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