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It’s one of the biggest questions in neuroscience: How do we take in a glut of sensory information from the environment and then transform it into a reliable perceptual representation that will allow us to successfully navigate the world around us? Decades of research have suggested that the foundation of good sensory perception and adaptation is brain specialization: unique areas grow to handle specific functions. A new study out of the Hebrew University of Jerusalem suggests that this specialization may be driven by task, not the type of sense used.
Seeking insights from the damaged brain
Historically, neuroscientists relied on patients with brain damage to better understand how the brain processed sensory information and formed it into functional representations. People with disorders like agnosia, the inability to recognize familiar objects, and aphasia, an inability to speak, read or write language, can have a hard time moving about in the world without assistance. By analyzing their difficulties, researchers have formed theories about how the brain is organized and how that organization might affect cognitive processes like recognition and recall. The study of such patient-volunteers still has remarkable value, even with the advance of neuroimaging techniques, says Marlene Behrmann, a co-director of the Center for the Neural Basis of Cognition and neuroscientist at Carnegie Mellon University.
“In functional imaging, you show someone a stimulus and then record some neural response. Essentially, this is just a correlation between the stimulus and that response. What we don’t know is what causes that response,” she says. “But what patient cases can give you is causality. When an area of the brain is essentially removed from the computation and you see the outcome—and those outcomes are often quite dramatic—you can start to really understand what the contribution of area may be. The perceptual system in normal humans is so optimized that it is really hard to get behind the scenes and really understand the mechanisms without patient data to guide you.”
So perhaps it isn’t too much of a surprise to learn that when Amir Amedi, a neuroscientist at the Hebrew University of Jerusalem, wanted to more closely examine how the brain specializes in order to perform different perceptual tasks, he looked to the congenitally blind to help guide him.
Substituting the senses
There is a brain region responsible solely for faces; another for vision motion perception. There are even specific brain areas associated with numbers and words: the visual-number-form area and visual-word-form area, respectively. Historically, people thought the organization or these specific functional regions was driven by the senses, vision in particular, says Shachar Maidenbaum, a graduate student in Amedi’s lab.
“It’s the obvious way to think about this. When you look at the nerves going from the eye, they go straight to the visual cortex. When scientists found motor sensory cortex, they did so by moving people around. So a specific sense seems the most obvious, easy way to activate these highly specialized areas,” he says. “And that’s what people have thought for a very long time.”
But Amedi wondered if there might not be more to it, especially since studies suggested that these so-called visual areas were also recruited when people who had been blind since birth read words or numbers using Braille. “These are areas that are activated by both looking at objects and touching objects. It was a real puzzle,” he says. “So we started to wonder if this were more areas that can decipher 3-D shapes, as opposed to a specific visual area. And it made some sense, because you can decipher 3-D shapes by vision or by touch.”
To test the idea, Amedi and colleagues used what is called a sensory substitution device, or a device that helps the blind “see” by representing visual stimuli with aural information. The SSD translates a visual image into a distinct soundscape that a blind person can learn to “read” over time. Researchers scanned the brains of congenitally blind people as they “read” different shapes and symbols, and found distinct activation and connectivity patterns when the participants were “reading” numbers or letters. The results were published in the January 23 issue of Nature Communications .
From sense to task
Maidenbaum argues that these results suggest that there is a basic layout in brain real estate that drives functional specialization in perception—not the senses themselves. “We see that you’ve dropped the sense, in this case vision, but people can still do the task,” he says. “It’s not that there’s no preference for a particular modality. Visual-word-form [area] would prefer vision. But it can still be activated through other senses.”
Amedi says the unique connectivity patterns observed between these specialized areas also may explain why the brain has been able to so quickly adapt to language—as well as new fast-paced technologies that, in theory, should tax our perceptual systems.
“We have only had a reading system for several thousand years. There was not enough time in evolution to develop a specialized reading system in the brain,” he says. “But these connections may have helped develop these specialized centers for reading and number symbol processing by their connectivity to quantity and language systems—systems that did have enough to develop in evolution. And those connections may be the things that helps our brains adapt quickly and pick up all these new technological inventions like computers as they come about.”
Constraints on rehabilitation
Behrmann says the study is quite compelling—and it supports the idea that cortical computation is quite constrained.
“What we see is that the brains of blind individuals are not turned upside-down because of a lack of visual input. They share the same broad characteristics as a person with normal vision,” she says. “It suggests that there are only so many ways in which the brains circuitry and computation can emerge—and, in the absence of input, the brain still adheres to those same modes of organization.”
That idea, Amedi and Maidenbaum argue, may mean that these results could herald new interventions that can help those with sensory disabilities better overcome their impairments.
“For years, people have been saying people who are blind don’t have the brain regions they need to fully enable their missing sense,” says Maidenbaum. “But we’re saying is, ‘Yes, they do.’ We just need to find new ways to train these people, reawaken these areas and help them regain that function through rehabilitation. That’s where the field needs to go in the future.”
Story revised 22 April 2015