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Chapter 5
SENSORY ENHANCEMENT AND SUBSTITUTION
Blind people must depend on nonvisual senses for information to
help them locate and identify objects and persons, to guide the
development of their personal relations, to regulate their motor
behavior in space, and to provide an overall conceptual organization of
their spatial environments. Determinina the rely or roll -.~ m~ ocher
senses in compensating for the loss of visual information should become
a major focus of scientific Inquiry. There Is an urgent need to
determine how and to what extent sensory substitution or enhancement
techniques can best be used to prevent or compensate for deficiencies
in spatial motor behaviors in persons with visual impairments.
Mobility involves more than just a resolution of the problem of
getting from here to there. Many kinds of sensory information have to
be dealt with in both the immediate or near environment and the more
remote environment, and, in both environments, there is information
that requires high resolution. In the immediate environment, within
touch by the body or with a cane, not much resolution is required to
determine if a person is present or if a chair is occupied; it requires
more resolution, however, to read the numbers on a hotel room door or
to know a person's identity. The same is true of the more remote
environment. It does not require much resolution to know that a
building is present or that one is near a street with traffic present,
but to know the name of a building or a street, and whether it is safe
to cross the street, requires higher resolution of the sensory
information. Both environments require appropriate orientation to the
stimuli present in them and certain mobility skills in order to move
about.
The visual system is capable of resolving information in both the
immediate and remote environments, but this is not always true for the
other sensory systems, such as the auditory, somatosensory, and
kinesthetic systems. Use of these other modalities requires that a
person pay more careful attention to the spatial stimuli present in the
environment and the use of sensory aids, especially for information
requiring high resolution. Although learning is necessary to use
spatial information acquired through any sensory modality, there are
special problems in the absence of visual information or with the use
of sensory aids. The information needed to locate something in the
environment may be quite different from the information needed to
53
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identify it. The problems posed by sensory enhancement are different
from the problems posed by sensory substitution, as are the problems
posed by age and by type of visual impairment.
THE VISUAL SYSTEM
What are the visual requirements of mobility? We consider
separately visual information gathered from immediate and more remote
environments.
The Immediate Environment
By immediate environment, we mean information within a stride or
two (perhaps the space defined by the extension of the long cane). Of
primary concern within the immediate environment is obstacle avoidance.
Experimental data exist concerning the visual requirements of obstacle
avoidance. Pelli and Serio (1984), at the Institute for Sensory
Research, Syracuse University, have studied how visual restrictions of
several types limit the ability of subjects to traverse a maze con-
structed of ~hree-dimensional obstacles. They have found that the
visual requirements of this task are very low, that is, substantial
restrictions can be tolerated before performance is affected--fields
down to 10 degrees, contrast reduction by more than a factor of 10, and
spatial-frequency bandwidth of less than 1 cycle per degree. These
findings indicate that very little spatial information is required for
obstacle avoidance and that most people with low vision will be able to
perform this task. Marron and Bailey (1982) have shown that visual
field and contrast sensitivity are better predictors of performance on
orientation and mobility tasks than acuity.
Some mobility-related tasks may require acquisition of more detailed
information from the immediate environment than is needed just for
obstacle avoidance. Examples include identifying objects (distinguish-
ing between a person and a mailbox or a tree and a lamp post or recog-
nizing an acquaintance in the hallway). In the extreme, the immediate
environment may contain high-resolution information, such as signs or
the labels on soup cans. Some laboratory data exist on the visual
requirements of these tasks--field, spatial-frequency and magnification
requirements for reading (Legge et al., 1985) and face recognition
(Iniui and Kensaku, 1984~. However, such data have not been collected
in the context of mobility, in which coordination of low- and high-
resolution tasks may impose added burdens, as described below.
Remote Environments
Vision is used to gather information about objects and spatial arrange-
ment at distances remote from the individual, that is, beyond the reach
of the long cane. Work by Gibson (1979) and more recently by Mar r
(1982) and colleagues suggests that information of this sort relies on
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a variety of visual cues, such as motion parallax, stereopsis, shading,
texture gradients, and optic flow patterns. Psychology has not yet
revealed in detail the circumstances in which these cues are used.
Moreover, we have only an elemental understanding of the physiological
mechanisms used for processing such information. We do not know how
visual impairments of different types and degrees affect the ability to
gather such information. Experimental psychologists could contribute
to the understanding of mobility by defining the involvement of these
cues in mobility tasks and by showing how their use is affected by
various visual impairments.
Vision of the remote environment may also be discussed in terms of
more global abilities, such as the ability to follow a path from
instructions, the ability to reverse a path traveled once, the ability
to locate oneself with respect to objects, and the ability to achieve a
memorial representation of the environment in which the travel task is
performed. We might ask, for example, how severity of field loss or
reduction in acuity affects the time needed to master a complex spatial
environment.
Information gathered from remote environments may also be discussed
in terms of low- and high-resolution tasks. An example of the former
is the use of buildings or other large structures as landmarks. An
example of a high-resolution demand of urban mobility is the need to
read signs at a distance. High-resolution tasks are a major problem in
low-vision mobility.
This brings us to the matter of coordinating low- and high-
resolution tasks. When individuals with normal vision arrive at
unfamiliar street corners, they rapidly locate street signs and traffic
lights by first locating them in peripheral vision and then employing
saccadic eye movements to bring them to the fovea. Individuals with
restricted fields and low acuity have a more difficult job. Although
there may be sufficient residual vision to follow the crosswalk across
the street, finding and reading the sign is another matter. Typically,
pedestrians who have low vision are obliged to use telescopes with
small fields. Even if it is successful, the scanning required to find
the sign is time-consuming, inconvenient, and, although magnification
may be adequate for reading, the demand for scanning and search
substantially impedes mobility. More accessible street signs and
traffic signals would help. In this connection, talking signs have
been explored. Other high-technology navigational aids might also be
explored.
It is important to recall that low vision cannot be characterized
simply as a loss in acuity. For example, in the case of reading, we
know that the presence or absence of central vision is a better
predictor of maximum reading speed than is acuity (Legge et al., 1985~.
With regard to the different component tasks of mobility, researchers
will have to determine the significance of subject variables like
acuity, field, and contrast sensitivity.
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Sensory Enhancement
Sensory enhancement refers to artificial manipulation of patterns
of sensory stimulation to make them more useful. The classic form of
sensory enhancement in low vision is magnification, which acts to
replace small retinal images with larger ones. This reduces the
resolving demands of the task. There are other examples of the
manipulation of visual stimulation to enhance its value for people with
low vision. For example, reversed telescopes can compress a large
visual field into a smaller one (minification). In some instances of
severe field restriction, this may have benefits for mobility. Careful
analysis of the value of such devices for mobility is in order. Other
examples of sensory enhancement within the visual domain include the
use of image intensifiers for people with night blindness or contrast
enhancement hardware in closed-circuit TV magnifiers. The use of
Corning CPF color filters for mobility is an intriguing but not yet
well-understood form of sensory enhancement. In these examples, some
properties of patterns of visual stimulation are transformed, while
other properties are invariant. It would be valuable to explore other
forms of sensory transformation.
In recent years, engineers and computer scientists have devoted
substantial effort to the development of digital techniques for image
enhancement and image restoration. Most of these techniques have not
been explicitly motivated by consideration of the sensory capacities of
the observer. However, it seems likely that image enhancement methods
could be tailored to optimize the residual capacity of observers with
low vision. Some preliminary work in this direction has been undertaken
by Eli Pelli and colleagues at the Retina Foundation in Boston (Pelli
and Pelli, 1984~. It is an open question as to how such techniques
could be implemented to aid mobility.
Finally, characterization of visual capacity is dominated by three
variables--field, spatial resolution (acuity), and contrast sensitivity.
It would be helpful to give an initial characterization of vision
substitution systems using the same variables. For example, we might
ask about the field in degrees, spatial resolution in cycles per
degree, and modulation sensitivity of a prospective tactile imaging
array. The use of common measurement metrics would help in the
comparative evaluation of vision substitution systems.
THE AUDITORY SYSTEM
With regard to the auditory system's ability to resolve signals
containing distance information, it has an enormous dynamic range (100
trillion to one) and distance information coded primarily through
intensity. Accordingly, the distinction between immediate space and
remote space is of relatively little importance. The near field flows
smoothly into open space with a surprising continuity, except for the
tendency of high-frequency components to be more susceptible to air
transmission loss, and this susceptibility provides an additional
timbre cue in distance estimation.
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~7
Given constant loudness of a sound source, sensory resolution in
terms of localization and identification remains relatively constant.
Median plane localization of sound sources is achieved through the use
of binaural time and intensity differences. The human auditory system
is capable of detecting binaural time of arrival differences as short
as 30 microseconds and binaural intensity differences in the single dB
range. The pinna also has as effect on the localization of sound
sources in space. The accuracy of localization is determined by the
spectral composition of the sound source with pure tones of 3,000 Hz
being the most difficult and noise bursts with rapid onset being most
effective.
Because most objects do not emit sounds, two other acoustic
mechanisms come into play. The first involves passive interaction with
sound stimuli emitted from another independent source. These ambient
sounds are attenuated, in part because of a reduction in the high
frequency components of the complex wave due to the inability of the
sound to bend around the object, and in part because of resonances
caused by the positioning of the object and the recipient's ear. The
second mechanism involves active emission of acoustic signals by the
observer and the subsequent detection of the echo reflected by the
object. The time delay between the production of the sound and the
detection of the echo reveals information primarily about the distance
of the object, but also about its position and its textural
characteristics.
In both of these cases, the auditory system must be regarded as
functioning in a low-resolution mode. The median plane localization of
an active sound source is rarely more accurate than 5 degrees. The
passive mechanisms are even cruder, providing the observer with little
more than presence and minimal position and distance indices.
The ability of the auditory system to identify and interpret
sounds, however, is the result of a very high capacity for resolving
acoustic signals. Variations in pitch, intensity, and timbre and
temporal variation in all three can be discriminated to a fine degree.
All of these parameters give natural sounds distinctive signatures and
can be easily manipulated in an electro-acoustic system, and this
ability provides tempting options for environmental coding. The
intrinsic utility of the uncompromised auditory system to gain
information about the environment is a very important consideration.
THE SOMATOSENSORY SYSTEM
The somatosensory system is complex system that provides a wide
range of information about the near environment and mediates the
perception of motion. In the natural mode (without artificial
stimulation), the sensory information may be categorized as a series of
progressively more complex functions classified roughly as: thermal
sensitivity, sensitivity to pressure and vibration, perception of
texture and form, and stereognosis. Appropriate sensory prostheses
might use any one or a combination of these somatosensory capacities.
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Each of these categories had been studied fairly extensively in
relation to the hand and fingers, but much less is known about these
sensory capacities in other areas of the body.
Pressure
In the hand, the sense of pressure depends on activity in slowly
adapting (Merkel) afferents (Mountcastle et al., 1966~. The relation-
ship between skin indentation and subjective magnitude is linear, as is
the relationship between indentation and impulse rate in the slowly
adapting afferents. Subjectively, one is more sensitive to edges than
to flat surfaces. Coincidentally, the slowly adapting afferents are
much more sensitive to edges than to flat surfaces (Phillips and
Johnson, 19813.
Vibration
Cutaneous vibration evokes a sensation that depends on neural
activity in the QA {cutaneous quickly adapting or Neissner) afferents
and/or the PC (Pacinian) afferents. At low frequencies, less than
40-50 Hz, the QA's dominate. At higher frequencies, the PC's dominate.
Subjective intensity is a monotonic function of vibratory intensity, as
are the impulse rates in the QA and PC populations (Mountcastle et al.,
1972~.
Tactile Form Perception
Tactile form perception is most highly developed at the fingertips,
where the limit of resolution is approximately 0.8 mm (Johnson, 19833.
This value is based on studies using gap detection, mechanical gratings,
and embossed letters as stimuli. This resolution appears to be based
on neural activity transmitted via slow adapting (SA3 afferents and for
complex patterns, like letters, it is not greatly different whether the
spatial stimuli are applied to the finger in a stationary manner or the
fingers are swept across the stimuli. It is also an interesting fact
that the relationship between resolution and the primary afferents is
the same in the skin and in the foveal region of the retina.
Spatiotemporal integration involving the fingertips occurs as the
fingers are moved across a stimulus or, conversely, a stimulus is moved
across the fingers.-This integration compensates in some ways for the
limited spatial field of the fingertips.
All the proven methods of high information transfer through the
skin (Braille, the Optacon, and the Tadoma methods employ the finger
pads. That is not to say that other skin areas could not serve equally
well, but there is no concrete evidence that they can.
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s9
Texture
Tactual perception of texture, which provides information about a
wide range of surfaces, is not understood at either a psychophysical or
neurophysiological level (Johnson, 19831. The sense of surface rough-
ness, which is one facet of texture perception, is understood to some
extent. Subjective roughness magnitude is a nonmonotonic function of
surface spatial frequency. At the neural level it appears to be
mediated by variation in the impulse rate of SA afferents (Fasman et
al., 1985~.
Stereognosis
Stereognosis is the appreciation of a three-dimensional form through
manual exploration. Recognition of a door handle or of a larger object,
such as a chair, are familiar examples. Stereognosis is a complex
function of joint angle sensation, which appears to be mediated largely
by muscle spindle afferents, tactile form perception, and vibration.
Beyond these simple observations, relatively little is known about the
physiological mechanisms of stereognosis. However, Stereognosis is
extremely important and lies at the heart of the success of the cane.
Implications for Sensory Aids
How might the somesthetic sense serve the purposes of a sensory
aid? We consider this question within the near field/far field, low
resolution/high resolution framework that we have adopted.
All the available evidence suggests that the appropriate sites for
the delivery of high-resolution information about the environment are
on the hands and fingers. The ideal device would be something like the
Optacon; that is, a portable device with a dynamic, two-dimensional
display. The user might carry it like a shoulder bag, slipping a hand
into the device when he or she wants high-resolution information. Such
a device might be used for orientation with a wide-angle view and then
zoomed to a specific object, e.g., a sign. When the user is seated, it
might be used to examine detailed materials of various kinds, although
the most important use may not be text reading; within the near future,
that function will probably be accomplished most effectively by
character recognition and speech synthesis hardware. The dynamic
display may serve most effectively to find and bring the appropriate
text into the view of these pattern recognition mechanisms.
The main problem in this area is the lack of appropriate instruments
for displaying spatiotemporal information to the skin. It seems clear
that the appropriate mode of presentation is the projection of iso-
morphic spatial patterns onto the skin. It is also clear from everyday
experience that the tactile sense of the hands and fingers provides
rich imagery concerning texture, form, remote vibration, etc. The
ability to read Braille (Foulke, 1982a) and recognize speech by the
Tadoma method (Snyder et al., 1982) are two examples that provide some
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quantitative indication of this capability. However, we cannot specify
with any certainty the form that the dynamic display should take. What
is needed is a research instrument that meets or exceeds the capacities
of the tactile system. The only instrument currently available for
studying the efficacy of dynamic, two-dimensional displays is the
Optacon, which falls far short of the physiological capacity of the
system. Its pin spacing is 3 times greater than the resolution limit
of the fingers, and its stimulus mode is poorly matched to the under-
lying receptor mechanism. For example, at 230 Hz, the frequency at
which the individual pins vibrate, the receptor system with the highest
spatial resolution, the SA population, is not even activated. Subjects
report that the sensations evoked are unnatural and poorly differen-
tiated. Most find it very difficult to use. The spatiotemporal
sensations are not robust and highly differentiated, like normal
tactile sensation.
What is needed is a device that spans the full intensive, temporal
and spatial range of the tactile system. Investigations with such a
device would provide a basis for determining the specifications of
working devices and display methods.
It seems unlikely that skin areas other than the hand and fingers
will be suitable for acquiring information of the sort that demands
high resolution. However, these skin areas might provide an effective
portal for low-resolution information.
EXI STING ENVIRONMENTAL SENSORS
A variety of sensory aids for visually impaired people have been
developed as technology has advanced, which are discussed in Chapter
6. Here we concentrate on aids that have been termed "environmental
sensors." An environmental sensor should be more than an obstacle
detector: it should convey the full dynamic character of visual
information. We concentrate on this class of aids because, to the
extent that they are designed to incorporate imminent advances in
electronics and optics, they are more likely to take advantage of the
capacities of nonvisual systems.
Sonar Substitution
The first kind of environmental sensor to be considered is the
sonar sensory aid. In aids of this type, a small transmitter irradiates
the field of view with acoustic waves of very high frequency that
encounter objects in space. The reflected waves that are returned to
the observer (echoes) are detected and made audible by suitable trans-
ducers. This display contains information about the directions,
distances, and surface textures of objects in the field of view. The
latest advance in this technology has been developed by Leslie Kay of
New Zealand (Kay, 1982; Easton and Jackson, 1983~. Kay's Trisensor is
a modified version of his earlier Binaural Sensory Aid (BSA). As in
the BSA, the Trisensor is fitted with widely angled receivers that
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sense reflected energy. Energy reflected from an object to either side
of the midline results in a transduced signal with an interaural
difference in amplitude that serves as a cue for the location of the
object. In addition, the Trisensor is fitted with a transmitter that
emits a very narrow beam of ultrasonic energy that is returned by
reflection and results in a signal that, because it does not exhibit an
intermural difference in amplitude, is a monaural signal. The signal
provides the user with more precise information regarding the size and
location of objects directly ahead. In addition to indicating object
direction, both the BSA and the Trisensor code object distance in terms
of the pitch of the acoustic display, while surface texture is signaled
by the timbre of the auditory signal. It should also be noted that
these aids also have adjustable range controls that create "windows" of
optimal localization from about 0.3 m up to 5 m.
Recent research has centered increasingly on the psychophysical
characteristics and the usability by humans of both the BSA and the
Trisensor (Warren and Strelow, 1984a; Easton, 19851. When blind
children or adults working under blindfold use the aids to locate small
cylindrical objects in near space (within arm's reach), the error of
estimation in judging the direction to a detected object is about 5
degrees, as opposed to approximately 1 degree in judging the direction
to the source of a natural sound. However, the error of estimation in
judging the distance to a detected object is about 5 cm, which is
substantially smaller than the error of estimation in judging the
distance to the source of a natural sound. This level of performance
is typically achieved after 3-4 hours of training sessions. When the
space observed by means of the Trisensor is expanded enough to allow
sensed objects to be as far as 5 meters from the observer, the error of
estimation in judging the direction of an object is approximately 6
degrees, and the error of estimation in judging the distance to an
object is approximately 0.3 meters in magnitude. One effect of using
the Trisensor appears to be a reduction over previous devices in the
magnitude of directional errors, and it is reasonable to attribute such
an effect to an increase in angular resolving power brought about by
the incorporation of a center channel. However, so far there has been
no direct psychophysical comparison of the Trisensor and the Binaural
Sensory Aid.
The next logical step in evaluating these aids is a comparison of
their usefulness in forming memorial representations of the spatial
layout of objects. Both the ability of blind pedestrians to move
through object-filled spaces and their ability to update their changing
spatial positions and relationships to objects in space need further
study.
Tactile Substitution
Bach-y-Rita and colleagues (Bach-y-Rita, 1972; Bach-y-Rita et al.,
1969) have developed a Tactile-Vision Sensory Substitution System
(TVSS}. Early research in the field entailed presenting television
camera images directly on the skin, on a point-by-point basis using
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tactile arrays worn on the abdomen. The results obtained with the TVS S
suggested that such systems can have educational value if the shapes of
objects are kept simple and the background against which they are
observed is carefully controlled. When the subjects were moving while
they observed complex, dynamic scenes, they found the TVSS display very
difficult to interpret. In addition, the original TVS S proved to be a
relatively cumbersome, uncomfortable system to wear and use while
moving about.
Recently, Bach-y-Rita and Hughes (1985) have developed a more
reliable and portable TVS S by modifying the Optacon, a device
originally intended for use as a reading aid. The modified Optacon
presents vibrotactile stimulation to the user's fingertip via the
transduction of optical images of distal objects picked up by an array
of photosensitive elements in a camera (an optician scanner fitted with
a suitable focal-length lens) under the direct motor control of the
user (mounted on a headband). The main advantages of the modified
Optacon is that it uses reliable engineered hardware (the Optacon),
which is readily available to most schools and institutes for blind
persons.
The feasibility of this approach rests on the as yet unproven
assumption that the skin is functionally similar to the retina in its
capacity to mediate information. Like the retina, the skin can sense
variation in two spatial dimensions and is capable of temporal
integration. Thus there is generally no need for complex topological
transformation or for temporal coding, although temporal display
factors are being explored with the goal of transmitting spatial
information across the skin more quickly than is possible with present
systems.
Psychophysical experiments are presently being conducted in order
to determine the sensory capacity of the skin with respect to the
variables represented in the Optacon display.
RECOMMENDATIONS
On the basis of our findings, we make the following recommendations
regarding physiological considerations in sensory enhancement and
substitution.
The Somatosensory System
If sensory aids are to take full advantage of the capacities of the
somatosensory system, they must effectively engage the mechanisms
responsible for tactile perception and stereognosis. Natural examples
of tactile perception are Braille reading and texture perception.
The necessary condition for tactile perception is cutaneous
deformation. There is, in principle, no reason why a device cannot
simulate the deformation patterns encountered in normal tactile
experience and recreate any tactile sensation of which the system is
capable. However, no currently available device comes close to
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achieving this objective. Such a device should have a dense array of
probes with a spacing of 0.8 mm or less. Individual probes should have
a dynamic range of a least 2 mm and a frequency range from 0-300 Hz.
Another form of an optic device might consist of a modifiable teas
relief display with protrusions of nonvibratory pins proportional to
the gray level. The image received by the fingertips would consist of
a TV frame, updated at will by the blind subject scanning the image
with the fingertips of one hand. Once such a device is developed, the
question of appropriate methods of stimulation can be pursued.
RECOMMENDATION: To speed up progress in mobility research, high
priority should be placed on the design and development of a device
that can simulate the tactile perception that results from cutaneous
deformation.
Since the devices currently available for stimulating skin are so
much more limited than the sensory system they address, research tends
to illustrate the limitations of the device rather than the sensory
system. To take a specific example, using the Optacon as a research
instrument is analogous to doing auditory research over the telephone.
The device has limited utility as a research instrument.
Stereognosis depends on the patterns of neural impulses that are
generated when, as a consequence of the movement of parts of the body,
the receptors in muscles, tendons, and joints and the receptors in the
skin are excited. It is stereognosis that largely accounts for the
success of the long cane as a mobility aid. The cane works because it
is effectively coupled to the stereognostic system. A hypothetical
example of the kind of device that we have in mind is an electronic
cane operating on some reflectance principle Beg., sonar or radar)
with an adjustable range. The electronic cane, which might be held in
the hand like a flashlight, could offer its user a menu of functions.
In one mode, it might simulate a rigid cane of fixed length and present
to the hand of its user the same pattern of stimulation that would be
presented by a real cane, like the jolting sensation that occurs when a
cane comes in contact with an object. In another mode, it might
function as a directional range finder and be used to detect obstacles
or openings such as doorways.
RECOMMENDATION: High priority should be given to the development
of mobility aids that engage stereognosis.
The Auditory System
As in the case of the TVSS, the performance enabled by existing
devices that substitute auditory stimulation for visual stimulation
should be assessed more thoroughly and carefully than it has been to
date. Closer interaction between engineers and psychologists with
specialization in human factors engineering would facilitate this
endeavor. A more thorough assessment of existing sensory aids is
needed, but there are more fundamental issues that must be addressed.
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RECOMMENDATION: We recommend that basic research be fostered
concerning the cues on which the auditory perception of distance
depends as well as research to develop transduction schemes that yield
acoustic displays whose cues to the perception of space are analogous
to the cues in natural acoustic displays.
Mobility and Low Vision
We need research that will make possible a better definition of the
relative importance of the several cues available for the perception of
depth, and how their use is limited by different forms of visual
impairment. The effect of visual impairment on spatial learning also
merits study. When the natural optical display from which visual
observers acquire spatial information is transduced to create an
acoustical display, extensive recoding takes place. Only one cue, the
directional cue provided by intermural differences in amplitude, is the
same in the transduced display and the natural acoustic display from
which auditory observers acquire information about space. The cue to
distance provided by differences in pitch, and the cue to surface
texture provided by differences in timbre, are arbitrary. what is
important to recognize is that a transduced display more closely
analogous to the natural acoustic display should be interpretable with
significantly greater speed and accuracy than the transduced displays
of existing devices. Of course, the auditory perception of space, in
many respects, is not well understood. For example, the cues for
auditory ranging are apparently provided by complex variation in the
amplitudes of several simultaneously occurring signals. The cues
provided by variations in the amplitude of a single signal would be
ambiguous. However, basic research concerning the cues on which
auditory ranging depends will be needed before such information can be
taken into account in the development of a transduced acoustical
display that is analogous to the natural acoustical display.
RECOMMENDATION: A careful experimental analysis is required to
identify the components of the mobility task. Once this is done,
further research should be carried out to clarify the demands on vision
made by each of these components.
Performing tasks that require changing from one level of resolution
to another, such as the task of first finding a street sign and then
reading it, poses a serious problem for pedestrians who depend on
mobility aids for the information they need to travel, and it may be
that current or imminent technology can offer a solution to this
problem. There are some possibilities for visual enhancement that
depend on techniques and instruments currently available or that could
be made available with little additional development. For instance,
tunnel vision might be enhanced by reversed telescopes or Corning CPF
filters, and digital image enhancement could be used to create displays
that compensate for various visual deficiencies Finally, it is
generally agreed that visual capacity is adequately defined by the
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measured values of three variables: spatial resolution, contrast
sensitivity, and extent of the visual field. The visual capacity
enabled by a mobility aid could be defined in the same way.
RECOMMENDATION: Research should be conducted to determine the
feasibility of defining the sensory capacity enabled by the use of a
sensory substitution system, such as a VTSS, in terms of the three
variables that define the capacity of the visual system. For example,
at the appropriate point in the development of a new VTSS designed to
display a tactile analogue of a visual image, its field of view,
spatial resolution, and contrast sensitivity could be measured. The
measured values thus obtained should give a fairly accurate indication
of expected performance. This approach would be useful, not only to
evaluate individual substitution systems, but also to compare different
substitution systems.
Detrimental Effects of Mobility Aids
The designer of a sensory substitution system should be mindful of
the natural ability of the sensory system to be addressed and the
interference that may be caused by substitute signals. For instance,
the auditory system has useful ability to acquire spatial information
from the natural acoustic display, and the benefits associated with the
use of a sensory substitution system must be weighed against the costs
incurred by compromising natural auditory ability.
A sensory aid with the intended function of enhancement may also
interfere with reception of the natural acoustical display. For
instance, in an effort to improve the signal-to-noise ratio in the
region of the audible spectrum in which speech signals occur, a hearing
aid may be designed to filter out both the low frequencies, on which
the detection of resonances depends, and the high frequencies, on which
the detection of sound shadows and distance cues depends. Automatic
volume level controls, commonly used in modern hearing aids, eliminate
distance cues provided by differences in loudness. A hearing aid with
these features might seriously interfere with the ability of a
pedestrian who is both visually and hearing impaired to acquire spatial
information.
RECOMMENDATION: An effort should be made to develop a hearing aid
that is effective with regard to the reception of speech, but not at
the expense of effectiveness with spatial information.
Animal Models
Sensory substitution experiments with animals, such as primates,
may be the only practical way to acquire an understanding of the
potential benefits and limitations of the devices currently available
or in the planning stage. Sensory substitution experiments using
animals provide the only means available to use for evaluating the
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possibility of negative effects on neurophysiological development as a
consequence of early and long-term use of sensory aids, and such
experiments should be conducted before the effects of their long-term
use by blind human infants are assessed.
Observations of behavioral changes that correlate with changes in
functional activity patterns of the central nervous system (Bach-y-Rita,
1972) should prove useful in identifying characteristics of device i net
that prove to be detrimental to the individual user, so that such
characteristics can be altered or eliminated. Beneficial character-
istics could also be identified and enhanced. Sensory substitution
devices may have to be tailored to meet the needs of the individual,
much as other prostheses are, and just as prescribed medicines are.
At the present time, we can only speculate about the physiological
. . .
basis of effective sensory substitution, because the data available
will not allow us to do otherwise.
However, we can make and test some
rudimentary hypotheses, most of which involve Plastic OrODerties of the
central nervous system.
, ~ ~ _ _,= ~
In essence, when we ask questions about the
basis of sensory substitution, we are also asking questions about the
nature of processes that depend on the plasticity and compensation
properties of the central nervous system. The possibility of being
able to control modifications or organizational processes in order to
enhance compensation in the advent of a sensory deficit presents a
significant challenge to science and technology.
RECOMMENDATION: Research should be encouraged on the use of animal
-
models to study the effects of device use on sensory development and
functioning and to assess the relative contributions of the various
kinds of information provided by sensory aids to the development of
spatial knowledge and spatial ability.
Representative terms from entire chapter:
sensory enhancement
