With 188 electrodes implanted in his skull, the man tried to put his vision into words: “How do I explain this...just like the previous one, I see an eye, an eye and a mouth, sideways.” Looking at a box, a volleyball, and a live human face, the man was seeing phantom faces and rainbows superimposed over the world around him as his brain was being stimulated.
The experiment, described In a study published in PNAS last week, was part of a greater effort to understand parts of the brain that researchers have long believed to be dedicated to helping humans "see" faces and color. The researchers studied a 26-year-old man from Japan, who, as part of previous treatment for epilepsy, had his brain wired with the same kind of electrodes researchers need to precise patches of brain cells. To find the source of his seizures, doctors placed electrodes throughout his brain. Some of those electrodes were coincidentally already in the Fusiform Face Area (FFA) and other areas believed to be involved in color perception.
It’s hard to study things like this in humans. There are ethical and practical roadblocks that keep researchers from surgically implanting electrodes in people’s brains. But the young man this group studied was already receiving treatment for epilepsy. Much of the research on the FFA has been done in primates.
And, as the researchers note, their study adds an unprecedented piece to this puzzle. Past experiments have shown that disrupting those areas in the brain interferes with a person's ability to put together the lines and dots that form a face into one coherent whole (think about how easily you recognize an emoji, or how eagerly humans read faces on pieces of toast and dog ears.) Other experiments have also suggested those brain areas play a role in recognizing faces and colors, but they haven't directly been able to pin down cause and effect.
“The only way to do this,” the researchers write, “is to directly intervene on the region and ask what happens perceptually.” Or, in other words, zap those parts of the subject's and find out what he sees.
And this study did just that, showing what appears to be a clear cause and effect. Crucially, the stimulation did not affect the man's ability to perceive faces and colors that were already part of the objects he looked at (a result achieved in similar experiments), but it caused him to see these additional “phantom” faces and rainbows.
When the face-specific cells were stimulated, the man saw them superimposed on a soccer ball, a box, and one of the researcher’s faces. When the color-specific cells were stimulated, the man saw rainbows. (The rainbow on the box was limited to the left side, he said, and it blinked when it appeared on the soccer ball, while the one on the researcher’s face was “glowing.”)
The fact that the stimulation added more faces to the picture rather than interfering with the man's ability to see the ones that were already there adds evidence that these brain areas play a very specific role in perception.
As the blogger Neuroskeptic writes, lots of previous work has shown that stimulating the FFA was “necessary to evoke face perception,” but other processes researchers weren’t aware of might have also been required to make that happen. This new experiment suggests that stimulating the FFA on its own is “sufficient.” And while this is just the case of one particular patient, the findings are striking.
But, as Neuroskeptic suggests, this is likely far from the last point of debate on a brain function that is still not well understood.