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OCR for page 36
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
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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.
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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
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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
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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.
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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.
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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. -
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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
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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
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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
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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
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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
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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-
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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
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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
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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:
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(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.
.
Representative terms from entire chapter:
mobility task
