Springer-Verlag 1998
Exp Brain Res (1998) 118:541±550
RESEARCH ARTICLE
Randolph D. Easton ´ Anthony J. Greene
Paul DiZio ´ James R. Lackner
Auditory cues for orientation and postural control
in sighted and congenitally blind people
Received: 11 November 1996 / Accepted: 29 July 1997
Abstract This study assessed whether stationary auditory
information could affect body and head sway (as does visual and haptic information) in sighted and congenitally
blind people. Two speakers, one placed adjacent to each
ear, significantly stabilized center-of-foot-pressure sway
in a tandem Romberg stance, while neither a single speaker in front of subjects nor a head-mounted sonar device
reduced center-of-pressure sway. Center-of-pressure sway
was reduced to the same level in the two-speaker condition for sighted and blind subjects. Both groups also evidenced reduced head sway in the two-speaker condition,
although blind subjects head sway was significantly larger than that of sighted subjects. The advantage of the twospeaker condition was probably attributable to the nature
of distance compared with directional auditory information. The results rule out a deficit model of spatial hearing
in blind people and are consistent with one version of a
compensation model. Analysis of maximum cross-correlations between center-of-pressure and head sway, and associated time lags suggest that blind and sighted people
may use different sensorimotor strategies to achieve stability.
Key words Balance ´ Auditory information ´ Vision ´
Blindness ´ Human
Introduction
It is well known that vision exerts a strong influence on
the postural stability of humans. During quiet, unperturbed stance, for example, normal subjects can maintain
balance when vision is denied, but body sway typically
increases by a factor of 2 (Dichgans and Brandt 1978).
)
R.D. Easton ( ) ´ A.J. Greene
Boston College, Department of Psychology,
Chestnut Hill, MA 02167, USA
Fax: +1-617 552-0523, e-mail: Randolph.Easton@bc.edu
P. DiZio ´ J.R. Lackner
Brandeis University, Waltham, MA 02154, USA
The effect is generally attributed to visions high resolution of spatial position, including body position, relative
to vestibular or proprioceptive inputs. Recently, it has also
been demonstrated that haptic cues at the fingertip can enhance postural stability in the absence of vision, even
though the forces between a stable surface and the finger
are far below those constituting mechanical support
(Holden et al. 1994; Jeka and Lackner 1994, 1995). The
haptic sensory cues, arising from cutaneous, kinesthetic,
and proprioceptive inputs, presumably also provide high
spatial resolution in terms of specifying body position
and change of body position. It has also been demonstrated that haptic cues facilitate postural stability in labyrinthine-defective individuals (DiZio et al. 1997) as well as
congenitally blind people (Jeka et al. 1996a), indicating
that the haptic information can substitute for the absence
of vestibular information or a visual spatial reference system, respectively.
Together these findings regarding the effects and interactions of different sensory cues on postural stability suggest an additional question: Can the auditory system, like
vision and haptics, provide sufficiently precise spatial information regarding ongoing body position to effect postural stability? There is reason to believe that auditory information is only minimally suited to influence postural
stability. Of relevance here are findings from the sensory
dominance or bias paradigm, where, in an attempt to discern a sensory control hierarchy, a discrepancy is created
between spatial information processed in two modalities
(see Welch and Warren 1980 for a review). The response
of the system typically reveals a biasing effect, that is, one
modality dominates the other in terms of perceptual experience. This is interesting, for the case of perceived object
position vision is found to strongly dominate audition and
proprioception, while audition and proprioception exert
only small biasing effects on vision. When a discrepancy
is created between audition and proprioception (with the
use of a pseudophone), audition is substantially dominated by proprioception and exerts little influence on proprioception. In the context specifically of perceived
self-position, it is additionally known that a rotating
542
sound field (external or internal) can induce an illusory
experience of self-rotation, just as a rotating visual field
does, but the effect is eliminated if a stable visual environment is present (Lackner 1977). As perceived self-position is complementarily related to perceived object position (Lee 1978, 1990), the auditory system may exert only
a limited influence on perceived body position and resultant postural stability.
The suitability of auditory information for control of
postural stability may be particularly important in the context of its effects in congenitally blind people, who typically tend to rely on auditory information to garner spatial
knowledge that would otherwise be gathered by vision.
Good examples in the current context of spatial orientation
are blind peoples use of traffic sounds for parallel or perpendicular alignment in street crossing (Hill and Ponder
1976) or the approach of a motor vehicle (Schiff and Oldak
1990). Two theoretical perspectives on the role of congenital blindness in the development of hearing can be identified: deficit and compensation models (Ashmead et al.
1996). The deficit model argues that nonvisual information
is customarily encoded within a visual spatial frame of reference, and individuals lacking such a reference system
will evidence impaired spatial hearing (Pick 1974; Warren
1984, 1970; Warren and Pick 1970). The compensation
model argues that, in the absence of typical visual inputs,
nonvisual areas of perception may become more highly developed than in sighted people (for a review, see Rauschecker 1995). In their strongest versions, these models
lead to opposite predictions about the differences in spatial
hearing in sighted compared with blind people and, by extension, the potential effects of auditory information on the
control and regulation of postural stability. That is, given
the lack of a visual-spatial frame of reference, the deficit
model would predict little or no effect of auditory information on the postural control of blind people, while the compensation model would predict that, especially in congenitally blind people, auditory information could prove capable of influencing postural stability.
In the present study the postural stability of sighted and
congenitally blind individuals was assessed using a tandem Romberg stance (heel-to-toe). Auditory information
was provided through either a single directional speaker
positioned directly in front of the subjects head, at the intersection of the median and transverse planes, or through
two speakers, one placed adjacent to each of the subjects
ears (see Fig. 1). Given that lateral plane (right-to-left)
body sway is enhanced in the tandem Romberg stance,
the single-speaker condition results in the traditional information regarding object-to-self direction ± interaural
intensity, time of arrival, and phase differences. In contrast, the two-speaker condition results in amplitude
changes at the two ears regarding object-to-self distance.
Moreover, intensity change at the two ears obeys the inverse square law, that is, intensity varies reciprocally with
the square of the distance. Thus the information available
is particularly pronounced at short distances (Ashmead et
al. 1995, p. 241), especially those used in the present
study. In addition to these auditory conditions, the effect
Fig. 1 Three auditory information conditions: (a) a single speaker
placed in front of the subject in the median plane, (b) two speakers,
one placed adjacent to each ear, or (c) a head-mounted sonar device.
All conditions were presented in a normal ambient environment with
sound-reflecting surfaces. A 2.54-cm-diameter (1-inch) vertical pole
was placed 30 cm in front of the subject in the sonar condition to
provide self and object cues
of a head-mounted sonar device known as the Trisensor
(L. Kay, 1985 and unpublished work) was assessed. The
Trisensor was designed for use as a mobility and orientation aid for blind people. It is purported to be an ªenvironmental sensorº (Brabyn 1985) in the sense that returning
echoes from a transmitted signal inform the user of the direction, distance, and texture of objects within the range
of the device, as well as providing acoustic flow during
locomotion. In addition, it has been demonstrated that
the Trisensor can improve the ability of blind people
(who received over 12 h of distributed practice with the
device in spatial localization) to perform a challenging
balance task on a moving (rocking side-to-side) stabilometer platform placed before three cylindrical, 2.54-cmdiameter poles (Easton 1992). In contrast, stationary directional speakers emitting highly localizable sounds
mounted on the poles did not increase stability on the
stabilometer platform, a result attributed to the superior
precision of the head-mounted sonar in specifying head
and body position relative to the environment during
543
Table 1 Characteristics of
blind subjects
a
Retinopathy of prematurity is
a vascular abnormality of the
retina characterized by neovasularization and resultant
sequelae, occurring almost exclusively in premature infants
b
Leber©s congenital amaurosis is
an inherited disorder characterized by retinal pigmentary degeneration
c
Usher©s syndrome is an inherited disorder, primarily affecting
males, which is characterized by
degeneration of the retinal pigment epithelium, cataracts, and
hearing loss
Participant
Age
(years)
Sex
Travel aid
Cause of blindness
B.M.
44
M
Long cane and
dog guide
Retinopathy of
prematuritya
M.S.
52
F
Long cane
Retinopathy of
prematurity
T.W.
28
M
Long cane
Retinopathy of
prematurity
P.M
44
F
Dog guide
Retinopathy of
prematurity
G.C.
36
F
Long cane and
dog guide
Retinopathy of
prematurity
J.B.
24
F
Dog guide
Agricultural
pesticide during first
of 3 months© gestation
M.P.
19
M
Long cane
Leber©s congenital
amaurosisb
M.B.
24
M
Dog guide
Probable Usher©s
syndromec
large, side-to-side head or body rocking movements (and
compensations) which typically occur for the stabilometer
task.1
For the present experiment, subjects were tested in the
dark with and without auditory information in a tandem
Romberg (heel-to-toe) stance, while center-of-foot-pressure as well as head displacement were monitored. In
our previous work on postural stability of blind people
in this situation, it was found that haptic cues reduced
center-of-pressure sway to the same level and extent for
both blind and sighted people, but blind people evidenced
significantly more head sway than their sighted counterparts (Jeka et al. 1996). One interpretation of the finding
is that head movement control in sighted humans and animals may be driven primarily by movements directed toward orienting and stabilizing the eyes in space (Goldberg
and Peterson 1986; Outerbridge and Melvill Jones 1971),
which may not develop normally in the congenitally
blind. Thus, the larger levels of head displacement in
blind subjects may not be subserving spatial orientation
and postural control, but may result from the inability to
coordinate head movements in terms of eye-head synergies and gaze control. An additional question addressed
therefore in the present research was whether auditory information, which is processed by receptors located on the
head (indeed, in the case of the sonar, receptors as well as
a transmitter are located on the head), would attenuate
head displacement in blind people, substituting as a kind
of gaze stabilization mechanism.
1
The choice of these forms of auditory information was not motivated theoretically. A 500-Hz square wave was chosen over a sine
wave because of the presence of transients and resultant localizability. The sonar was chosen because of its stabilizing effect on the
stabilometer task (Easton 1992). However, other auditory information could prove particularly effective at stabilizing posture, especially highly localizable, broad-band acoustic wave forms.
Additional information
Mild hearing loss.
Active participation in
outdoor activities such
as skiing and climbing
Materials and methods
Subjects
Eighteen individuals participated in the experiment, ten sighted and
eight congenitally blind, ranging in age from 25 to 50 years (see Table 1). They were healthy and physically active, with no known
musculoskeletal or neurological disorders that might have affected
their ability to maintain balance. Human subject use was approved
by the appropriate ethics committee and all participants gave informed consent prior to participating.
Apparatus and measurement
Figure 2 depicts our test situation. The participants stood in the tandem Romberg position (heel-to-toe) on a force platform, with their
arms at their sides. The force platform measured the reaction forces
generated by the feet, indicating displacement of the center of pressure. Medial-lateral (CPx) and anterior-posterior (CPy) coordinates
of foot pressure were computed from the medial-lateral (Fx), anterior-posterior (Fy), and vertical (Fz) force components registered by
piezo-electric crystals in the corners of the force platform (Kistler
model 9261A) and the distances of the crystals from the center of
the platform. Medial-lateral (Hx) and anterior-posterior (Hy) head
movements were measured with an ISCAN video system. An
LED was attached to a headband and the ISCAN camera tracked
the movements of the LED to measure Hx and Hy directions of head
sway. The ISCAN system measures two-dimensional (2-D) movement in a field of view 512 pixels (Hx)”256 pixels (Hy). Because
of differing subject heights, we normalized the field of view across
participants by measuring the distance between the camera and LED
and computing a calibration factor for each participant. The mean
resolution across subjects was 0.48 mm (Hx) and 0.96 mm (Hy).
All signals were sampled at 60 Hz and collected in real time on a
personal computer instrumented with a Data Translation analog-digital board.
Figure 1 depicts the configuration of the different auditory information conditions. Auditory information was presented in a normal
laboratory room in the following manner: For a single-speaker condition, a speaker was mounted 30 cm in front of participants and level with their ear canals. For a two-speaker condition, speakers were
544
Fig. 2 Participant in the tandem Romberg position on the force platform. Primary sway is in the lateral, side-to-side direction
mounted approximately 5 cm to the left and right of the participants
pinnas, level with their ear canals. The speakers were driven by a
500-Hz square wave and delivered at 73 dB.
In addition to these auditory conditions, participants wore a
head-mounted sonar, the Trisensor (Kay 1985 and unpublished
work), which is an air-borne sonar patterned after its predecessor,
the Binaural Sensory Aid or BSA (Kay 1974). The basic principle
on which this aid operates is that a small, head-mounted transmitter
irradiates the field of view (essentially a 60 cone) with very high
frequency acoustic waves (50±100 kHz), which encounter objects
and surfaces in the environment. Reflected waves, which return to
the observer as echoes, are detected by head-mounted transducers
and these are transformed into an audible acoustic display (essentially, returning echoes are multiplied with the transmitted signal) that
potentially informs the user about objects distances, directions, and
surface textures. Distance of objects is coded by the pitch of the audible input. The display also receives binaural inputs from angled
receivers that yield interaural differences in amplitude level, creating a sense of spatial direction and movement. A unique feature
of the Trisensor sonar is that it adds monaural signals derived from
a narrow beam (10) in front of the observer. This latter feature provides the user with higher acuity information regarding objects directly ahead. For the present situation, a 2.54-cm-diameter,
182.88-cm-tall pole was placed 30 cm from the participant in the
median plane, matching the placement of the one-speaker condition.
Given lateral sway in the Romberg-tandem stance, the sonar primarily created an interaural amplitude cue to object or self direction.
Procedure
Participants stood with their right foot placed directly behind their
left foot along the center of the anterior-posterior axis of the force
platform. Adhesive tape was used to mark the positions of the feet
on the platform, so that the same configuration could be precisely
repeated during each trial. The experimental trials included five conditions: eyes open (sighted participants only), eyes closed and no
sound, one speaker, two speakers, and sonar feedback sound.
Participants began each trial by assuming the tandem stance and
positioning their arms at their sides with their heads level, facing
forward, with their eyes open or closed, depending on the condition.
The visual environment provided a rich, complex scene with many
vertical and horizontal cues. Participants were told to take as much
time as desired to assume a comfortable stance. Their instructions
were to maintain the tandem stance as comfortably as possible while
Fig. 3 Raw data time-series wave forms of center-of-pressure sway
in the lateral direction (CPx) and head sway in the lateral direction
(Hx) for a blind and a sighted subject for the no-sound control and
the two-speaker auditory input conditions. The two-speaker condition significantly attenuated both center-of-pressure and head sway
in sighted and blind subjects
swaying as little as possible. Once they felt ready, subjects said ªgoº
and the experimenter initiated data acquisition. Practice trials were
given for each condition before the experiment began.
The experimental trials were run in four blocks of 5 (sighted) or
4 (blind) trials (one of each condition per block) for a total of 20 or
16 trials. Conditions were randomized within a block. Trial duration
was 24 s. If a participant was unsuccessful in a particular trial (e.g.,
lost balance), then the trial was repeated until performed correctly.
After each trial, the participant stepped off the platform and sat comfortably for at least 1 min. The experiment lasted approximately 1 h.
Analysis
Raw data consisted of time-series wave forms of CPx, CPy, Hx, and
Hy. Figure 3 presents examples from a typical sighted and blind subject for the no-sound control condition and the most stabilizing auditory condition ± two speakers. The four time series were reduced
to four summary values as follows. First, 4 s were cut from the beginning and end of each series to minimize possible contamination
from anticipation of the start and finish of a trial. The absolute deviation of each data point from the mean of the entire 16-s raw signal was found, and these were averaged to yield an index of mean
sway amplitude (MSA). Cross-correlations between CP and H sway
were calculated to determine the temporal relation of body displacement and head sway. Jeka et al. (1996) found greater head sway in
blind than sighted subjects, suggesting the possibility that the two
groups use different coordinative strategies to achieve body sway
stability. Correlations were performed at each of 100 steps
(15.624 ms/step) in both the forward and backward directions to determine whether correlations were strongest at times other than t=0
(i.e., in phase). Finally, power spectral density analyses were performed to determine the component frequencies of CPx and Hx displacement, with a frequency resolution of 0.0586 Hz. Mean power
spectra were calculated by collapsing across subjects and trials for
each condition.
To determine the effects of the different auditory input conditions as well as to directly compare sighted and blind participants,
we conducted a 2”4 mixed ANOVA (subject groups, between; sen-
545
sory input condition, within). We used the omnibus MSe for withinsubjects planned comparisons and the pairwise MSe for betweensubjects planned comparisons. Correlations between CP and H sway
were transformed to the Fischers exact z-test for statistical analysis,
because raw correlations are not normally distributed (Senders
1958).
Results
CPx mean sway amplitude
Mean lateral center-of-foot-pressure sway is presented in
Fig. 4 (top) for sighted and blind subjects under the different sensory input conditions. (As a point of reference,
mean CPx displacement for sighted subjects with eyes
open was 0.25 cm (SE 0.02). An ANOVA revealed a significant effect of sensory input (F3,48=4.29, MSe 0.004,
P<0.01) and no effect associated with the groups factor
or the interaction between groups and sensory input.
Planned contrasts between the auditory input conditions
and the control condition, collapsed across groups, revealed that only the two-speaker condition resulted in significantly reduced lateral body sway (F1,48=8.77,
P<0.01). As an index of individual differences or the consistency of the two-speaker effect, it was found that eight
of ten sighted and seven of eight blind subjects showed
reduced CPx sway in the two-speaker condition compared
with the no-sound condition.
Hx mean sway amplitude
Mean lateral head sway is presented in Fig. 4 (bottom) for
subject groups under sensory input conditions. (Mean Hx
sway for sighted subjects with eyes open was 0.33 cm (SE
0.03). An ANOVA revealed a significant effect for the
group factor (F1,16=8.9, MSe 0.25, P<0.01), with the
sighted evidencing smaller head sway than their blind
counterparts. Planned contrasts between the auditory input conditions and the control condition, collapsed across
groups, indicated that both the two-speaker and the sonar
conditions resulted in significantly reduced, mean lateral
head sway (F1,48=3.1, P<0.08 and F1,48=2.9, P<0.09, respectively). As an index of individual differences and the
consistency of the two-speaker effect, it was found that
seven of ten sighted subjects and six of eight blind subjects showed a reduction in Hx sway in the two-speaker
condition compared with the no-sound condition. For
the sonar condition, five of ten sighted subjects and seven
of eight blind subjects showed reduced Hx sway compared
with the no-sound condition.
CPy mean sway amplitude
Mean anterior-posterior center-of-foot-pressure displacement is presented in Fig. 5 (top) for the subject groups
across sensory input conditions. (Mean CPy for sighted
subjects with eyes open was 0.25 cm (SE 0.03). An AN-
Fig. 4 Top Means and standard errors for center-of-pressure sway in
the lateral direction (CPx). Bottom Means and standard errors for
head sway in the lateral direction (Hx)
OVA revealed a significant effect for sensory input conditions (F3,48=3,58, MSe 0.004, P<0.02) and no effect associated with the groups factor or the interaction between
groups and sensory input. Planned contrasts between the
auditory input conditions and the control condition, collapsed across groups, revealed that only the two-speaker
condition resulted in significantly reduced anterior-posterior body sway (F1,48=9.14, P<0.01).
Hy mean sway amplitude
Mean anterior-posterior head sway is presented in Fig. 5
(bottom) for subject groups across sensory input conditions. (Mean Hy for sighted subjects with eyes open was
0.39 cm (SE 0.03). An ANOVA revealed no significant
main or interaction effects. Planned contrasts between
the auditory input conditions and the control condition,
collapsed across groups, nevertheless revealed that both
546
Fig. 5 Top Means and standard errors for center-of-pressure sway in
the anterior-posterior direction (CPy). Bottom Means and standard
errors for head sway in the anterior-posterior direction (Hy)
Fig. 6 Top Maximum correlations and standard errors for CPx and
Hx. Bottom Associated time lags and standard errors for CPx and
Hx. Negative time lags mean that CPx leads Hx temporally
the two-speaker and sonar conditions resulted in reduced
anterior-posterior head sway (F1,48=5.3, P<0.025 and
F1,48=3.5, P<0.06, respectively).
indicate that center-of-foot pressure was leading head
sway. An ANOVA comparing blind and sighted subjects
across sensory input conditions revealed no main or interactive effects. The high CPx and Hx correlations and
negative time lags mean that head movements were
strongly related but temporally behind center-of-pressure
movements.
Cross-correlations between CPx and Hx
Maximum cross-correlations between CPx and Hx, as
well as the time lags associated with the correlations
are presented for both groups of subjects in Fig. 6. As
can be seen, the maximum correlations for both groups
of subjects were high across all conditions, averaging
about 0.80. A mixed two-way ANOVA comparing subject groups across sensory input conditions revealed no
main or interactive effects. The time lags corresponding
to the correlations (Fig. 6, bottom) were uniformly negative for both groups of subjects across all conditions,
ranging from about 50 to 100 ms. The negative time lags
Cross-correlations between CPy and Hy
Maximum cross-correlations between CPy and Hy, as well
as the time lags associated with the correlations, are presented for both groups of subjects in Fig. 7. Again the correlations for both groups were high across all conditions,
averaging in this case about 0.65. An ANOVA comparing
subject groups across sensory input conditions revealed
no main or interactive effects.
547
For the most part, maximum mean power occurred at
about 0.15 Hz for both CPx and Hx across the sensory input conditions. The most striking difference in the power
spectra for sighted compared with blind people appears in
Hx displacement, where blind people evidenced substantially more power than sighted subjects across the frequency spectrum for all four sensory input conditions.
This finding, of course, is related to the greater overall
Hx sway for blind subjects. Separate 2”10 ANOVAs
(group”frequency bin) were conducted on Hx for each
sensory input condition, and in each case the effect of
group as well as the interaction was significant. As can
be seen, the interaction was attributable to proportionately
greater Hx power by blind people at the lower frequency
bins.
Overall, for all four dependent measures, CPx, CPy, Hx,
and Hy, the anterior-posterior components generally mirrored those of the lateral components. Given that lateral
sway is the primary component of body sway in the tandem Romberg stance, the discussion of the results will be
focused on the lateral center-of-pressure and head sway.
Discussion
Fig. 7 Top Maximum correlations and standard errors for CPy and
Hy. Bottom Associated time lags and standard errors for CPy and
Hy. Negative time lags mean CPy leads Hy temporally
The time lags corresponding to these correlations(Fig. 7, bottom) were uniformly negative for both
groups of subjects across all sensory input conditions,
ranging from 25 to 125 ms. An ANOVA comparing blind
and sighted subjects across sensory input conditions revealed a significant interaction between groups and input
conditions (F3,48=3.02, P<0.04). The interaction was attributable to significantly greater time lags for blind subjects for the no-sound and sonar conditions. In general,
however, like CPx and Hx relations, the high CPy and
Hy correlations and negative time lags indicate that head
movement is strongly related but temporally behind center-of-pressure movements.
Mean power spectra for CPx and Hx
The mean power spectra in each frequency bin are presented in Figs. 8 and 9 for CPx and Hx, respectively.
The primary goal of this research has been to determine
whether auditory information regarding a stable environment can, like visual and haptic information, enhance
postural stability. The effect of auditory information on
postural stability was assessed in sighted as well as congenitally blind people, who lack a visual reference system
to encode spatial information. Using a tandem Romberg
(heel-to-toe) stance, it was found that body sway was significantly attenuated when two stationary speakers, one
placed adjacent to each ear, were present for both sighted
and blind people. Neither a single speaker placed in the
median plane directly in front of the subject nor a headmounted binaural sonar device attenuated body sway for
either group of subjects. Blind people evidenced greater
overall head movement than sighted people; but both
groups of subjects exhibited significantly less head sway
in the presence of either the two-speaker or sonar auditory
information.
In discussing these results, it is useful to compare the
magnitude of the effects to the magnitude of visual and
haptic influences on postural stability. As can be seen in
Fig. 4, center-of-pressure or body sway was reduced by
about 10% for the two-speaker condition relative to the
control condition. Recall from the results that sway in
sighted subjects with eyes open averaged 0.25 cm. Thus
vision attenuates sway about 60%, far greater than auditory information. In our prior work assessing the effects of
haptic cues on postural stability of blind and sighted subjects (Jeka et al. 1996), we found that the most stabilizing
condition consisted of subjects holding a cane in a slanted
orientation (relative to the body) against the floor while
applying a very small force (<0.4 N of applied force), a
condition which does not afford mechanical support, indicating the attenuation of body sway must be sensory in or-
548
Fig. 8 Means and standard errors for CPx power spectra
across sensory cue conditions
Fig. 9 Means and standard errors for Hx power spectra across
sensory cue conditions
549
igin. The attenuation of sway under these conditions was
found to be about 40% relative to a sighted control condition. Thus, while auditory information from a stable environment can significantly improve stability, its effects are
small compared with those of vision and haptics.2 This
pattern of sensory influences parallels the pattern found
using the sensory dominance or bias paradigm, for which
a modality precision hypothesis has been proposed to account for a sensory control hierarchy for the perception of
spatial position (e.g., Pick et al. 1969). Likewise, the customary precision of processing spatial information in a
modality may determine the influence different sensory
information will have on perceived self-position and stabilization of posture.
The relatively small influence of auditory information
on postural stability is also attested to by the fact that neither the one-speaker nor sonar condition attenuated sway.
The greater influence of the two-speaker condition relative to the one-speaker condition is probably explained
in terms of the sound intensity changes at the ears which
occur with lateral sway and signal body position through
distance change. According to the inverse square law,
sound intensity changes would be particularly pronounced
for the 5-cm speaker distance used in this condition (Ashmead et al. 1995, p. 241). Given that the mean head sway
magnitude under the auditory conditions was 1±1.5 cm
(see Figs. 1, 4), quite substantial sound intensity changes
would occur for even these small head movements. In
contrast, the 1- to 1.5-cm head sway did not serve to generate sufficient interaural difference information about
object to self direction (i.e., intensity, time-of-arrival,
and phase) for a single speaker located 30 cm in front
of the subject.3
The lack of an effect for the sonar device relative to the
control condition appears surprising, as the Trisensor sonar has been found to improve stability on the stabiliometer platform task by approximately 30% (Easton 1992).
However, two differences between the present study and
the earlier assessment of sonar input on postural stability
should be noted. First, blind subjects in the stabilometer
study received extensive training in spatial localization
with the sonar (12 h of distributed practice). Second, information regarding spatial position is quite pronounced
with the sonar in the stabilometer task, as large side-toside head and body movements (about the knees) occur
2
In a related finding it has recently come to our attention that ambient auditory clicks fail to reduce the variance in sway when
gastrocnemius muscles are vibrated (Petersen et al. 1995, 1996)
3
It is important to note another factor that could contribute to the
stabilizing effect of the two-speaker compared with the one-speaker
condition. The one-speaker condition yields inherently ambiguous
auditory information regarding lateral head translation compared
with rotation (about its vertical axis). That is, for the one-speakercondition, either a head translation or a rotation would yield similar
interaural differences, whereas for the two-speaker condition the two
types of head movement are uniquely specified, with translation resulting in an intensity increase in one ear and a decrease in the other
ear, and rotation resulting in an intensity decrease in each ear. As
head translation is the crucial variable to stabilize in the tandem
Romberg stance, the two-speaker condition would unambiguously
specify its occurrence.
which yield large interaural amplitude differences regarding object to self direction relative to stable objects in
front of the subject (i.e., three, 2.54-cm-diameter poles
separated by30.48 cm). By contrast, for the Romberg
stance, balance is irretrievably lost after the occurrence
of much smaller head and body movements than occur
on the stabilometer. Thus, either a lack of practice with
sonar information or the relative lack of information
about body position, given that only relatively small head
movements can occur in the Romberg stance, could account for the lack of a sonar influence on postural stability.
Sonar input did significantly affect head stabilization,
however, as did the two-speaker condition, for both
groups of subjects. Although, with the sonar, head sway
was reduced for both blind and sighted people, centerof-pressure or body sway was not reduced. Thus while
the sonar had a small stabilizing effect on head sway it
was not accompanied by reduced center-of-pressure sway.
The analysis of center-of-pressure and head sway
cross-correlations and associated time lags can be used
to suggest coordination strategies used by subjects to
achieve postural stability. As will be recalled (Fig. 6),
the maximum correlations were similar and high for both
groups of subjects, and the associated time lags were negative and relatively small. At the small amplitude and low
frequency of sway observed in the present test situation, it
is reasonable to assume that all body segments were essentially moving together as a single unit in blind and
sighted subjects (i.e., an inverted pendulum), even though
the head movements of the blind subjects were larger than
those of sighted subjects. Indeed, for the blind subjects,
Hx sway was about twice as large as their CPx sway,
which would be predicted by an inverted pendulum model. It is as though blind participants, perhaps because of
their inherently unstable head posture (see Jeka et al.
1996), ªlockº the head to the body to form a single rigid
unit in order to achieve center-of-pressure stability. It is
also worth noting that blind subjects evidenced head sway
that was twice as large as body sway only in the lateral
and not in the anterior-posterior direction, which would
be precisely what was needed to stabilize posture in the
tandem Romberg stance. Sighted subjects performance,
in contrast, may not have so closely resembled an inverted
pendulum, perhaps because of greater head stability fostered by reflexive or learned gaze stabilization mechanisms (Goldberg and Peterson 1986; Outerbridge and
Melvill Jones 1971). This analysis suggests that blind
peoples inherently less stable head position requires them
to adopt a more passive body stabilization strategy than
sighted people.
Overall, however, the general effects of auditory information on postural stability in this experiment are strikingly similar for blind and sighted subjects. This pattern
of results is clearly not consistent with a deficit model
of auditory perception, which would have predicted superior performance by sighted people. The results are also
inconsistent with a strong version of the compensation
model, which would predict superior performance by
550
blind subjects, but are consistent with a weaker version of
the compensation model, which would predict simply that
blind subjects are capable of compensating nonvisually
for a lack of vision (see Rauschecker 1995). The present
findings are consistent with a recent comprehensive assessment of spatial hearing in blind and sighted subjects
that also found no striking differences between the groups
on a wide range of auditory tasks, including minimum audible angle and distance, reaching to sounds, walking to
sounds and perceiving the path of a moving sound source
(Ashmead et al. 1997). It should be noted, however, that,
consistent with a strong version of the compensation model, Schiff and Oldak (1990) have reported that blind people are more accurate at judging time to arrival of an approaching vehicle using auditory information than are
sighted subjects and are comparable with sighted subjects accuracy on the task using visual information. Even
though judging vehicle approach, using auditory information, and the stabilizing effect of the two-speaker condition on postural stability in the present study both involve
using auditory intensity changes regarding object or self
distance, blind people may not customarily use auditory
information for precise postural control as extensively
as for vehicle or object approach. On the other hand, Jeka
et al. (1996) report no differences between sighted and
blind subjects for the effects of a hand-held cane on postural stability, a task with which blind people have substantially more experience.
In sum, the present results clearly indicate that a deficit
model of auditory spatial perception in blind people is untenable. Additional research is needed to assess the strong
version of the compensation model of auditory perception
in blind humans. One possible means of doing so would
be to provide blind people with extensive spatial orientation training over an extended period in an environment
rich in reliable, high-energy acoustic information (Easton
and Bentzen 1997), in order to determine whether they
can develop superior auditory perception.
Acknowledgements The research reported has been supported in
part by NIH grants EY04907 and EY05887 to R. D. Easton and
by AFOSR grant F49620-95-1±0390, and NASA grants NAGW
4374 and NAGW 4375 to J. R. Lackner. We are grateful to Eli Rabin for assistance in data collection and analysis, and Beezy Bentzen
for discussion regarding the perceptual world of blind people and for
assistance in blind subject recruitment.
References
Ashmead DH, Davis DL, Northington A (1995) The contribution of
listeners approaching motion to auditory distance perception. J
Exp Psychol Hum Percept Perform 21:239±256
Ashmead DH, Wall RS, Ebinger KA, Hill M, Yang X, Eaton S
(1997 Manuscript submitted for publication) Spatial hearing in
children with visual disabilities.
Brabyn JH (1985) A review of mobility aids. In: Warren D, Strelow
E (eds) Electronic spatial sensing for the blind. Martinus-Nijhoff, Dordrecht, pp 13±27
Dichgans J, Brandt T (1978) Visual-vestibular interaction: effects on
self-motion perception and posture control. In: Held R,
Leibowitz HW, Teuber HL (eds) Handbook of sensory physiology. Springer, Berlin Heidelberg New York, pp 755±804
DiZio P, Jeka JSJ, Horak FB, Lackner JR, Krebs D (1997) Reduction of postural sway due to precision contact of the finger tip
in patients with bilateral vestibular loss. (Manuscript submitted
for publication)
Easton RD (1992) Inherent problems of attempts to apply sonar and
vibrotactile sensory aid technology to the perceptual needs of
the blind. Optom Vis Sci 69:3±14
Easton RD, Bentzen BL (1997) The effect of spatial localization
training in an acoustically rich environment on spatial updating
in the blind. (Manuscript submitted for publication)
Goldberg J, Peterson BW (1986) Reflex and mechanical contributions to head stabilization in alert cats. J Neurophysiol 56:
857±875
Hill EW, Ponder P (1976) Orientation and mobility techniques: a
guide for the practitioner. American Foundation for the Blind,
New York
Holden M, Ventura J, Lackner JR (1994) Stabilization of posture
by precision contact of the index finger. J Vestib Res 4:285±
301
Jeka JJ, Lackner JR (1994) Fingertip contact influences human postural control. Exp Brain Res 100:495±402
Jeka JJ , Lackner JR (1995) The role of haptic cues from rough and
slippery surfaces in human postural control. Exp Brain Res
103:267±276
Jeka JJ, Easton RD, Bentzen BL, Lackner JR (1996) Haptic cues for
orientation and postural control in sighted and blind individuals.
Percept Psychophys 58:409±423
Kay L (1974) A sonar aid to enhance spatial perception of the blind:
engineering design and evaluation. Radio Electron Eng 44:605±
627
Kay L (1985) Sensory aids to spatial perception for blind persons:
their design and evaluation. In: Warren D, Strelow E (eds) Electronic spatial sensing for the blind. Martinus-Nijhoff, Dordrecht,
pp 125±139
Lackner JR (1977) Induction of illusory self-rotation and nystagmus
by a rotating sound field. Aviat Space Environ Med 48:129±131
Lee DN (1978) The functions of vision. In: Pick HL, Saltzman E
(eds) Modes of perceiving and processing information. Erlbaum,
Hillsdale, NJ, pp 159±170
Lee DN (1990) Getting around with light and sound. In: Warren R,
Wertheim AH (eds) Perception and control of self-motion.
Erlbaum, Hillsdale, NJ, pp 487±505
Outerbridge JS, Melvill Jones G (1971) Reflex control of head
movement in man. Aerosp Med 42:935±940
Petersen H, Magnusson M, Johansson R, ¾kesson M, Fransson P-A
(1995) Acoustic cues and postural control. Scand J Rehab Med
27:99±104
Petersen H, Magnusson M, Johansson R, Fransson P-A (1996) Auditory feedback regulation of perturbed stance in stroke patients.
Scand J Rehab Med 28:217±223
Pick HL (1974) Visual coding of nonvisual spatial information. In:
MacLeod RB, Pick HL (eds) Perception: essays in honor of
James J. Gibson. Cornell University Press, Ithaca, NY, pp
153±165
Pick HL, Warren DH, Hay JC (1969) Sensory conflict in judgments
of spatial direction. Percept Psychophys 6:203±205
Rauschecker JP (1995) Compensatory plasticity and sensory substitution in the cerebral cortex. Trends Neurosci 18:36±43
Schiff W, Oldak R (1990) Accuracy of judging time to arrival: effects of modality, trajectory, and gender. J Exp Psychol Hum
Percept Perform 21:303±316
Senders V (1958) Measurement and statistics. Oxford University
Press, New York
Warren DH (1970) Intermodality interactions in spatial localization.
Cognitive Psychol 1:114±133
Warren DH (1984) Blindness and early childhood development.
American Foundation for the Blind, New York
Warren DH, Pick HL (1970) Intermodality relation in blind and
sighted people. Percept Psychophys 8:430±432
Welch RB, Warren DH (1980) Immediate perceptual response to intersensory discrepancy. Psych Bull 88:638±667