What drives brain changes in macular degeneration?

March 3, 2009 by Cathryn Delude

In macular degeneration, the most common form of adult blindness, patients progressively lose vision in the center of their visual field, thereby depriving the corresponding part of the visual cortex of input. Previously, researchers discovered that the deprived neurons begin responding to visual input from another spot on the retina — evidence of plasticity in the adult cortex.

Just how such plasticity occurred was unknown, but a new MIT study sheds light on the underlying neural mechanism.

"This study shows us one way that the brain changes when its inputs change. Neurons seem to 'want' to receive input: when their usual input disappears, they start responding to the next best thing," said Nancy Kanwisher of the McGovern Institute for Brain Research at MIT and senior author of the study appearing in the March 4 issue of the Journal of Neuroscience.

"Our study shows that the changes we see in neural response in people with MD are probably driven by the lack of input to a population of neurons, not by a change in visual information processing strategy," said Kanwisher, the Ellen Swallow Richards Professor of Cognitive Neuroscience in MIT's Department of Brain and Cognitive Sciences.

Macular degeneration affects 1.75 million people in the United States alone. Loss of vision begins in the fovea of the retina — the central area providing high acuity vision that we use for reading and other visually demanding tasks. Patients typically compensate by using an adjacent patch of undamaged retina. This "preferred retinal locus" (PRL) is often below the blind region in the visual field, leading patients to roll their eyes upward to look at someone's face, for example.

The visual cortex has a map of the visual field on the retina, and in macular degeneration the neurons mapping to the fovea no longer receive input. But several labs, including Kanwisher's, previously found that the neurons in the visual cortex that once responded only to input from central vision begin responding to stimuli at the PRL. In other words, the visual map has reorganized.

"We wanted to know if the chronic, prior use of the PRL causes the cortical change that we had observed in the past, according to what we call the use-dependent hypothesis," said first author Daniel D. Dilks, a postdoctoral fellow in the Kanwisher lab. "Or, do the deprived neurons respond to stimulation at any peripheral location, regardless of prior visual behavior, according to the use-independent hypothesis?"

The previous studies could not answer this question because they had only tested patients' PRL. This new study tests both the PRL and another peripheral location, using functional magnetic resonance imaging (fMRI) to scan two macular degeneration patients who had no central vision, and consequently had a deprived central visual cortex.

Because patients habitually use the PRL like a new fovea, it could be that the deprived cortex might respond preferentially to this location.

But that is not what the researchers found. Instead, the deprived region responded equally to stimuli at both the preferred and nonpreferred locations.

This finding suggests that the long-term change in visual behavior is not driving the brain's remapping. Instead, the brain changes appear to be a relatively passive response to visual deprivation.

"Macular degeneration is a great opportunity to learn more about plasticity in the adult cortex." Kanwisher said. If scientists could one day develop technologies to replace the lost light-sensitive cells in the fovea, patients might be able to recover central vision since the neurons there are still alive and well.

Source: Massachusetts Institute of Technology

Explore further: Researchers find a simple explanation for the typical patterns of nerve cells inside neural maps

Related Stories

Retraining the brain to see after stroke

April 12, 2017

Patients who went partially blind after suffering a stroke regained large swaths of rudimentary sight after undergoing visual training designed by researchers at the University of Rochester Medical Center's Flaum Eye Institute.

An innovative model for the study of vision

April 11, 2017

A new study shows for the first time that the progressive processing of the visual signal underlying human object recognition is similarly implemented in the rat brain, thus extending the range of experimental techniques, ...

Recommended for you

People match confidence levels to make decisions in groups

May 26, 2017

When trying to make a decision with another person, people tend to match their confidence levels, which can backfire if one person has more expertise than the other, finds a new study led by UCL and University of Oxford researchers.

Optic probes shed light on binge-eating

May 26, 2017

Activating neurons in an area of the brain not previously associated with feeding can produce binge-eating behavior in mice, a new Yale study finds.

Study finds gray matter density increases during adolescence

May 26, 2017

For years, the common narrative in human developmental neuroimaging has been that gray matter in the brain - the tissue found in regions of the brain responsible for muscle control, sensory perception such as seeing and hearing, ...

Game study not playing around with PTSD relief

May 26, 2017

Post-traumatic stress disorder (PTSD) patients wrestling with one of its main symptoms may find long-term relief beyond medication thanks to the work of a Western researcher.

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.