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19.7 Concluding Remarks on Phosphene Mapping Techniques
Various absolute and relative mapping procedures were discussed in this chapter. Absolute mapping provides estimated phosphene coordinates, while relative mapping provides phosphene positions only with respect to each other. Due to eye movements, mapping by sequential electrode activation may still yield unreliable relative phosphene coordinates, but absolute mapping is inherently subject to position errors if gaze is not monitored. Relative mapping of closely spaced phosphenes yields more reliable information about phosphene positions with respect to each other, which will be important when trying to present arbitrary shapes to a prosthesis wearer.
We reviewed more than a dozen absolute mapping techniques using a variety of pointing, drawing, verbal, and eye movement methods. Advantages of most of these absolute mapping procedures are their technical simplicity and the short time required to obtain a phosphene map. Especially when performing acute experiments in the operating room with time and equipment restrictions, absolute mapping by verbal communication may be the most convenient method. Data can be digitized on the spot by a drawing tablet, or recorded by the experimenter in the form of crude coordinates. With chronic implant wearers or visually impaired subjects in a laboratory setting, detailed information can be obtained using a touch screen or a dart board or clock face with tactile markings. Tactile markers and training improve accuracy. Drawings can be advantageous when phosphene shape is of interest.
Disadvantages of these techniques are their inaccuracy and the difficulty resolving phosphenes located closely together, especially by subjects with long-standing vision loss. Visuo-motor translation may affect the results, especially when phosphene location or shape is indicated by drawing. Disadvantages of verbal descriptions, paper drawings and pointing to a surface other than a touch screen include the need to re-draw the data in a visual field map, or to digitize them into a computer. Finally, some of these methods can only be successfully employed by individuals with functional residual vision (e.g., using a laser pointer).
Relative mapping methods require subjects to provide details about the relationship between different phosphenes. The techniques we reviewed varied in phosphene presentation, using timing or other attributes to distinguish two or more phosphenes, but also in response modalities and analysis methods. All these techniques tended to be more complex and time consuming than the absolute techniques. This may not be a serious problem in subjects with long-term implants, as the benefits of careful mapping in increased ability to convey visual information to the prosthesis wearer will far outweigh the cost in time.
Finally we learned that a combination of well-chosen absolute and relative mapping methods may yield accurate maps with acceptably small distortions. There is still a need to further elaborate some of the techniques beyond what has been described in the literature thus far, but many of the elements for reliable and efficient phosphene mapping procedures appear to be available. The principal remaining task is to perform comparisons of promising techniques, and choose optimal combinations.
Acknowledgment Supported in part by PHS grant # EY012843.
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Chapter 20
Prosthetic Vision Assessment
Marilyn E. Schneck and Gislin Dagnelie
Abstract As visual prostheses continue to evolve, assessing their efficacy assumes paramount importance. This chapter identifies some of the key questions and issues that arise when planning and designing such assessments, in order to help point the way forward.
High quality evaluations will naturally follow basic scientific principles such as including pre-operative as well as post-operative testing. Evaluations should include both visual function and visual task performance. Improved visual function tests may need to be developed or adapted that are suitable for the levels of vision afforded by current and near-term prosthetics. In assessing task performance, the choice of tasks to be assessed is critical, and can greatly influence the results.
Longer-term follow-up testing after periods of acclimatization and training are also necessary, with control groups receiving alternative training such as more conventional rehabilitation or interventions.
Self-assessment of difficulty in performing daily living tasks is also important, as are the more subjective assessments of user satisfaction.
As the technologies continue to evolve, there will be a changing dynamic involving the steadily improving capabilities of the technology and the unique needs of a growing number and more diverse target population.
Abbreviations
ADL |
Activities of daily living |
ALS |
Activities of life satisfaction |
BaLM |
Basic light and movement test |
M.E. Schneck (*)
Rehabilitation Engineering and Research Center, The Smith-Kettlewell Eye Research Institute, 2318 Fillmore Street, San Francisco, CA 94115, USA
and
Vision Sciences Program School of Optometry-2020, University of California at Berkeley, Berkeley, CA 94720-2020, USA
e-mail: mes@ski.org
G. Dagnelie (ed.), Visual Prosthetics: Physiology, Bioengineering, Rehabilitation, |
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DOI 10.1007/978-1-4419-0754-7_20, © Springer Science+Business Media, LLC 2011 |
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