Plasticity in Sensory Systems

2011 International Conference on Plastic Vision

Edited by Jennifer K. E. Steeves and Laurence R. Harris

The term “plasticity” in neuroscience means that the brain can change and discoveries over the last couple of decades have proven that we retain a degree of neuroplasticity into old age. This volume is a collection of papers from the presentations at the meeting. None of the scientists are optometrists but all of their research relates to vision development and vision therapy. Visual skills and the processing of visual information develop through experience and can be modified through directed, intensive rehearsal. This happens as people improve at a craft, hobby, music, art, job, or sport. Visual skills and visual processing can also be developed through vision therapy. The following excerpts are taken directly from the scientific papers.

This revolutionary finding indicated that critical periods are not, in fact, critical and that the brain is not, completely hard wired in adulthood.

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Up to now I have neglected the most frequently occurring learning of visual patterns that has no evolutionary significance: reading. The presentation of visual letter strings is associated with activity in a region referred to as the visual word form area (VWFA). Given the lack of evolutionary significance, the mere presence of a region found only in literates is already suggestive of an important role for learning. The responses of the VWFA are indeed modulated strongly by manipulations of expertise. Strikingly, the VWFA can still develop in adulthood. This was demonstrated by comparing illiterates with literates that learned to read as adults (Dehaene, et. al., 2010). Reading in the Brain: The Science and Evolution of a Human Invention

In this study, we seem to have many factors coming together – as is predicted given that informativeness is related to the many maps that together make up the organization of the visual object representations: bottom-up shape characteristics matter, the cognitive processes involved during training matter, and top-down task factors matter. We divide vision into ocular motor skills and perception to aid understanding, testing, and communication. But ocular motor skills (tracking, aligning, and focusing) and visual perception cannot really be divided. When children have visual perceptual problems, we expect them to also have ocular motor problems since the development of perception is dependent on the intake of consistent, accurate information which, in turn is dependent on accurate eye movements and clear, single vision. And enhancing their ocular motor skills is dependent on visual attention and the processing of feedback. One of the most basic examples is what is known as visual crowding. Is having difficulty counting these lines / / / / / / / or these lines /////// a tracking problem or a perceptual problem? To count them, you need very good control of your eye movements. You also need to see more than one line at a time to keep your place which requires the integration of information across your visual field which is a fundamental perceptual skill.

Directing the eyes to a target prior to initiation of hand movement allows the central nervous system to obtain a high-resolution image of the target before the reach is initiated, which can facilitate programming of the reaching movement. In addition, when the eyes fixate on the target early during the trajectory, visual feedback can be used to update the initial motor plan…. In contrast, patients with mild or severe amblyopia initiated reaching prior to directing the eyes to the target in significantly more trials when viewing with the amblyopic eye. This behavior is not limited to children with amblyopia and is commonly seen in children with learning-related vision problems and in those who are viewed as clumsy. These poorly learned skills can be enhanced with commensurate outcomes in performance.

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The accuracy and precision of our movements depend on our ability to predict the consequences of our own actions and use sensory feedback. In directing movement, vision is actually predicting the future. The ways that you use vision to do this are endless. As examples, consider how vision functions to enable you to catch a ball or to drive a car.

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Robert Hess presented a very unusual scientific paper. Hess was mentored by Fergus Campbell and in the 1970s they developed a device based on sound theory to treat amblyopia. (We purchased one right away and used it for a number of years in vision therapy.) Hess discusses the device with a perspective and humility which is refreshing. This was the beginning of the CAM treatment of amblyopia which was to have an interesting, if not checkered, history over the next 40 years. The device consisted of a rotating wheel with stripes which were designed to stimulate edge detectors in the visual system. Edge detectors were discovered by Hubel and Weisel which changed the course of neuroscience and won them a Nobel Prize. To maintain the child’s attention on the wheel, the children did tasks such as “naughts and crosses” on a piece of plastic in front of the rotator. Subsequent research demonstrated that it was the minutes of engaged eye-hand coordination which improved the visual function in amblyopia, not the rotating stripes. The device worked, but for a serendipitous reason. In recent decades, numerous studies have shown that a range of visual functions in normal adult subjects can be improved as a result of intensive training (termed perceptual learning).

Hess goes on to write: It has long been assumed that the primary problem in amblyopia is monocular loss of vision and that loss of binocularity is a secondary consequence. The traditional treatment is patching, which is directed to restore the vision in the amblyopic eye with the assumption, using the preceding logic, that binocular function will eventually follow suit. Patching does have a level of success, although, at a cost in terms of a child’s social and emotional development. However, the binocular expectation is often not realized, and one is tempted to think that keeping a child monocular for a significant part of early visual development may not be the ideal way of going about restoring binocularity.

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Sue Barry is a neuroscientist who successfully became binocular with stereopsis for the first time at age 48 through vision therapy. This was assumed to be impossible by the scientific community and most of the medical community. She states: We must ask whether poor success in establishing binocular vision in adult strabismics and amblyopes has resulted, not because of irreversible changes in neural circuitry, but because standard clinical treatments do not address the underlying causes of these disorders…. The underlying problem in amblyopia is not poor acuity in one eye but rather a poor ability to use the two eyes together…. Surgery for the human patient is an entirely passive experience. Although an operation may reorient the eyes in the sockets in a way that makes fusion more likely, it does not teach the patient how to fuse…. Active learning must be involved. 

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