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Perceptual learning may improve visual function in people with central vision loss
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Eye movement behavior and certain spatial properties that have adapted to vision loss may be evidence of the plasticity of the visual system, even late in life, according to the American Academy of Optometry’s 2012 Glenn A. Fry award winner. This suggests that it may be feasible to improve visual function of people with central vision loss through perceptual learning, she said.
“Imagine you are learning to play the piano. You will get better if you practice more. Vision is the same,” Susana T. L. Chung, OD, PhD, FAAO, said in an interview with Primary Care Optometry News. “Many patients with reduced vision still believe that vision is a limited quantity of resource, like how much money you have in the bank, and they worry that if they use that up, they will have nothing left. This is simply not true. Actively using our vision could actually improve it.
“And this works because of cortical plasticity, which refers to the changes in the connections among neurons in the visual cortex, so that signals encoding visual stimuli can be better represented in the visual cortex. This helps us become more sensitive to certain signals, or to be able to see a certain stimulus better or easier,” she continued
Plasticity is not completely understood, Chung said. Some believe it is the brain learning how to better discriminate between a signal and noise, and that with enough repetition, the brain can better extract the relevant information for a given task or can better suppress the irrelevant information, or noise.
“One way to make use of plasticity to help patients, as in the studies in my lab, is to use perceptual learning, or visual training,” she said.
According to a paper authored by Chung and published in Investigative Ophthalmology & Visual Science in 2011, perceptual learning can enhance reading speed in individuals with central vision loss.
“Neural plasticity in adults with sensory visual deficits is not at all a new concept,” Chung wrote in the paper. “It is well known that after the loss of central vision, many of these individuals eventually adopt a retinal location outside the afflicted macular area to serve as the preferred retinal locus. This in itself is strong evidence that even for people with central vision loss, the visual cortex is still malleable and able to adapt to unfavorable visual experience. Therefore, it is not surprising that these people can benefit from perceptual learning.”
To evaluate that hypothesis, Chung had six observers with long-standing central vision loss practice reading aloud sentences presented sequentially using rapid serial visual presentation (RSVP), according to the paper.
Visual acuities, RSVP reading speeds for six print sizes, preferred retinal locus for fixation locations and fixation stability were all measured before the onset of perceptual learning. Participants then attended six weekly sessions of RSVP consisting of 300 sentences per session.
At the end of the training period, all observers showed improved RSVP reading speed, with an average improvement of 53%, and a comparison of pre- and post-test measurements revealed little change in visual acuity, critical print size, preferred retinal locus location and fixation stability.
“The specificity of the learning effect, and the lack of changes to the preferred retinal locus for fixation location and fixation stability suggest that the improvements are not due to observers adopting a retinal location with better visual capability, or an improvement in fixation,” Chung wrote. “Rather, the improvements are likely to represent genuine plasticity of the visual system despite older ages of the observers, coupled with long-standing sensory deficits.”
Chung said in the interview: “Based on the perceptual learning literature, once training stops, the improvement will retain for a while, but will also slowly go back to the baseline. However, if the improved visual function — in this case, reading — is important to them, one would expect them to keep doing this at home even after training. In this sense, it’s almost as if they have additional ‘maintenance’ training. For those who do not do the task at home after training, it’s probably because they don’t care or they don’t read anymore. For these people, whether the improvements are permanent or not is not crucial, since they are not going to benefit from it.” – by Daniel R. Morgan