Sharpening the Focus on Brain Function

Is your brain like a Swiss Army knife? No doubt it’s sharp (after all, you’re a member of APS), but the real question is, how does your brain operate? Is it jam-packed with specialized tools that are unfolded only when a specific situation arises? Or is it more all-purpose, with a few parts that tackle many different situations? Convention Keynoter and APS Fellow Nancy Kanwisher (Massachusetts Institute of Technology) is attempting to find out.

Following centuries of debate about specialized brain regions — from the phrenologists to Broca — the development of fMRI technology has ushered in a new era of studying brain regions. By monitoring the blood flow necessary to support neural activity, fMRI has allowed researchers to track which regions of the brain are involved in processing specific stimuli. With this new weapon in her arsenal, Kanwisher performed a now-classic study published in 1997. In it, participants sat in an fMRI scanner while looking a series of faces and objects. She and her colleagues identified an area in the fusiform gyrus on the bottom surface of the temporal lobe that responded more strongly when the participants viewed faces than when they viewed objects. Dubbed the fusiform face area, this region seemed like it could be specialized for processing faces.

But the researchers could not yet be sure. What if the area responded to everything animate, or everything round? A decade of more detailed research confirmed their original hypothesis — the fusiform face area lived up to its name. At the same time, Kanwisher’s lab discovered two other specialized areas: the parahippocampal place area, which specializes in processing places, and the extrastriate body area, which specializes in processing images of the body.

These answers only lead to more questions. These areas are involved in processing certain categories, but do they merely process perceptual input or actually reflect conscious experience? What are the roles of genes and experience in wiring up these areas? And finally, how much of the brain is like this? Is our entire cortex broken up into small pieces, each with their own special domain? Kanwisher and her colleagues are tackling these questions head on.

Nancy Kanwisher delivers her Keynote Address

Do these areas only engage in their respective categorical processing or do they perform other functions as well? For example, take the fusiform face area. It is most active when viewing faces, but it also shows lesser activity when the participant is looking at other visual stimuli, like objects. Something in the pattern of this lower activity could be crucial in processing input other than faces. Evidence against this idea comes from research on patients with neurological trauma, who sometimes lose face perception abilities without losing object perception. But, the low chance of finding subjects with a lesion in just the right spot make this research limited. Other researchers have turned to transcranial magnetic stimulation (TMS), a method that uses magnetic fields to transiently disrupt neural activity. The fusiform face area is too deep in the brain to be affected by TMS, but the extrastriate body area is closer to the scalp and susceptible to the TMS disruption. When the neural activity in this region is disturbed, participants are impaired in the ability to recognize bodies but have no difficulty recognizing faces or other objects. Although these specialized areas may collect information about other types of stimuli, it seems that they are only necessary for processing information of their specific type.

Are the functionally specific regions merely perceptual processers or do they reflect our conscious experience? To illustrate the difference between perception and experience, Kanwisher instructed the audience to pick up the 3-D glasses left on the seats. But, before we could put them on, she showed us two images, a red-tinted image of a face and a green-tinted image of a house. Then she superimposed the house on the face creating a red/green face/house jumble. But, when looking at this jumble through glasses with one red-tinted and one green-tinted lens, so that the house image goes to one eye and the face mage to the other, you don’t experience a jumble — you experience a red face that fades to a green house and back and forth as your brain attempts to make sense of this new situation. Even though your experience of what you are seeing is changing, the image beamed to your retina is constant the whole time. Work from Kanwisher’s lab showed that in this situation, activity in the fusiform face area corresponds with one’s experience, not with the actual perceptual input.

Further, not only does the activity in specialized areas correspond with what we consciously see, it also corresponds with what we imagine. Kanwisher has put people in the fMRI machine and asked them to imagine familiar faces and places. The same areas are active when participants are imagining faces and places as when they are actually looking at faces and places. It’s not just what you are physically seeing, but what you are consciously aware of that is processed by this area.

So, where do these specialized areas come from? What role do genes and experience play in their construction? Twin studies have shown that face-perception abilities are heritable. In addition, infant monkeys raised in a rich visual environment and given plenty of nurturing, but not shown faces, had no impairment when viewing faces for the first time. But, you can’t count the environment out just because genetics clearly play a role. Studies have found small selective regions for processing words and letter strings, even though reading is a relatively recent human experience and evolution hasn’t had time to hardwire it. Kanwisher’s lab has run further studies in this area with bilingual readers: Both English and Hebrew readers show a relatively strong response to English words and letters, but only the Hebrew readers show a strong response to Hebrew words and letters. So, as Kanwisher said, “there is a very significant role of genes, at least for the face system, and a very significant role of experience, at least for the word or letter system.”

More than a decade of research has shown there are specialized regions for consciously processing certain visual stimuli, but there is also suggestive research that we have specialized brain regions for more complex thought processes, including language. Although Broca introduced language as one of the first probable candidates for specialization, identifying the location of such regions has been tricky. When taken together, the many language neuroimaging studies implicate much of the left hemisphere of the brain in language processing. This may simply be a problem of aggregating data when the exact locations of smaller language regions vary from person to person. Colleagues in Kanwisher’s lab are currently conducting studies in which they identify a specific participant’s likely language areas and then put them through a variety of tasks to find out if these regions are specific to language.

Kanwisher’s former grad student Rebecca Saxe, now a faculty member at MIT, identified an area that is specialized for thinking about what others are thinking. This concept may at first seem strange, but it makes immediate sense once you realize how much thought you put into understanding others’ thoughts. Think about any conversation — you have to judge how much the other person already knows, if they understood your last statement, how they might react based on how you phrase the next statement, etc. And this skill would have been crucial to our ancestors.

Although Kanwisher and others have come a long way in understanding specialized brain regions, there are still many unanswered questions. Why do some functions get an area while others don’t? Are these regions neuroanatomically different in addition to functionally different? Can they move after brain injury?

While our brain has some functionally specialized regions, they may be limited to a few key tasks. As Kanwisher reminded the audience, Swiss Army knives may be filled with specialized tools, but they also have that trusty general blade that can be used for many different tasks, from “opening cardboard boxes to performing an emergency tracheotomy to spreading peanut butter on bread.” Our brains are likely to have similar general-purpose machinery in addition to the specialized components that Kanwisher’s research has illuminated.