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Tracking and shifting gaze are important skills for personal care and mobility. For example, applying make-up involves eye-hand coordination and the ability to shift gaze from a cosmetic to the reflection of one’s face in the mirror. In mobility the ability to track a moving vehicle to assess time-to-contact (impact) and shifting gaze to acquire information at a two-way street crossing are common mobility tasks.
21.3 Measuring Functional Outcomes
It is important to determine the effects of prosthetic vision on functional performance. Did the treatment make a difference? If so, what kind and how much of a difference? Is there a difference in the patient’s posture, body position, and/or head position? Did the treatment result in greater safety or was the level of independence improved? Does the patient experience greater visual independence when preparing dinner? Have additional responsibilities in the home been absorbed by the patient such as sorting laundry or selecting the proper placemat and plate while setting the dinner table?
It is critical to understand what each patient’s goal is for prosthetic vision. If the goal is for prosthetic vision to replace or supplement the individual’s need to travel with a long cane, then the resulting prosthetic vision will need to be of sufficient resolution to allow the person to reliably and safely detect changes in elevation (curbs, stairs), confidently travel in a variety of light levels, accurately detect objects in the travel path, and consistently identify changes in the travel surface (gravel, grass, pavement). If the goal is simply to enhance the currently existing travel skills, then the resolution provided by the prosthetic vision can be more modest.
Another prosthetic vision recipient may have sufficient functional vision to travel safely in familiar indoor and outdoor areas, but use a long cane when traveling in unfamiliar areas. In this example, the question is what type and how much prosthetic vision will be required to improve travel skills (light perception to light projection or hand motion)?
Assuming the subject has some amount of independent mobility prior to the treatment, the effect will either be to change the mode of travel (non-visual to visual) or to enhance the current approach to travel. Herein is the challenge. If the patient is an independent traveler prior to the introduction of prosthetic vision, prosthetic vision may not increase their level of independence. The best possible outcome would be an enhancement of independent travel, that is, improved orientation in unfamiliar areas. It is quite difficult in general to measure the enhancement of travel. If the expected outcome of prosthetic vision is to change the mode of travel (cane or guide dog to vision only), the technology is not yet capable of providing that level of visual input. Both examples present challenges to measuring outcomes. For example, if the patient is an independent cane traveler or guide dog user, taking the cane or dog away may result in a degradation of performance
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until the new visual approach has been mastered, assuming the future of prosthetic vision provides for better visual acuity and visual field. In this example, a degradation of performance would be expected for the short term with the hope that performance will improve as mastery of prosthetic vision occurs. However, even in this example, the best we can hope for is for the patient to transition from being an independent blind traveler to an independent visual traveler. When vision is considered as an enhancement to the current mode of travel the measurement challenge is more extreme. Attributes such as ease, anticipation, or previewing are difficult to quantify.
One approach to measuring mobility performance would be to isolate the visual aspects of mobility and to concentrate testing on these visual tasks. For example, walking down a hallway while following the ceiling lights could be compared to walking down the hallway without the ceiling lights, identifying the location of windows in a room, or visually identifying the fourth intersecting sidewalk are all discrete elements of O&M that can only be done visually.
Another approach to measuring mobility performance with prosthetic vision is to measure mental effort. The underlying assumptions of this approach are:
1. Patients with low vision rarely bump into obstacles
2. It requires more cognitive attention to the environment to travel with no vision than low vision
3. This attention can be measured as mental effort
Experiments have shown that measures of mental effort through the use of a secondary task are responsive to variation in environmental complexity [10] and to varying extent of visual field [5]. Assessing mental effort may be an approach with potential for measuring outcomes with prosthetic vision. We assume, however, that the introduction of prosthetic vision would itself impose a secondary task and at least initially result in an increase of mental effort until the patient adapted to the new visual input. We have observed patients with recent implants whose performance is initially degraded as they adjust to the prosthetic vision. At times we have also observed that patients ignore other sensory information as they strive to utilize their new vision. Time is required for patients to complete their adjustment to the prosthesis and reintegrate all the sensory information it provides.
In the context of ADLs, the issue of independence of people who are blind is also present. Since many patients can perform ADLs without sight, it is challenging to measure the effects of a prosthetic implant on living skills. One approach is to isolate the visual elements of a task and to concentrate the performance measure on those specific elements. For example, the identification of a white shirt may be done tactually via the feel of the cloth, the location of the shirt in a closest, or a tactual marking on the inside of the collar. However, it may also be possible to sort shirts based upon visual input, separating white shirts from dark shirts, and this is what should be evaluated in people with prosthetic vision. Another example is the height of flame on a stove. The common approach is to attach tactual markers to the stove dials that indicate the relative height of the flame. Prosthetic vision may be used to visually detect the flame.
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21.4 The Future
Although prosthetic vision does not currently offer those who receive it much assistance with tasks that require good functional visual acuity, we anticipate that the next generation of this technology, if it offers a modest improvement in visual acuity, may be of benefit for tasks that require high contrast and do not require good visual acuity. For example, a black hairbrush on a white countertop in the bathroom might be located through the use of artificial sight. In the kitchen, counting and placing strawberries on a white cutting board, working with peeled white potatoes on a dark cutting board, locating food such as beef on a white plate, determining how much liquid is in a glass (see Fig. 21.3), and sorting laundry into dark and light piles are activities that could be aided by the use of artificial vision.
Anticipating that the technology will improve over the next decade, we would expect that tasks requiring higher levels of visual acuity will become possible through the use of prosthetic vision.
Prosthetic vision has been described as being analogous to cochlear implants. Although this analogy may prove to be accurate, we provide a different analogy that offers a word of caution. Electronic travel aids (ETAs) have been designed to provide information about the environment with the goal of offering an improved preview of the environment. Common examples of ETAs include the laser cane and the sonic guide. These two devices provide detailed information about the environment. Users of such devices find that they compete with the naturally occurring sensory information and tend to only use them in specific and isolated situations. For example, the sonic guide is effective for following the barrier that separates paid customers from the general public in a subway station, allowing the user of the sonic guide to follow the barrier without touching it. In other situations such as a grocery store, users describe the overwhelming amount of information it provides
Fig. 21.3 Effect of contrast on visibility: dark liquid in a clear glass
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as competing with the naturally occurring sensory input. Mike May’s descriptions of his experience with regaining vision are similar to the reactions of users of the sonic guide in sensory-rich environments. We wonder if improvements in the level of resolution of prosthetic vision will result in clients receiving too much visual information to process, inhibiting their functional performance.
As the quality and quantity of prosthetic vision improves, we assume that patients who are congenitally blind will receive implants in greater numbers. The history of adult-onset vision, as previously mentioned, suggests there will be challenges. A few of these may include
•Learning to interpret the image
•Adapting to the new sensory input
•Integrating vision into a lifestyle of non-visual independence
We believe it will be necessary to educate low vision rehabilitation service providers in how to work with individuals who have prosthetic vision. We do not know if the standard low vision rehabilitation techniques will apply to this population or if entirely new strategies will need to be developed or if the modifications to the existing approaches is all that will be required. Low vision rehabilitation professionals working with recipients of prosthetic vision will need to be familiar and competent with non-visual techniques and strategies in the performance of ADL tasks as these skills may continue to be essential for the individual with prosthetic vision. We have emphasized addressing mobility skills with this population, but as the technology improves applications for near-point activities (reading and writing) may also involve changes to the current regime of rehabilitation strategies. Assuming the technology will ultimately be funded by third-party payers, if the rehabilitation strategies are highly specialized, it will be necessary to add new certification requirements and a greater body of knowledge including the non-visual strategies and techniques for rehabilitation specialists serving this population.
In conclusion, the use of prosthetic vision must be understood in the context of the client and his goals, lifestyle, and ability to adapt to change. Realistic expectations and a high level of independence will enhance the chances of a positive outcome with prosthetic vision. Patients who are properly selected to participate in prosthetic vision rehabilitation need extensive education in regard to what prosthetic vision intervention can and cannot do to assure they have realistic expectations, which are vital to the success of rehabilitation. A realistic understanding of the potential of the technology, in combination with rehabilitation instruction, is one key to a successful outcome.
References
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