Rewiring Your Brain - the Part that Sees

A mind is a terrible thing to waste, but wasting your brain is just as bad. Nature seems to realize that, for when people are blind from birth or a young age because of damage to their eyes, the part of the brain that ordinarily processes visual signals doesn’t just sit idly by. The visual cortex, which takes up about one-third of the entire brain, makes a nimble career switch. Receiving no signals from the eyes, it instead begins processing signals from the fingertips (to read Braille) or even from the ears (which accounts for the greater auditory acuity of many blind people).

This is yet another manifestation of neuroplasticity, the ability of the brain to rewire itself. Now a treatment that exploits the malleability of the visual cortex has taken another step forward. Veterans and active-duty military personnel who have lost all or part of their vision due to stroke or traumatic brain injury (of which the war in Iraq is producing tragically many cases: an estimated 10 to 30 percent of returning service members suffer traumatic brain injury) will be offered a new therapy to induce healthy regions of their brain to take over the job of seeing that injured regions once did.

Called Vision Restoration Therapy, it was developed by a company called NovaVision, Inc., and will be offered at the Tampa Polytrauma Rehabilitation Center, part of the Department of Veterans Affairs.

In Vision Restoration Therapy, a physician first determines which region of the visual cortex has been knocked out by injury or stroke. The company then devises a custom-made rehabilitation program that patients can use on their own. A computer screen shows a green dot in its center, and the patient is to stare at this “fixation point,” resisting the urge to glance at white dots that appear on the screen. The patient is instructed to watch for the white dots only with his peripheral vision, and when he sees one to click the mouse. At irregular intervals, the central green dot changes color, and the patient is to click the mouse again (it’s a check to make sure the eyes remain front and center, so only the peripheral vision is involved in glimpsing the white dots).

The idea is to stimulate peripheral vision around the blind spot, engaging the “border zone” in the visual cortex between damaged neurons and healthy ones, explains neurologist Randolph Marshall of Columbia University, who was one of the first to offer the system to patients. Only neurons adjacent to those damaged by the stroke register the white dots. The hope is that if these “transition zone” neurons are activated often enough, they will take over the function of the injured neurons, and the patient’s blind spot will disappear.

Experiments in the late 1990s suggested that this repeated stimulation can accomplish that, and larger studies are now confirming it. Marshall and colleagues recently studied  patients, aged 35 to 77, who had lost sight on the same side of both eyes due to stroke or traumatic brain injury. As they trained surviving neurons to pinch-hit for the damaged ones, the visual cortex rewired itself, beginning a month after starting the vision restoration treatment, they report online in the journal Neurorehabilitation and Neural Repair.

A study in the September issue of the Journal of Neuroscience adds to the growing pile of evidence that the visual cortex can rewire itself after stroke. Scientists led by Daniel Dilks of the Massachusetts Institute of Technology studied a 51-year-old man in whom a stroke had destroyed neurons carrying signals from the eyes to the visual cortex. The visual cortex itself was fine. But he couldn’t see anything in the top left of the visual field. Objects just below that blind area looked distorted, as if they were bleeding upward into the blind area. What was happening, fMRI scans showed, was that regions of the visual cortex that usually responded to signals about the top left quadrant of the visual field apparently got tired of waiting for signals that never came. They switched careers, and began vibrating to the arrival of signals from the lower left visual field.

Okay, not as dramatic as visual cortex starting to hear or feel. But a strong hint that the plasticity of this region of the brain can be tapped to restore sight to those who have had it stolen by stroke or war injury.

Comments