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Chapter 4 PERCEPTUAL, COGNITIVE, AND ENVIRONMENTAL FACTORS In order to build more effective aids to mobility, we must gain a much better understanding of the mobility problem than we currently have. We need to know what spatial information blind and visually impaired pedestrians need, how spatial information should be displayed, to what sense or senses, and whether preprocessing of the data acquired by electronic travel aids will be required. This chapter deals with the perceptual, cognitive, and motor functions of the blind and visually impaired pedestrian who performs the mobility task and identifies the questions that must be answered in order to specify the functions of a successful travel aid. THE MOBILITY TASK Mobility is undertaken with a purpose in mind, that of reaching a destination, and, if pedestrians are to reach destinations, they must know not only where those destinations are, but also where they are themselves. 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 (1954) 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, including internal somesthetic feedback and external feedback in the form of information acquired from the space in which the task is performed. Furthermore, the requirements of the task are not usually adequately specified by information acquired from the space in which the task is performed while it is in progress, and the blind pedestrian must supplement this information with information retrieved from memory. PERCEPTUAL, COGNITIVE, AND MOTOR FUNCTIONS Pedestrians acquire some of the information they need for the mobility task directly from the space in which the task is performed while it is in progress, by way of perception. They also acquire some of the spatial information they need by consulting their memorial 36
37 representations of the space that they have experienced and their knowledge of generalized aspects of spatial structure. Hereafter, we use the term perceptual information for the information acquired directly from the surrounding space, and the term cognitive information for the information retrieved from memory in order to perform the mobility task. It should be stressed, however, that this distinction is more an organizational convenience than a theoretical division-- perceptual issues merge gradually into cognitive issues, and no sharp separation is possible or desirable (Strelow, 1985~. Perceptual Information Perception informs pedestrians about the layout of the space within the range of observation. Through perception, they discover where paths are and where they are not. They discover obstacles, and, provided they have had enough experience with a space to have some memory of it, they may identify useful landmarks. Perceptual information is distributed over time as well as space, and perception of temporal changes may be as important as perception of stable spatial structure. Physical Features In describing the contents of space, it is customary to mention objects. However, for our purposes, the term object is not inclusive enough. What is needed is a term that denotes any part of a space that can be distinguished from other parts and remembered. The term feature is more appropriate because it is of more general significance. Thus, a parking meter, which would usually be called an object, is a feature, but an opening between trees, a curve in the path, a slope, or an irregularity in the surface underfoot, if they are discriminable, are features, too. The path segments in the currently observable space, which, if remembered, become the elements that are integrated to provide knowledge of layouts, are also features. Pedestrians need to know what features are in space and where they are, because features may be obstacles, and, with learning, may become landmarks (Armstrong, 1977; Foulke, 19831. Many features of the environment make characteristic sounds, and because pedestrians can form associations between these features and their sounds, sounds can become meaningful. Consequently, such features can often be identified by the sounds they make. For instance, because car horns make sounds that are rarely confused with other things, the sound of a car horn warrants the inference that there is a car at the locus of the sound source. However, the acoustical energy that is perceived as a car horn can contain no information about spatial exten- sion, and so it cannot specify the size and shape of the car. As a matter of fact, unless the sound of the car horn is so distinctive that it can be distinguished from all other car horns and can therefore be associated with a particular car, the sound of the car horn can only identify the category that includes those things we call cars.
38 Even when an association between a feature and its sound has been formed, the sound does not, in most cases, provide a dependable means of identification, because the sound and the feature with which it is associated are usually not continually coincidental. For example, because a car horn is silent most of the time, it does not provide a dependable indication of the car's presence and position. Events Events, such as movement, have temporal extension and must be perceived by a system that is capable of temporal analysis. In order to perform the mobility task, pedestrians must know not only what things are in space and where they are, but also whether they are in motion and, if so, how fast and in what direction they are moving. For example, blind pedestrians sometimes regulate their own direction of motion by keeping it in agreement with their perception of the direction of motion indicated by the sounds of moving traffic in a parallel street. Both blind and sighted pedestrians must use estimates of the rate and direction of motion of moving things in order to make decisions about when and when not to move and, if to move, in what direction and how fast. Blind pedestrians get the information they need for these decisions by hearing audible things in motion. Sighted pedestrians probably get some information by hearing audible things in motion, too, but they undoubtedly get more information of this kind by seeing visible things in motion. Spatial Zones In order to get the spatial information they need, pedestrians must know where in the surrounding space to find it. If blind pedestrians are using mobility aids for that purpose, those aids must be so designed that they can examine the spatial zone or zones in which the information is to be found. This is an important issue. One of the decisions reflected in the design of any mobility aid is a decision, implicit or explicit, con- cerning the spatial zone to be examined. That decision, once made, sets some limits on the kind of information that can be acquired. There are four primary zones in which important information is found: (1) The travel surface. Blind pedestrians need information about the surface on which they walk, including changes in surface texture, particularly drop-offs or step-ups and potholes or other hazards that may occur in the surface. (2) The path ahead. Blind pedestrians need information about obstacles that may occur in the spatial area through which they will be moving. (3) The ad joining space. Blind pedestrians may need information about features of the space that adjoin the travel path, particularly
39 so that such features may serve as landmarks. The adjoining space is that which is immediately perceptible to both sides of the intended travel path as well as, possibly, overhead. (4) The extended space. Sighted pedestrians often need information about the larger space in which they travel, whether there is a bridge or a building ahead, for example. Blind pedestrians have the same need, although the limit of their immediately perceptible space is often restricted to the limits of the cane or other travel aid. Serious attention must be given to the need for blind pedestrians to have acces to the important features of the extended space in which they travel. Perceptual Anticipation Numerous experiments (Barth, 1979; Grossman, 1960; Hershman and Hillix, 1965; Levin and Kaplan, 1969; McLean and Hoffman, 1973; Poulton, 1954) have demonstrated the dependence of skillful mobility performance on the ability to anticipate behavioral requirements by observing the characteristics of a situation in advance of the time at which some action will be required. It is useful to distinguish two kinds of anticipation, perceptual anticipation and cognitive anticipa- tion (Barth and Foulke, 1979; Foulke, 1982; Poulton, 1952~. Perceptual anticipation is made possible by direct observation of the space in which it is performed while the task is in progress. Blind pedestrians who are informed in advance by perception that a curb is coming up have time in which to prepare for the sequence of movements that, when executed with proper timing, will ensure that the curb is dealt with successfully. Cognitive factors are discussed in the next sectin. The Sufficiency of Perceptual Information If pedestrians can gather enough perceptual information while the mobility task is in progress, if it is relevant and sufficiently accurate and specific, and if it can be acquired soon enough to allow time for planning the behavior it dictates, then perceptual information can regulate task performance. The spatial information that can be acquired by visual perception usually meets these requirements. How- ever 9 because blind pedestrians, using the perceptual processes avail- ab~e to them, acquire less spatial information than sighted pedestrians observing the space in which they are performing the mobility task as they are performing it, they must depend more heavily than sighted pedestrians on information retrieved from memory. Cognitive Information There is, at present, a lively debate over how spatial patterns are represented in memory (Anderson, 1978; Corballis, 1982; Pylyshyn, 1973~. Current research suggests that cognitive representations are hierarchical in structure; that information is clustered into chunks
40 throughout these hierarchies; and that the clustering exhibits both spatial (analog) and non-spatial (semantic) attributes tHirtle and Jonides, 1985~. Many experiments show that memory is selective with regard to the spatial information it stores (Appleyard, 1970, 1976; Brewer and Treyens, 1981; Evans et al., 1981; Lynch, 1960; Shagen, 1970; see also Strelow, 1985, for a discussion of these and related issues). It seems reasonable, therefore, to regard the cognitive representations constructed by pedestrians as they gain experience with the spaces in which they travel as schematic in character. Just as a road map preserves the information needed by travelers to find their way through a network of roads, while omitting most of the detail that would be present in an aerial photograph of the territory described by the map, it is reasonable to hypothesize that pedestrians preserve the information they need to move through space independently and safely and to reach their destinations, while omitting the abundance of irrelevant detail they may have observed. Memorial representations of perceived features and events are the elements that are integrated to form cognitive representations. Cog- nitive representations contain information not immediately present in the elements from which they are synthesized. For instance, a cognitive representation may supply knowledge of the shape of an object that is too large to be observed by touch from one position; for the shape to become evident, it is necessary to integrate haptic observations made from different positions at different times. Likewise, the spatial relationship of a number of things perceived separately will not be evident until it has been revealed by integrating them. Thus, some of the information that is essential for pedestrians is the product of cognitive processes. In most of the spaces through which pedestrians move, there are affordances--that is, movement along some courses is easy, while movement along other courses is difficult or impossible. These affordances are simply paths. The paths in a space usually intersect. Intersections are connected by path segments. The entire collection of path segments and intersections in a space constitutes its path structure. A route is a sequence of connected path segments that affords passage from a starting place to a destination, and in most spaces, a destination may be reached by more than one route. The layout of a space is, broadly speaking, the arrangement of all of the things in that space whose locations are fixed. It is not usually possible to perceive the pattern formed by the paths in the spaces through which pedestrians move, because in most cases not enough space can be observed from one position and at one time to make the pattern evident. Consequently, the cognitive representations are often syntheses that have been achieved by integrating spatial information resulting from observations made from different positions and at dif- ferent times. Evidence presented by several investigators (Appleyard, 1970; Beck and Wood, 1976; Lynch, 1960) indicates that integration is not always complete and that cognitive representations often exhibit discontinuities, oversimplifications, and errors. The completeness and accuracy of the integration probably depends, in part, on the extent of the space that can be observed from one position and at one time, as well as on the nature of the individual's experience in the space.
41 Because blind pedestrians can observe much less space from one position and at one time than sighted pedestrians, they must do more integrating, and the syntheses they achieve are accordingly less accurate and complete. Furthermore, because they cannot make use of many of the landmarks that keep sighted pedestrians oriented in the larger spaces, the blind may have to place more reliance on cognitive representations than sighted pedestrians. Ordinarily, route knowledge includes landmarks (e.g., distinctive characteristics of the surface under foot or features in the spaces bound by path segments) which pedestrians use to confirm their route positions. However, in the absence of other information, pedestrians can, with just the spatial knowledge contained in a cognitive repre- sentation, traverse a route without error. Imagine a route like that learned by a white rat in a maze. Path surfaces are smooth and undifferentiated. The spaces bounded by path segments are devoid of distinguishing landmarks. Nevertheless, with practice the route can be learned. What is learned is a sequence of correct actions at choice points (turn left, turn right, go straight ahead). Pedestrians who rely on this kind of knowledge alone must remember their actions at preceding choice points in order to know what to do next. If they have a lapse of memory, they will be lost. If for some reason they stray from the path, they will also be lost, because their cognitive repre- sentat~on does not include any landmarks that could be used for geo- graphical orientation. Furthermore, with such a simple representation they cannot select an alternate route if they find a path blocked. Blind pedestrians rarely find themselves in situations in which they must depend entirely on route knowledge with no landmarks what- soever, although a network of corridors in a large hotel can come close to providing this situation. However, they often find themselves in situations in which landmarks are scarce, and, when this is so, they must depend on more generalized or stereotyped cognitive representations. Most people spend most of their time in built environments, and in built environments certain patterns are repeated over and over again. For example, in cities, streets cross other streets, and they often intersect at right angles. Many of them have been given contours that assist drainage: from the center, they slope downward to either side. They are usually bounded by curbs, so that one must step up in order to pass from the street to the adjacent land. On both sides of streets are frequently found sidewalks, and in residential areas these sidewalks are often separated from the streets by grassy verges on which trees, utility poles, and lamp posts are scattered. Because constructed environments exhibit pervasive reiteration, the cognitive representation incorporates what may be regarded as spatial stereotypes. These stereo- types are the generalizations on which pedestrians can base predictions concerning what they will encounter in spaces not previously experi- enced. Of course, actions will be more effective if they are mediated by the information acquired by direct observation of the space in which the task is performed or by consulting an accurate schematic representa- tion of that space, but, in the absence of better information, these generalizations are useful. The extent of their actual use, however, is an unresearched issue.
42 When pedestrians enter new spaces, they have no schematic repre- sentations of those spaces to consult, and, if they are blind, the information they can acquire by observation on first encounter will generally not be sufficient to direct their actions. Their only recourse is to rely on relevant spatial stereotypes until they can integrate enough direct observations, made serially, to form accurate schematic representations. Recourse to spatial stereotypes is also available to sighted pedestrians, but the perceptual information provided by visual observation is generally sufficient to guide their performance, and they do not have as much need to supplement it with cognitive information. Thus, on first encounter with unfamiliar space, the performance of the blind pedestrians is relatively poor but improves with practice, whereas the initial performance of sighted pedestrians is relatively good and does not improve much with practice (Hollyfield, 1981). Perceptual-Motor Skills Of course, any discussion of mobility skills would be incomplete without attention to the set of perceptual-motor skills that enable the blind pedestrian to maintain an appropriate relationship between his or her body and the path immediately ahead. Developmentally, muscle tone, range of motion, flexibility, crawling, creeping, and upright walking are examples of important prerequisite motor behaviors that enable blind persons to move about. Blind pedestrians need such gross motor skills as strength, endurance, coordination, and balance in order to perform the mobility task efficiently. In addition, fine motor skills are needed to explore an object, use an orientation aid, or use a mobility aid such as the long cane or an ETA. In performing the mobility task, blind pedestrians must constantly update their direction and distance to relevant features of the environ- ment. Several orientation skills (Hill and Ponder, 1976), along with corequisite perceptual motor skills, are used to accomplish effective spatial updating. For example, consider the skills of establishing alignment, maintaining a straight line of travel, executing turns, and judging distances. All these skills facilitate spatial updating. However, good posture, gait, coordination, and walking speed are crucial to execution of these skills. In addition, the proprioception (body awareness/image), tactile {use of the hands, feet, and posterior plane of body), auditory (localization and judging trajectories), and kinesthetic (time/distance estimation through movement) senses are integral to performing the above orientation skills. The process of spatial updating, along with its complementary skills, also facilitates the acquisition of spatial layout knowledge (object-to-object relationships). Learning a large novel space requires the concurrent use of the previously mentioned orientation and perceptual-motor skills, along with using systematic search patterns, establishing landmarks, and identifying clues. -
43 MOBILITY SKILLS Having outlined the nature of the perceptual information and cognitive processes available to the blind pedestrian, we now describe how the information from these sources is put to use in order to deal with a specific mobility task. These applications of knowledge about the environment can be conceptualized as a set of mobility skills, each of which has its own developmental sequence and degree of importance for an individual blind pedestrian. Before describing these mobility skills one by one, however, it is useful to keep in mind several common features. First, the goal motivating the application of each skill is the same: successful solution of a mobility problem. But success, as we stated earlier, includes much more than just the act of reaching point B from point A. One mark of a successful traveler is the ability to reach point B in a way that minimizes to an acceptable degree the physical and psychological risks associated with travel. As Foulke (1971) has stated, the goal is to move safely' gracefully, comfortably, and efficiently from A to B. The mobility skills described below are all strategies that make obtaining this goal considerably more likely. Second, the application of these specific mobility skills is particularly critical for blind pedestrians due to the limited degree of perceptual anticipation possible in the absence of vision. Sighted pedestrians who can gather perceptual information in ample time for the guidance of their locomotor behavior need not rely so heavily on inferences about the environment. What sighted pedestrians can see for themselves, blind pedestrians must often infer, and many if not all of the mobility skills described can be thought of as types of inferential reasoning about what is present in the environment and how one should move as a result. In the sections that follow we describe the mobility skills, or applications of knowledge, that are to a greater or lesser extent available in the repertoire of the blind traveler. For purposes of discussion these skills are divided into two major categories: (a) skills that depend on the application of general knowledge about environments and (b) skills that depend on the application of knowledge about a specific environment. Obviously, one implication of this division is that the first set of skills will have a particularly important role to play in solving mobility problems in unfamiliar environments. Applications of General Knowledge Inferences Based on Spatial Stereotypes As noted earlier, one type of information available in the knowledge base is a set of expectations about what a given environment should be like. These expectations may be characterized as probability statements that arise through a variety of sources, including one's own previous spatial experience and direct tutelage by travel instructors. These
44 expectations are applied by blind travelers, sometimes successfully and sometimes not, to help them plan the locomotor behavior that will be most likely to advance them toward their goal. For example, the expectation that streets in an urban setting will be laid out in a grid pattern may lead the blind pedestrian to infer that a 90-degree turn at a corner will allow him or her to maintain the appropriate relationship to the sidewalk. The feedback that the traveler obtains once this inference is acted on will obviously enter the data base of knowledge on which the traveler will depend for a subsequent trip through that environment. Inferences Based on Logical Principles The traveler for whom minimizing effort is a major criterion for successful travel may learn to apply various logical principles. One example is a technique called baselining, which is useful in crossing open spaces such as going from a corridor across an open hotel lobby to the continuation of the corridor on the other side of the lobby. In baselining, a decision is made to steer toward a point that is certain to be wide of the target in one direction (e.g., to the right). Once that point is reached, the traveler can be sure what direction of travel (e.g., left) will result in reaching the goal. Had the traveler instead chosen from the start to try steering directly toward the target, a failure to hit the target accurately would leave him or her uncertain about which direction to move to regain the desired path. Such a cognitive strategy can be applied in a variety of environmental situations in order to increase the probability that the target will in fact be found. Applications of Specific Knowledge Landmarks At the simplest level, landmarks are features that, when recognized, serve only to assure travelers that they are on course. At this level, landmarks need not serve an orienting function. In fact, they need not even have been observed before. Knowledge of such landmarks may have been conveyed by maps, pictures, spoken or written words, and so forth. At a higher level, the identification of a landmark that is on or close by a path segment that is part of a known route may serve as confirmation of one's current position. For example, the unusually sharp curve in a path or the distinctive bump in a sidewalk caused by a tree root underneath may tell blind pedestrians who have traversed the route a number of times exactly where they are. A landmark is most useful when its relationship to other landmarks in the same space is known. Pedestrians whose spatial knowledge includes a knowledge of the interrelation of several landmarks establish more than their route positions by recognizing one of those landmarks. They also establish their positions in the entire space that encompasses
45 the interrelated landmarks. Although they have probably acquired a good deal of route knowledge and knowledge of path structure in the course of learning landmarks and their interrelations, they could, if necessary, find their way through the space that encompasses the interrelated landmarks without drawing on their knowledge of path structure and routes. It is this aspect of landmarks in which most blind pedestrians are lacking. The value of a landmark is, in part, a function of the extent of the space in which it is observable. The skyscraper just north of the main shopping district that can be seen for miles in all directions allows sighted pedestrians who know about it to maintain orientation in a very large space; as a result of its presence, they need not learn about many other landmarks that cannot be observed from so great a distance. Furthermore, pedestrians need not ever get close to a landmark of this type. It will still serve its orienting function from a distance. The implication is that the number of landmarks pedestrians need to maintain orientation varies inversely with the extent of the space within which landmarks are observable. The extent of the space within which the landmarks used by blind pedestrians can be observed is relatively small. They can identify distinctive characteristics of the surface underfoot, they can find things within arm's reach or the reach of the cane or ETA, they can find things a few feet off the path by echo location--but they are largely unaware of the contents of the spaces bounded by the path segments along which they walk. They must, instead, depend on a large number of landmarks that denote small segments of the space. Clues As the blind pedestrian travels through a specific environment, he or she will inevitably encounter objects or events that act as clues to guide decisions about the appropriateness of a particular movement. The skill that is desirable for the blind traveler includes both the ability to identify the objects or events and the ability to draw the proper inferences from them. For example, from the sound of water splashing the blind traveler might infer the existence of a puddle in the street about to be crossed. Clues furnish information whose bearing on the mobility task is not completely certain; inference must be brought to bear to make use of this information. INDIVIDUAL DIFFERENCES In order to understand the mobility task and the human skills and abilities enlisted in the performance of the task, it is necessary to consider the individual differences among those who will be performing the task. With regard to blind pedestrians, those differences are numerous and of considerable importance and magnitude. There are those factors on which all humans vary, such as intelligence and motor facility, which affect the performance of any of the tasks in which
46 humans engage. Beyond this, there are individual differences brought about by the blindness that is responsible for the mobility problem. Risk Estimation and Risk Taking Humans vary widely in regard to their ability to estimate risk. Some seem unaware of the risk that may be inherent in a situation. Others characteristically overestimate risk. In addition to the matter of estimating risk, there are those who seem to court risk and those who choose courses of action intended to minimize risk, regardless of the personal price in loss of freedom, missed opportunity, and so forth. Those who perform the mobility task face genuine risks, such as the risk of getting lost, the risk of falling on a slick surface or down a flight of stairs, and the risk of being hit by a car. For the most part, blind and sighted pedestrians face the same risks, but the probability of engaging the dangers is greater for blind pedestrians. They have less ability to acquire from the surrounding space the information they need to assess risk; consequently, they must contend with a higher level of uncertainty. They also have less ability to acquire information on which to base actions that avoid risk. The seriousness of these problems can be reduced by training of the sort provided by mobility specialists and by the use of effective travel aids. There should be a positive correlation between the ability of blind pedestrians and their willingness to travel in unfamiliar situations; evidence presented by Russo (1985) suggests that this is the case. However, risk can also be reduced by restricting travel to familiar situations; this strategy is adopted by more blind pedestrians than those who have a vested interest in the ability of blind persons to travel independently would care to admit. There are blind pedestrians who, like gamblers at a casino, seem to enjoy risk and seek the challenge of traveling in unfamiliar situations, but most blind pedestrians, including many apparently skillful travelers, learn routes to the places they must reach with some frequency and avoid unfamiliar situations. They find acceptable the risks they incur when their performance is regulated by both perceptual and cognitive information, but they are reluctant to accept the risks they incur when only perceptual information is available. Thus, travel in novel environments may be avoided. Early Versus Late Blindness There is evidence that age at onset of blindness is an important variable in determining the ability of blind pedestrians (see Warren et al., 1973; Warren and Rocon, 1974). The literature indicates that those who have had vision for several years before becoming blind acquire the concept that there is a spatial relationship among the objects of experience, a relationship that can be grasped and remembered. Once this concept has been incorporated as a part of the cognitive operating system, it is available for organizing spatial
47 experience, even when vision is no longer a perceptual resource. Those who are blind from birth or who have lost vision before having had the opportunity to acquire this concept, are not served by the perceptual system that is most effective in providing information about the spatial extension of features or about the relationship among features in space. They must rely on other perceptual systems that are not as well suited for acquiring information of this kind. For example, although the reach of the auditory system permits the observation of a considerable extent of the surrounding space, acoustical stimulation does not generally contain reliable information about the spatial extension of spatial features, and most spatial features are not associated with distinctive sounds that could disclose their distance and direction from the observer. Although the haptic system can provide information about the spatial extension of spatial features, physical contact is a requirement for perception. The reach of the haptic sytem is therefore too limited to permit direct perception of the relationship among many important spatial features. It may be that appropriate perceptual experience can provide compensation for these perceptual limitations, but, in the absence of such experience, those who are congenitally blind may exhibit a reduced ability to grasp spatial relationships that persists throughout life. The possibility that early use of mobility aids to serve as environmental sensors (Strelow and Warren, 1985) may help overcome this limitation is an exciting one that should be pursued. Overprotection It is understandable that those responsible for the care of blind infants will want to protect them from harm and will imagine that movement through space without vision is dangerous. Guided by this premise, many parents restrict the activity of blind infants, but, by so doing, they deprive them of the perceptual experience that could lead to spatial comprehension. Beyond this, they may be making it difficult for blind infants to learn where to turn for the stimulation that contains the information they need to deal effectively with the world in which they live. As the work of Fraiberg (1977) suggests, infants who are perceptually deprived in this way may have difficulty in distinguishing themselves from other features in the surrounding space and in acquiring the concept of object permanence O Because they are relatively inattentive to stimulation in the surrounding space, they do not have the perceptual experience that could compensate for the reduced ability of the perceptual systems available to them to provide information about space. Neurological Damage It is commonly believed by teachers, mobility specialists, and others who have the opportunity to observe the mobility of blind children that those who are congenitally blind by reason of retinopathy
48 of prematurity (ROP; previously called retrolental fibroplasia) are much worse off in terms of mobility than other blind children (except the multiply handicapped blind child). There is only sparse evidence bearing on this issue (e.g., Fletcher 1981~. If it is true, then there is the possibility that the condition responsible for the retinal destruction that characterizes ROP may also be responsible for other damage in the central nervous system that is too diffuse in character to be directly observable. That damage may also be too diffuse to have clearly observable behavioral consequences from which it might be inferred but may nevertheless be responsible for the poor spatial ability of individuals who are blind because of ROP. Remaining Vision Most of those who fall within the def inition of legal blindness have some remaining vision, and that remaining vision may be very useful in acquiring information that facilitates performance of the mobility task. Even when only light perception remains, the ability to detect shadows can provide useful information. Macular degeneration may reduce central acuity enough so that the af fected individual is regarded as legally blind, yet the individual may have relatively good peripheral vision. One of the important uses of peripheral vision is the detection of movement, and information about objects in motion is an important kind of information for any pedestrian (Raymond and Leibowitz, 1985~. Individuals are legally blind for many reasons, and the cause of a visual impairment is an important determinant of the spatial information that can be acquired with the remaining vision. Conclusion The purpose of the discussion has been to make the point that any mobility aid or training effort must be applied in the context of individual differences, and these individual differences may play a major role in determining the effectiveness of the mobility aid or training effort. If we are to apply what we know about mobility to the solution of human problems, our application must be guided by a better understanding of the differences among the individuals who might be able to use what we know to their advantage. RECOMMENDATIONS Research involving blind individuals, whether on mobility or other topics, has unfortunately received far too little sustained attention by experienced investigators. There are, of course, notable exceptions, but promising leads have not been pursued, and the research literature is choppy and fragmented as a result. Perhaps the most important recommendations, therefore, in this area are that support and encourage-
49 ment must be provided for sustained research programs and that means should be provided for drawing young research scholars into mature and sustained research settings. Research needs in the perceptual and cognitive areas of mobility are considerable. It is not the case that the traditional themes of research in perception and cognition are devoid of material bearing on issues of blind mobility; indeed, much of the literature cited in earlier sections of this chapter deals primarily with traditional themes and only secondarily or by implication with issues of visual impairment. To some degree, the theoretical underpinnings of mobility in the blind are weak because there has not been a sustained attack on issues related to blind mobility, and there is no self-sufficient theory of blind mobility. In the long run, the theoretical foundation for blind mobility will be stronger for its roots in the traditions of perceptual and cognitive psychology. In the meantime, there is a need to develop sustained research attention in several areas of direct bearing on blind mobility. We make recommendations in four major areas in which active research should be emphasized beyond current activities and give examples of specific project topics. In each area, ongoing methodological attention should be devoted whenever possible to two issues. First, the population of blind individuals is extremely heterogeneous, and thus individual differences must be evaluated. Individuals vary on a wide range of characteristics, such as remaining visual function, early visual history, etiology, and additional handicaps, all of which have potential implications for mobility. To have optimal impact on the mobility of blind people, research must be adequately attuned to the variations in mobility skill and its components that are produced by these variables. Second, while research on the perception and cognition of blind people as well as the sighted has been reasonably good at describing the end product of a perceptual or cognitive process, it has been notably weak at elucidating the nature of the process itself. This is a serious gap. Research designed to evaluate the nature of the perception and understanding of space often requires of its subjects the acquisition of that perception or understanding, and every opportunity should be taken to assess the acquisition process as well as the product. This is generally a difficult research issue from a methodological standpoint, but a grasp of the processes involved in these areas is critical if we hope eventually to intervene in ways that facilitate mobility. Making Information Available to the Blind Traveler Current mobility aids are reasonably effective in making available information from the adjoining space (e.g., shoreline information) and from the path ahead. The travel surface and the extended space pose more difficult problems. The long cane, when properly used, provides good information about the surface ahead, but it does so only for a relatively short extent {about 1 m). No aid that is currently available extends this range
50 effectively, although the Lasercane does provide drop-off information. The blind traveler needs more preview of the surface than this: how much more is not clear but certainly depends on such variables as walking speed and the variabilitiy of the terrain itself. RECOMMENDATION: Research should be addressed to the nature of the information a blind traveler needs to preview the travel surface and to the corresponding question of the best manner of presenting such information to the traveler. The sighted traveler typically (although not always) has perceptual information available about a far more extended region than does the blind traveler, such as from landmarks that are visible from consider- able distances. Such information is most obviously useful when alternative routes must be taken (for example because of an obstruction in the path), but it also serves to lessen the cognitive load for sighted travelers--they can forget temporarily where they are and can usually reestablish their positions in the space quickly and easily. The blind traveler is at a serious disadvantage by not having such information available. It should be an extremely high priority to explore ways of overcoming this disadvantage. RECOMMENDATION: In research on ways to assist blind travelers establish their positions in space quickly and easily, two approaches should be used: (1) the use of fixed information sources in the environment, such as beacons to enable the establishment of position (such as the VOR system for aircraft) and {2) the training of larger-scale geographic orientation capabilities in the blind traveler him- or herself. Forms of Information Display In what form might these kinds of information most effectively be presented? Information from the environment may be presented in many forms, and it is not obvious which forms are more or less effective. There are several critical questions in needs of answers from research: Is the use of a natural cue correspondence, such as auditory IAD for azimuth direction in the Sonicguide, superior to a more arbitrary matching of dimensions, such as signal rate for distance in the Mowat Sensor (see Chapter 61? Although the use of natural matches seems intuitively desirable, some evidence (e.g., Strelow and Warren, 1985) suggests that this may be an area in which intuition is not an effective guide. In any case, the issue is amenable to empirical investigation, and it is a very important question. To what extent is preprocessing of information desirable? One approach, as exemplified by the Sonicguide, is to present a relatively complex signal that is not artificially simplified for the user. The other extreme is to have the device abstract from a complex array of stimulation a few pieces of salient information to be delivered to the user in highly simplified form, as in the computer vision system
51 described by Deering (1985). Some very fundamental questions of human information processing capabilities and perceptual-cognitive function are involved here, and the effective design of future mobility aids will depend more on empirically generated answers than on simple conviction that one approach or the other is best. Is redundancy of information useful and desirable, and if so to what extent? The sighted observer often has several sources of perceptual information available, for example, about the distance of objects (stereopsis, convergence, interposition). Is the introduction of such functional redundancy desirable for the blind? The desirability of backup information must be weighed against the demands for increased information processing that a device designed to provide it would impose. While clearly a pragmatic issue of mobility aid design, this issue is also of interest to theories of human information processing and performance, quite aside from the question of mobility. RECOMMENDATION: More research should be carried out on the relationship of natural cue to human processing capabilities, on the desirability of the information preprocessed by travel aids, and on the usefulness of information redundancy in the use of travel aids. The Relationship Between Perceptual and Cognitive Information Although a sharp distinction between perceptual and cognitive levels is impossible and probably not desirable to draw, there is a significant area of interaction that must be explored with respect to blind mobility. Blind travelers acquire some immediate perceptual information about the environment and their positions in it, but generally they must also make use of stored information about that environment (or about environments in general) to engage in effective mobility. The referencing of current position and orientation to some stored representation of the environment is critical for effective mobility as long as the travel demands exceed the limits of dead reckoning. That is, travelers must reference currently obtained perceptual-motor information to a stored representation in order to monitor their current activities in relation to the space in which they stand. The means by which this reference updating is accomplished is not well understood. RECOMMENDATION: It is critical to understand better how blind travelers reference and update information about their position and orientation in space, and we recommend that more research be directed to that issue. Perceptual Learning Principles It is unlikely that a mobility aid can be designed that is so spontaneously useful that no learning process is involved in using it effectively. Instead, a perceptual-learning process must be involved that has at least the following aspects:
52 (a) Understanding of the variables of the physical world that the mobility aid is designed to convey, (b) Discrimination of the variables of stimulation presented by the mobility aid, (c) Learning the correspondence between the physical world variables (a) and the aid-presented variables (b). All this must be accomplished in the context of a stimulus environment that goes beyond the user-device interface: natural stimuli also occur that may facilitate performance if they can be effectively attended and processed, and, perhaps just as important, other perceptual-cognitive-motor tasks must be carried on simultaneously that are vital but that have no immediate relevance for the task of mobility RECOMMENDATION: Research should be fostered on the perceptual- motor learning processes that underlie the use of mobility aids. .
