4.8  A Look at What Is Ahead

Several studies have highlighted that following the loss of vision the brain undergoes profound neuroplastic changes. This plasticity takes place at a variety of levels, from the synaptic interactions among single neurons and the circuits in which neurons interact, to large-scale systems comprising those circuits. Furthermore it has been also suggested that glial cells could have central roles in the adaptation to blindness [13]. The precise understanding of these changes will be crucial in developing and projecting the success of novel visual neuroprosthetic strategies will certainly have implications for rehabilitative training and device development. This endeavor will require strong interactions between basic scientists, clinicians, engineers and rehabilitation experts to help make decisions about (a) whether potentially residual capacity for vision exists; (b) how this plasticity can be driven and (c) what the inputs should be to maximize this restitution. These issues are central to the development of any visual neuroprosthesis approach and will provide a mechanistic rationale for understanding therapeutic interventions and teaching strategies for the blind.

New evidence about experience-dependent plasticity of the adult brain together with the achievements of other neuroprosthesis efforts allows cautious optimism about the possibility to restore some functional vision to profoundly blind individuals, but there are still several important issues that should be taken into account. Case studies of surgical sight restoration following long-term visual deprivation [35, 39] provide a relevant insight. For example, patients blinded for many years experience profound difficulty in various visual tasks, particularly those requiring the identification and recognition of objects following ocular surgical procedures aimed at regaining some degree of functional vision. Interestingly, if these patients were allowed to explore the same object through touch, they can recognize it immediately as to register their newly acquired visual percepts with their existing senses. These results suggest that the simple restoration of the lost sensory input may not itself suffice for achieving a functional sense. One possibility to overcome this problem might be to develop a patient controlled system that coordinates and registers the visual perceptions generated by a visual prosthesis with the identification of objects perceived through other senses (such as touch and audition). Patients could then learn to integrate these concordant sources of sensory stimuli into meaningful percepts [61].

Finally, although the effects of neural plasticity are prominent in the context of any visual neuroprosthesis, they are usually unrecognized or greatly underestimated. Therefore, it is essential that future research explore the mechanisms that underlie brain plasticity following the loss of vision and that research studies in the field of visual prosthesis learn to integrate these new findings to enhance the translation of this knowledge to clinical research and practice. We have now an unprecedented number of tools for the restoration of sight through artificial means but we have to use these tools to select appropriate candidates for implantation, to develop suitable rehabilitative strategies for each particular type of visual neuroprosthesis and to achieve the best possible behavioral outcome for a given person using these devices.

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Chapter 5

Visual Perceptual Effects of Long-Standing

Vision Loss

Ava K. Bittner and Janet S. Sunness

Abstract  This chapter focuses on the changes in vision experienced by patients with RP and AMD. The specific aspects of vision that are reviewed include progressive changes in central acuity, contrast sensitivity, visual field, color vision, night vision, glare, and light and dark adaptation. Emphasis is on patients’ perspectives, including the impact on functioning and performance of activities of daily living, as well as rates, patterns of vision loss, and day-to-day visual fluctuations experienced by those with retinal degenerative diseases. Several types of visual phenomena are presented, including Charles Bonnet Syndrome hallucinations in AMD, perceptual completion or filling-in of scotomas in AMD, remapping visual cortex in AMD, the preferred retinal locus in AMD, and photopsias or light show type flashes in RP. The proposed implications of these visual changes and phenomena as they apply to retinal prosthetic vision are discussed.

Abbreviations

A.K. Bittner (*)

Lions Vision Research & Rehabilitation Center, Wilmer Eye Institute, Johns Hopkins University School of Medicine, 550 N. Broadway, 6th floor, Baltimore, MD 21205, USA

e-mail: abittne1@jhmi.edu