ARTICLE IN BRIEF
Noninvasive brain stimulation seemed to aid post-stroke visual rehabilitation in a new study, but some experts question the technique and the presumed mechanism on which it is based.
BALTIMORE—Transcranial direct current stimulation (tDCS) increases the brain's ability to benefit from a visual retraining program during stroke rehabilitation, improving visual field deficits in hemianopia, according to a new study presented here at the annual meeting of the American Neurological Association in October.
Noninvasive brain stimulation has become a major tool in post-stroke motor rehabilitation, but this is the first test of its ability to aid visual rehabilitation, said Alvaro Pascual-Leone, MD, PhD, director of the Center for Noninvasive Brain Stimulation and professor of neurology at Harvard Medical School. Dr. Pascual-Leone and Lotfi Merabet, OD, PhD, were principal investigators on the study.
In the retraining program, called Vision Restoration Therapy (VRT), patients focus on a central point on a computer screen and respond when they see a light stimulus. The stimulus is directed toward the part of the visual field that corresponds to the brain region bordering the blind zone. Initially, the patient cannot reliably see stimuli in the border zone, but with repetition over several months, stimuli here become more consistently visible.
tDCS provides subthreshold stimulation, “priming” neurons to fire so that they respond to weaker stimuli. In post-stroke motor retraining, it is combined with standard physical rehabilitation to facilitate brain plasticity and promote motor learning.
The goal of the current study, led by Ela Plow, PT, PhD, of Beth Israel Medical Center in Boston, was to test if the same might hold true for visual learning.
Six patients were randomly assigned to VRT plus real tDCS or VRT plus sham dTCS, for 30 minutes per day, three days per week, over three months. Current was delivered over the region of the occipital lobe corresponding to the border zone. In sham stimulation, patients donned the same headgear, but didn't receive stimulation during the VRT procedure. Patient responses at the end of the study indicated they could not guess whether they received real or sham stimulation.
By the end of the trial, patients who had received real stimulation had improvement in their visual field by 5.2 degrees over baseline, versus 1.9 degrees for those receiving sham stimulation. They were also better able to detect motion of a single dot within a masking field of stationary dots, another standard test of visual perception, and they were better able to read better on the Minnesota Reading Test as well.
The results from this small study appear to confirm that the same benefit from tDCS seen in motor retraining is possible in the visual system, according to Dr. Pascual-Leone. “I think it is going to make a real difference in therapy. This has the potential to become a powerful tool in rehabilitation,” especially since the protocol is simple, and the equipment has been designed to be portable and lightweight, meaning it may be useful not just in a research setting, but in physical and occupational therapy clinics and even in the home.
WHY DOES VRT WORK?
But if it works, why does it work? NovaVision, the company that markets VRT, and for which Dr. Pascual-Leone serves as an advisor, suggests the improvement is based on “recovery of low-level connections” in the border zone, he said. “But we really don't know.”
Increased activation of individual cells in the border zone is “one possibility.” But other work in Dr. Pascual-Leone's lab and elsewhere suggest the recovery process may involve contributions from much more extensive regions of the brain. “Brain cells, in order to do any one thing, need to be connected in functional networks.”
Two exciting findings have come out in the past decade or so, he said. “One is that we have a sense of the capacity of the brain to compensate for injury, through plasticity harnessing this process to promote recovery. At the same time, plasticity is a double-edged sword. It also contributes to disability, to the development of maladaptive strategies.”
Functional imaging of aphasia patients has shown elevated activity in the non-dominant hemisphere when stroke damages the language areas in the dominant hemisphere. “Some of that additional activity is promoting recovery, but some appears to be holding it back, and when it is shut down with stimulation, patients get better. And it appears that similar finding are present in the visual system.”
Both low-level and higher-level processes may be at work in recovery, he said. Low-level improvements may account for the increase in the visual field, while higher mechanisms allow the brain to use more of the information it is receiving, allowing the patient to “functionally see better.”
“Visual acuity tests only one limited aspect of the visual system. The brain is capturing a lot more information than we are aware of. If we could expand on that, we could help our patients see better.”
He suspects the recovery seen with VRT may also be drawing on these higher level activities, “but we are not optimizing that in this stimulation protocol — that's a different area for stimulation.”
Commenting on the study, Randolph Marshall, MD, professor of neurology and director of the Division of Cerebrovascular Diseases at Columbia University Medical Center in New York City, said:“Transcranial direct current stimulation looks promising for enhancing the behavioral effects of motor recovery in hemiparesis.”
Its application in visual therapy is novel, he said in an interview with Neurology Today, but the visual system does look like it is amenable to plasticity-based changes. “Neuroscientists are fairly convinced that neuroplasticity can take place in the visual system,” he said, although clinical ophthalmologists have been “reluctant” to accept the concept.
However, the current study is small. “They need more patients,” he said, in order to more firmly establish its utility.
Dr. Marshall also noted that VRT “is somewhat controversial in its own right. It appears to be the one approach to visual therapy to expand the visual field, but we don't know how it is working.”
Nonetheless, he said, “Our best estimate is that enhancing a therapy with tDCS will increase the gain on therapy.”
He agreed with Dr. Pascual-Leone about the possibility of involvement of higher brain centers. “We really don't have a good understanding of the border zone. It may not be just a local phenomenon. Recovery may be broader, with cross-domain contributions.”
On the other hand, Steven Galetta, MD, professor of neurology at the University of Pennsylvania, has doubts. “There have never been convincing studies of Vision Restoration Therapy that adequately control for fixation,” he said, and some evidence suggests the benefits stem from compensatory eye movements, not from any reorganization of the brain. “When you combine one questionable methodology with another, you bring out all the skeptics. I'm all for whatever helps people with field deficits, but you want to get the mechanism right. At the moment, it's a large leap of faith.”