on a much debated field of neuroscience.
Though some experts promote the aforementioned Swiss-Army-knife view of the brain, others say that mental processes are performed in a widely distributed way. They argue that the regions involved in face recognition are really used for identifying all sorts of objects.
Margaret Livingstone, a neuroscientist in the same laboratory as Tsao, says that the brain's expertise at recognizing a range of objects may follow from this ability to recognize faces.
"The fact that we found that virtually all the cells are responsive to faces says that it can't just be general expertise," Livingstone said. "There's no machinery left to be expert at [identifying] these other things," such as birds and cars and so on.
However, people might use the face selectivity of these cells to recognize objects that in some way resemble a face, Livingstone says.
Knives, Apples, and Clocks
Some of the monkey brain cells responded, though weakly, to round shapes such as clocks and apples.
This finding, according to Tsao, suggests that cells in the macaque's middle face patch are involved in some intermediate level of face coding.
"[The cells are] not yet coding a particular identity, but they are coding the basic structure and measurement of a face," Tsao said.
There are three face patches in each side of the macaque brain. The human brain has a similar distribution.
According to Winrich Freiwald, a former Harvard postdoctoral student who co-led the study with Tsao, it remains a mystery why the brain has multiple face-recognition areas, rather than just one.
"Ultimately the answer, I think, will depend on what the areas surrounding these patches do and which are the areas they're connected to," said Freiwald, now with the Brain Research Institute at the University of Bremen in Germany.
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