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Chapter 1 INTRODUCTION Problems related to mobility and written communication account for most of the disability experienced by those who have little or no vision. Mobility problems are serious because of the instrumental character of mobility. Most human activities are carried on in particular places, and no matter how skillful a person may be in other respects, he or she will be excluded from participation in those activities by the inability to get to where they are carried on. With the introduction of the long cane in the 1940s (described in Chapter 6) and of techniques for using it (Hoover, 1968), it became apparent that the mobility of blind pedestrians could be improved significantly with training. To provide this training, specialists of a new kind, called mobility specialists, soon appeared. Although the improvement in mobility made possible by proper use of the long cane is undeniable, a long cane is, at least at first glance, a rather simple travel aid of limited capability. Since the 1960s, many have expected that by exploiting the possibilities offered by a burgeoning electronics technology, it should be possible to build an electronic travel aid (ETA) that could provide much of the critical information about space that accounts for the ease of mobility of sighted pedestrians and that is not available to blind pedestrians. However, as the years passed and attempt followed attempt, it became apparent that the potential offered by ETA s was not being fulfilled. Although some improvement in performance could often be attributed to their use, in most cases the improvement was modest (Shingledecker and Foulke, 1978; Brabyn, 1982~. The mobility of a blind pedestrian using an ETA should, in the opinion of some, approach the mobility of a sighted pedestrian (Leonard, 1968~. Though many would regard this criterion as unrealistic, they would at least insist that the improve- ment enabled by an effective ETA should be more than modest. The ETA should make a difference that is obvious and indisputably significant. A successful ETA should enable independent, efficient, effective, and safe travel in unfamiliar surroundings. To date, no ETA has been built that can meet even the less demanding of the criteria just mentioned, and it is commonly recommended that an ETA be regarded as only an ancillary aid to supplement a primary aid: the long cane or the dog guide. 4 -
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s In recent years, although the power of electronics technology has continued to grow at a geometric rate, few have been tempted by the possibility of building a better ETA. The fact that a long cane, costing perhaps $10, has proved to be more useful by far than an ETA costing as much as $4,000 suggests that we do not yet know enough about the task of mobility and the human beings performing that task to specify the functions of an ETA. Before any further attempts are made to build ETAs, we must gain a much better understanding of the mobility problem than we currently have. We need to know what spatial informa- tion blind and visually impaired pedestrians need, whether preprocessing of the data acquired by ETAs will be required, how spatial information should be displayed, and to what sense or senses. The chapters that follow constitute an effort to render an account of the mobility task and of the perceptual, cognitive, and motor functions of the blind and visually impaired pedestrians who perform that task. Our account is consistent with the data currently available and suggests the questions that must be answered in order to specify the functions of a successful travel aid. In these chapters it will often appear that more attention has been given to blind pedestrians and the task they perform than to pedestrians with useful remaining vision and to the task they perform. We are well aware of the important perceptual differences between blind and visually impaired pedestrians and the important differences between the tasks they perform. Many investigators believe, however, that an initial emphasis on the problems confronting blind pedestrians is the best approach to take in seeking a basic understanding of the mobility problem in general. THE PROBLEM OF MOBILITY FOR BLIND AND VI SUALLY IMPAI RED PEDESTRIANS The Mobility Task The task confronting blind pedestrians is the task of independently traveling from a starting place to a destination. This may appear, at first glance, to be a simple definition, but when the information and abilities needed for its performance are taken into account, the apparent simplicity vanishes. Mobility is more than the ability to avoid obstacles while moving aimlessly through space. Mobility worthy of serious interest is that undertaken with a purpose in mind, that of reaching a destination, and, if pedestrians are to reach destinations, they must know not only what those destinations are, but also where they are. They must be oriented, and they must be able to maintain the currency of orientation as they move through space. Performance of the mobility task demands what Poulton (1957) has called an open skill. Because the task is performed in an environment that is changing and only partially predictable, its performance must be guided by feedback--both internal, somesthetic feedback and external feedback in the form of information acquired from the space in which the task is performed. Relevant information must be differentiated
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6 from a background of irrelevant, often masking stimulation. The requirements of the task are not adequately specified by information acquired from the space in which the task is performed while the task is in progress, and the blind pedestrian must supplement this informa- tion with information retrieved from memory. Finally, the observation of task performance is often difficult because the behavior to be observed is embedded in highly integrated sequences of behavior, and because critical task components must often be inferred from their behavioral consequences. Abilities and Skills To say that a person exhibits an ability is to say that he or she is able, by some action or combination of actions, to produce an intended effect. An effect may be overt and observable, as when a person's actions produce some environmental change, or not directly observable and established by inference from its behavioral conse- quences, as when information acquired by perception is stored in memory. The performance of a task usually requires the production of more than one effect, and its performance usually demands the proper timing and sequencing of effects. To understand the performance of a task, more than one level of analysis may be required. At the first, most immediate level, an attempt is made to identify the overt and observable effects on which accomplishment of the task depends. However, explanations at this level of analysis are often inadequate, and it becomes necessary to move to a second level of analysis, at which many of the actions and the effects of actions that help to account for the performance of a task must be established by inference. Explanations of task performance at this level are usually in terms of perceptual, cognitive, and motor functions. A skill is a developed proficiency or mastery, an ability that has been improved by practice or training or both. Many skills, when examined, are found to be composite skills; that is, they are the integrated expressions of component skills that are either directly observable or inferred from their behavioral consequences, or both. The skill required to perform the task of moving from a starting place to a destination is such a composite skill. In his analysis of the mobility task, Brambring (1985) divides the skills on which performance depends into three groups: prerequisite skills, necessary skills, and desirable skills. Prerequisite skills are skills that are not unique to the task but that must be developed before the task can be attempted. For example, a pedestrian must be able to walk or at least have some means of locomotion before attempting the mobility task. Necessary skills are those skills without which the task cannot be performed. Obstacle avoidance is one example, and the actions it implicates are observable. The use of landmarks to maintain orientation is another example, but the cognitive functions implicated by this skill are not directly observable and must be inferred from their behavioral consequences.
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7 Desirable skills are skills that, although they enhance performance of the mobility task, are not necessary for its performance. The skill required to walk at a speed that does not cause interference with other pedestrians who are performing the same task, at the same time, and in the same setting is an example of a desirable skill. The elimination of interference is not a condition on which task performance depends. If the blind pedestrian performing the task walks at a much slower speed, he or she may interfer with other pedestrians, and the cost in time to the blind pedestrian may be considerable, but the task can still be performed. The task confronting blind pedestrians, as we have suggested, is the task of independently traveling from a starting place to a destination. Successful mobility involves much more, however, than just the act of reaching a destination. The fact that an obstacle has not been hit does not always warrant the inference that a pedestrian has avoided it by the exercise of perceptual, cognitive, and motor skills--he or she may simply have been lucky. Measures are needed to determine the success with which the blind pedestrian has performed the mobility task. These measures involve the development of performance · ~ criteria. Establishing Performance Criteria A performance criterion is a standard against which a performance to be evaluated is compared. If the performance to be evaluated is equal to or greater than the standard, the criterion has been met. The criterion may be a standard of excellence: Rudolf Serkin's performance of a Beethoven sonata would serve, for instance, as a standard of excellence against which to compare the performances of aspiring pianists. More commonly, a criterion denotes a level of performance that marks the boundary between adequate and inadequate performance. A performance variable may be found that is believed to be an adequate indicator of the composite performance of the whole task, because the value it assumes is jointly determined by a number of component variables, and a criterion could be established for this indicator variable. Motor ability is one determinant of walking speed; an elderly person with weak musculature or arthritis, for example, will walk more slowly than a person who does not have these limitations. Walking speed is also determined by the perceptual anticipation of which a pedestrian is capable. If little perceptual anticipation is possible, the time required to plan the actions that will be needed to cope with upcoming situations must be gained by walking more slowly. A pedestrian with some perceptual anticipation afforded by some residual vision, or by a travel aid that provides information about the char- acteristics of the path some distance ahead, will have more time for planning actions and will therefore be able to walk faster. The amount of perceptual anticipation needed is also a function of the environ- mental conditions (i.e., complexity, irregularity, congestion, etch. These and other such variables act in combination to determine walking speed, and if their values are appropriate, walking speed should be
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8 adequate. A walking speed that is at or above the criterion speed provides some assurance, therefore, that the component skills required for the mobility task are adequate. Component Skill Criteria Indicators of composite performance provide no enlightenment concerning the particular combination of component variables that are responsible for the observed value of the indicator variable. There could be many such combinations, including some in which there are values indicating unacceptably low levels of some of the component skills. Leonard (1972) and Armstrong (1975) have proposed the estab- lishment of criteria for the component skills on which performance of the mobility task depends. Obstacle avoidance, measured by counting unintended bodily contacts with objects in the surrounding space, and skill in navigating, measured by counting unintended departures from the path, are examples. The pedestrian whose performance of each of these skills reaches criterion also meets the composite criterion of safety, which applies to the mobility task as a whole. By the same token, the whole-task criterion of efficiency is satisfied by reaching the criterion for economy of time on task, as indicated by the time spent walking divided by total time en route, continuousness of progress or the distance walked toward the destination divided by the total distance walked, and the skill that is demonstrated by choosing the most direct route. Even if ways have been found to measure all the component skills that contribute to performance of the mobility task, there remains the problem of how to derive from these measures an indication of the skill with which the whole task has been performed. Is composite skill indicated by the number of component skills performed at or above criterion, or by the contour of a profile of the measured values of component skill variables, or-by weighing and combining the measured values of component skill variables in some way to produce a single indicator? GUI DE TO THE REPORT The chapters that follow describe the research that has been done, that is now in progress, and that must be undertaken in order to resolve many of these issues. The report specifies the information that will be needed to improve the design, development, and use of mobility aids. Following this introduction, Chapter 2 presents a profile of the target population and what information is available concerning the factors that contribute to the use or rejection of available travel aids. Chapter 3 addresses the difficult problem of assessing mobility performance and points to important new directions for developing measures in that area. Chapter 4 outlines the dimensions of mobility from perceptual, conceptual, and environmental perspectives. Chapter 5 reviews the physiological mechanisms underlying sensory enhancement and substitution
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9 and considers the success of environmental sensors in meeting the needs of the visually impaired or blind traveler. Chapter 6 provides a description of the technology of information acquisition and display, highlights the limitations of existing technology, and recommends new directions for the design and development of electronic travel aids.
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