How do the brain’s sensory regions adapt when input signals are cut off? How swiftly do deprived neurons become receptive to alternative inputs? And how does this adaptability—or “plasticity”—change as the brain grows older? Such questions have been contentious among neuroscientists, many of whom have theorized that neural plasticity greatly weakens with age. Now a new study hints that the brain is wired for a very fast-acting type of plasticity, even in adulthood.
The research, published in the July 15 Journal of Neuroscience, was prompted by an earlier evaluation of a man who had suffered a stroke. The loss of blood flow in his brain had destroyed nerve fibers that send visual information from his retina to his primary visual cortex. Daniel Dilks, then a doctoral student at the Massachusetts Institute of Technology, led a study showing that six months after the stroke, the information-deprived neurons in the man’s primary visual cortex had begun to respond to inputs from adjacent visual neurons instead. The “blind” part of the visual field thus displayed information from nearby, non-blind fields, resulting in a measurable distortion of perceived images.
Having developed the visual distortion measurement technique as a way to study plasticity in the cortex, Dilks, now a postdoctoral researcher, decided to find out how quickly this plasticity could manifest itself. “Does it in fact take six months for your cortex to change, or might it happen within hours?” he remembers asking.
To test the issue, Dilks and his colleagues exploited a quirk of vision involving the retina’s “blind spot,” where the bundled optic nerves depart the eye for the primary visual cortex, creating an area of the retina with no light-receptive cells. The cortical neurons corresponding to this “blind” part of the visual field, however, normally and seamlessly fill the perceptual gap by taking their input from a portion of the other eye’s visual field.
Dilks found that when he put an eye patch over one eye of a research subject, that person would experience an image distortion similar to that seen in the stroke patient. The subject’s “blind spot” neurons would lose their usual inputs, would start to take their inputs from adjacent neurons instead, and thus would effectively stretch a perceived image into their part of the visual field, turning a simple shape such as a square into an apparent rectangle.
Expecting this distortion to manifest only after some hours, as the neurons adjusted to the deprivation of their usual input stream, Dilks put eye patches on each of 48 volunteers and began perceptual tests to find the earliest point at which the distortion could be detected. “To my surprise, it wasn’t hours—it was seconds,” he says.
Too quick to be nerve growth
Such a rapid adjustment suggests that the blind field neurons were not relying on the extension of new connections to their neighbors, which would have required weeks of nerve growth. Instead they were strengthening pre-existing connections as needed on an almost instantaneous basis. “So at least part of this adult cortical change involves the changing of connections,” Dilks says.
Mark Huebener, who researches cortical plasticity at the Max Planck Institute of Neurobiology outside Munich, suggests that the fast adaptation seen in the Dilks study could represent “the unmasking of already present inputs, which were otherwise inhibited.”
Dilks plans to study the phenomenon further with functional magnetic resonance imaging, or fMRI, and he is working on an fMRI system with fine enough resolution to distinguish, for example, activity in the ordinary blind-spot portion of the primary visual cortex from activity in adjacent regions. So far the imaging isn’t sensitive enough to separate the two groups of neurons, he says. “But I’m not willing to give that [fMRI] up.”
Dilks also plans to do a follow-up study in children, to see whether the strength of their visual distortion in the blind-spot test is greater than that measured in adults—as one might expect from the theory that younger brains are more plastic. “Am I uncovering something that is left over from development,” he asks, “or is this something about the adult cortex?”