Sound Localization Under Perturbed Binaural Hearing
Marc M. Van Wanrooij1,2and A. John Van Opstal1
1Department of Biophysics, Radboud University Nijmegen, Nijmegen; and2Department of Neuroscience,
Erasmus University Rotterdam Medical Center, Rotterdam, The Netherlands
Submitted 10 March 2006; accepted in final form 20 October 2006
Van Wanrooij MM, Van Opstal AJ. Sound localization under
perturbed binaural hearing. J Neurophysiol 97: 715–726, 2007. First
published October 25, 2006; doi:10.1152/jn.00260.2006. This paper
reports on the acute effects of a monaural plug on directional hearing
in the horizontal (azimuth) and vertical (elevation) planes of human
listeners. Sound localization behavior was tested with rapid head-
orienting responses toward brief high-pass filtered (?3 kHz; HP) and
broadband (0.5–20 kHz; BB) noises, with sound levels between 30
and 60 dB, A-weighted (dBA). To deny listeners any consistent
azimuth-related head-shadow cues, stimuli were randomly inter-
leaved. A plug immediately degraded azimuth performance, as evi-
denced by a sound level–dependent shift (“bias”) of responses con-
tralateral to the plug, and a level-dependent change in the slope of the
stimulus–response relation (“gain”). Although the azimuth bias and
gain were highly correlated, they could not be predicted from the
plug’s acoustic attenuation. Interestingly, listeners performed best for
low-intensity stimuli at their normal-hearing side. These data demon-
strate that listeners rely on monaural spectral cues for sound-source
azimuth localization as soon as the binaural difference cues break
down. Also the elevation response components were affected by the
plug: elevation gain depended on both stimulus azimuth and on sound
level and, as for azimuth, localization was best for low-intensity
stimuli at the hearing side. Our results show that the neural compu-
tation of elevation incorporates a binaural weighting process that
relies on the perceived, rather than the actual, sound-source azimuth.
It is our conjecture that sound localization ensues from a weighting of
all acoustic cues for both azimuth and elevation, in which the weights
may be partially determined, and rapidly updated, by the reliability of
the particular cue.
I N T R O D U C T I O N
Sound localization relies on the neural processing of acous-
tic cues that result from the interaction of sound waves with the
torso, head, and ears. Directional hearing in the horizontal
plane (azimuth) depends on binaural differences in sound
arrival time and ongoing phase for relatively low (?1.5 kHz)
frequencies (so-called interaural time differences [ITDs]). At
higher frequencies (?3 kHz) the head-shadow effect causes
differences in sound level (interaural level differences [ILDs]).
Localization in the vertical plane (elevation) and front–back
discrimination require an analysis of spectral shape cues that
arise from direction-dependent reflections within the pinna
(describedby so-called head-related
[HRTFs]). The latter mechanism essentially constitutes a mon-
aural localization cue for sound frequencies exceeding about
3–4 kHz. However, several studies suggested that the compu-
tation of sound-source elevation also involves binaural inter-
actions, as a monaural perturbation of the spectral cues (e.g., by
inserting a mold in one pinna), and has a systematic detrimen-
tal effect on elevation performance contralateral to the manip-
ulated ear (Hofman and Van Opstal 1998; Humanski and
Butler 1988; Morimoto 2001; Van Wanrooij and Van Opstal
A large body of experimental evidence supports the notion
that the processing of the azimuth and elevation components of
a sound’s location is embedded in independent neural path-
ways. In mammals, the ITDs emerge in the medial superior
olive (MSO), whereas the ILDs are extracted in another nu-
cleus of the superior olivary complex, the lateral superior olive
(LSO; for a recent review see Yin 2002). Evidence also
suggests that the first neural correlates of spectral shape anal-
ysis may be found in the dorsal cochlear nucleus, which
receives monaural input from the ipsilateral ear (Young and
Psychophysical evidence supports the hypothesis of inde-
pendent processing of the acoustic cues. Experimental manip-
ulations can considerably degrade elevation localization per-
formance, whereas azimuth localization is far more robust:
e.g., by inserting molds, either binaurally (Hofman et al. 1998;
Oldfield and Parker 1984) or monaurally (Hofman and Van
Opstal 2003; Morimoto 2001; Van Wanrooij and Van Opstal
2005), by introducing background noise (Good and Gilkey
1996; Zwiers et al. 2001) or by extensively varying sound
levels and sound duration (Hartmann and Rakerd 1993; Hof-
man and Van Opstal 1998; MacPherson and Middlebrooks
2000; Vliegen and Van Opstal 2004).
Yet, the idea of independent pathways for the processing of
a sound’s azimuth and elevation coordinates may be too
simple. Clearly, at more central neural stages, such as the
midbrain inferior colliculus (IC) and beyond, the outputs of the
different cue-processing pathways converge (Chase and Young
2006) and are therefore likely to interact.
Recent psychophysical evidence suggests that the computa-
tions underlying the extraction of azimuth and elevation may
indeed interact. In a study with unilateral deaf listeners we
demonstrated that only listeners who used the spectral-shape
cues from their intact ear to localize azimuth could also
localize elevation (Van Wanrooij and Van Opstal 2004). In
particular, failure of the other listeners to localize elevation
was remarkable because these results indicated that under
chronic monaural conditions the azimuth and elevation com-
ponents are not processed independently.
To study the mechanisms underlying the integration of the
different acoustic cues, this paper reports on the acute effects
of a monaural plug on localization performance of normal-
Address for reprint requests and other correspondence: A. J. Van Opstal,
Department of Biophysics, Radboud University Nijmegen, Geert Grooteplein
21, 6525 EZ Nijmegen, The Netherlands (E-mail: J.vanOpstal@science.ru.nl).
The costs of publication of this article were defrayed in part by the payment
of page charges. The article must therefore be hereby marked “advertisement”
in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
J Neurophysiol 97: 715–726, 2007.
First published October 25, 2006; doi:10.1152/jn.00260.2006.
715 0022-3077/07 $8.00 Copyright © 2007 The American Physiological Societywww.jn.org
hearing listeners in the two-dimensional (2D) frontal hemifield.
Earlier studies assessed the effect of a monaural plug on human
sound localization performance (Flannery and Butler 1981;
Musicant and Butler 1984b; Oldfield and Parker 1986; Slattery
and Middlebrooks 1994). They all reported as a major effect a
horizontal shift (“bias”) of localization to the side of the
unplugged ear. However, it has remained unclear whether the
spectral cues contribute to the localization of azimuth. Al-
though Musicant and Butler (1984a,b, 1985) reported that
localization of far-lateral targets relied on monaural spectral
cues, none of the plugged listeners in the study of Slattery and
Middlebrooks (1994) was able to localize in azimuth. More-
over, Wightman and Kistler (1997) showed that a complete
removal of the binaural cues in a dichotic setup abolished
sound localization performance altogether. That study there-
fore suggested that the spectral cues are not sufficient for sound
localization in the horizontal plane.
In this study we took a different approach, by studying the
effect of a plug on localization responses to a variety of
acoustic stimuli that varied both in bandwidth and over a
considerable range in sound level. The insertion of a plug
perturbs the binaural difference cues in normal listeners in a
frequency- and level-dependent way and is thus expected to
affect the ITDs, ILDs, and HRTFs in different ways. We
measured localization across the 2D frontal hemifield imme-
diately after inserting the plug and quantified the changes in
localization responses as a function of the acoustic parameters.
Our analysis shows that the shift in azimuth responses
depends on sound-source location, the sound spectrum, and on
sound level. Moreover, performance in elevation is influenced
by both sound level and the perceived azimuth location, rather
than by the quality of the spectral cues defined by actual
stimulus azimuth in the plugged condition. Our data therefore
support the hypothesis that the processing of both sound-source
azimuth and elevation involve weighted contributions from
binaural difference cues as well as from spectral-shape cues.
The relative weights of the acoustic cues are adjusted under
acoustic perturbations that render a given cue unreliable.
M E T H O D S
Five listeners (ages 25–47 yr) participated in the experiments
(including both authors, listeners MW and JO). All listeners were
experienced with the type of sound localization studies carried out in
the laboratory and all had normal hearing (within 20 dB of audiomet-
ric zero) as determined by an audiogram obtained with a standard
staircase procedure (10 tone pips, 0.5-octave separation, between 500
Hz and 11.3 kHz). None of these listeners had any auditory or
uncorrected visual disorder, except for listener JO who is amblyopic
in his right eye.
During the experiments, the listener was seated comfortably in a
chair in the center of a completely dark, sound-attenuated room (H ?
W ? L ? 2.45 ? 2.45 ? 3.5 m3). The walls, ceiling, floor, and every
large object present were covered with black acoustic foam that
eliminated echoes for sound frequencies ?500 Hz. The room had an
ambient background noise level of 25 dB, A-weighted (dBA).
The seated listener faced an array of 58 small broad-range loud-
speakers (MSP-30; Monacor International, Bremen, Germany) con-
taining light-emitting diodes (LEDs) in their center. These speakers
were mounted on a thin wooden frame that formed a hemispheric
surface 100 cm in front of the listener, at polar coordinates R ? [0, 15,
30, 45, 60, 75] deg and ? ? [0, 30, . . . , 300, 330] deg. R is the
eccentricity relative to the straight-ahead viewing direction (defined in
polar coordinates as [R, ?] ? [0, 0] deg) and ? is the angular
coordinate, where ? ? 0 deg is rightward from the center location and
? ? 90 deg is upward. The lower three speakers (at R ? 75 deg, and
? ? [240, 270, 300] deg) were left out to allow room for the listener’s
legs (see also Van Wanrooij and Van Opstal 2005; their Fig. 2 shows
an illustration of the speaker setup).
Head movements were recorded with the magnetic search-coil
induction technique (Robinson 1963). To that end, the listener wore a
lightweight (150 g) “helmet” consisting of two perpendicular 4-cm-
wide straps that could be adjusted to fit around the listener’s head
without interfering with the ears. On top of this helmet, a small coil
was attached. From the left side of the helmet a 40-cm-long, thin
aluminum rod protruded forward with a dim (0.15 cd/m2) red LED
attached to its end. This LED could be positioned in front of the
listener’s eyes by bending the rod. Two orthogonal pairs of 2.45 ?
2.45-m2coils and one pair of 2.45 ? 3.5-m2coils were attached to the
room’s edges to generate the left–right (60 kHz), up–down (80 kHz),
and front–back (40 kHz) magnetic fields, respectively. This arrange-
ment allows for a precise recording of head orientations in all
directions, including the rear hemifield. The head-coil signal was
amplified and demodulated (Remmel Labs, Katy, TX), after which it
was low-pass filtered at 150 Hz (model 3343; Krohn-Hite, Brockton,
MA) before being stored on hard disk at a sampling rate of 500
Hz/channel for off-line analysis.
Acoustic stimuli were digitally generated using Tucker-Davis Sys-
tem II hardware (Tucker-Davis Technologies, Alachua, FL), with a
TDT DA1 16-bit D/A converter (50-kHz sampling rate). A TDT PA4
programmable attenuator controlled sound level, after which the
stimuli were passed to the TDT HB6 buffer, and finally to one of the
speakers in the experimental room.
All acoustic stimuli consisted of Gaussian noise and had 0.5-ms
sine-squared on- and offset ramps. The auditory stimuli were either
broadband (BB, flat characteristic between 1 and 20 kHz) or high-pass
(HP, high-pass filtered at 3 kHz) stimuli with a duration of 150 ms.
Sound levels ranged from 30 to 60 dBA (see following text). Absolute
free-field sound levels were measured at the position of the listener’s
head with a calibrated sound amplifier and microphone (BK2610/
BK4134; Bru ¨el & Kjær, Norcross, GA).
Listeners were equipped with a precisely fitting plug in their left ear
canal to perturb their binaural cues. The plugs were manufactured by
filling the ear canal with rubber casting material (Otoform Otoplastik-
K/c; Dreve, Unna, Germany).
Measurement of audiograms
To determine the attenuation provided by the custom-made plugs,
audiograms (10 tone pips, 0.5-octave separation, between 500 Hz and
11.3 kHz) were taken of the listeners’ ears, with and without the plug
(Fig. 1A). Although some plugs attenuated more than others, the
attenuation was always considerable (?20 dB). For high frequencies
the mean attenuation provided by the plugs (?3 kHz: 25–50 dB) was
equal to or higher than that for low frequencies (?3 kHz: about 25 dB,
procedure were obtained by instructing the listener to make an
Head-position data for the calibration
716 M. M. VAN WANROOIJ AND A. J. VAN OPSTAL
J Neurophysiol • VOL 97 • JANUARY 2007 • www.jn.org
Although the effects were smaller than the dramatic localiza-
tion deficits for azimuth, they were systematic and consistent
across listeners. Butler et al. (1990) showed that under plugged
hearing, advance knowledge of the sound’s azimuth may
enhance elevation performance. In line with this, findings
obtained with a monaural mold (Hofman and Van Opstal 2003;
Humanski and Butler 1988; Morimoto 2001; Van Wanrooij
and Van Opstal 2005) indicated that the localization of eleva-
tion involves binaural interactions, the strength of which varies
gradually with azimuth. The present study extends these results
by showing that the binaural weighting depends on the per-
ceived azimuth location, which in the case of plugged hearing
may be quite erroneous (Fig. 11, A and B). This is a remarkable
finding given that the spectral shape cues, the sound level, and
the SNR of the peaks and notches in the sound spectra at the
normal-hearing side were all unaffected by the plug. When
localizing elevation, acutely plugged listeners showed a
marked decrease in performance on the plugged side for the
lower sound intensities (Fig. 8A). In contrast, listeners per-
formed better for low-intensity sounds at their normal-hearing
side (Fig. 8B). We believe that two different factors underlie
these seemingly different behaviors. First, the spectral cues of
the plugged ear have a low SNR, which affects low-intensity
sounds more than high-level sounds (Fig. 8A). Second, the
elevation percept comes about by fusing the spectral cues from
each ear with a weight determined by the perceived azimuth
(see above), which on the normal-hearing side is perturbed
more at higher intensities than at low intensities (e.g., Figs. 5,
6, and 8B).
Our results extend recent findings obtained from monaurally
deaf listeners (Van Wanrooij and Van Opstal 2004). That study
showed that all monaural listeners relied heavily on the head-
shadow effect (i.e., absolute sound level at the hearing ear) to
localize sounds in the horizontal plane, whereas half of the
listeners also incorporated the spectral-shape cues of their
intact ear to estimate azimuth. Interestingly, only listeners who
had learned to use spectral cues to localize azimuth could also
localize elevation on the side of their intact ear. Monaural
listeners who did not make use of the spectral cues for azimuth
could not localize elevation either, which hinted at the possi-
bility that the ability to localize elevation strongly depended on
the performance in azimuth.
Plugging the ear of an otherwise normal-hearing binaural
listener is very different from real monaural hearing of the
unilaterally deaf for a number of reasons: First, although the
plug strongly attenuates sounds, the acoustic input will not be
entirely abolished as in the monaurally deaf. Thus for sounds at
a sufficiently high intensity, all localization cues are still
present, albeit heavily perturbed. Second, the acoustic effect of
the plug typically depends on frequency (Fig. 1), yielding
ambiguous localization cues when compared with normal hear-
ing. For example, not only will the ILDs differ for different
frequency bands (and thus point to different azimuth loca-
tions), but the ITDs and ILDs are also affected in a different
way. Thus the outputs of the two binaural localization streams
will often not agree on sound-source azimuth either. For
sufficiently low sound levels, however, plugged hearing ap-
proaches real monaural hearing. The plug’s intensity- and
frequency dependencies therefore pose an interesting and non-
trivial challenge to the sound-localization system. Third, the
monaurally deaf have had long-term exposure to their hearing
condition, allowing ample time for adaptive processes to re-
shape their localization behavior. This contrasts with the im-
mediate and complex effect of a plug on localization of the
normal-hearing listener. Conversely, binaural listeners have
had ample experience in using the binaural difference cues and
the detailed complex spectral shape cues from either ear.
Taken together, it is reasonable to expect that the central
organization of the sound localization systems of the monau-
rally deaf and of binaural listeners may be quite different (see
also Bilecen et al. 2000; Ponton et al. 2001; Scheffler et al.
Integration of acoustic cues
We propose that azimuth and elevation are both computed
on the basis of evidence from all available acoustic cues, but
that their relative weights depend on the acoustic conditions.
Figure 12 depicts this conceptual model, which extends the
classical idea that the stimulus coordinates are determined by
independent, noninteracting pathways. The localization of az-
imuth is based on a weighting of binaural difference cues as
well as of spectral shape cues. When the binaural cues become
unreliable or ambiguous (e.g., for weak sounds at far-lateral
locations or after plugging one ear) their weights are reduced,
and the contribution of the spectral cues from the contralateral
ear increases. In turn, the computation of sound-source eleva-
tion involves a weighting of the spectral shape cues from both
ears. The strength of the binaural weighting is modulated by
the perceived azimuth location.
At the initial stages in the auditory system, the localization
cues are processed by independent brain stem pathways, both
muth (AZp) is determined by weighting all available localization cues [inter-
aural time differences (ITDs), interaural level differences (ILDs), and spectral
cues from both ears, head-related transfer functions HRTFcand HRTFi]. An
elevation percept (ELp) is obtained by binaural weighting of the spectral cues,
which is modulated by the azimuth percept. Weights are updated as soon as a
given cue is absent or unreliable. Under normal binaural hearing, azimuth
performance is nearly perfect because the binaural difference cues are reliable.
With a plug, however, the binaural difference cues break down in a complex
way. This induces an increase in the gain of the spectral cues of the contralat-
eral normal-hearing ear (HRTFc) and a decrease of the contribution from
binaural difference cues (arrows). Likewise, the spectral cues from the ipsi-
lateral, plugged ear (HRTFi) become less reliable, so that the elevation
weighting is biased toward the normal-hearing ear. Processing of the elevation
percept is further perturbed by the influence of the incorrectly perceived
Cue-integration model for 2D sound localization. Perceived azi-
725 PERTURBED BINAURAL HEARING
J Neurophysiol • VOL 97 • JANUARY 2007 • www.jn.org
in birds (e.g., the barn owl; for review see Takahashi 1989) and
in mammals. In mammals, the medial superior olive (MSO)
constitutes the ITD pathway, whereas the ILD pathway is
processed in the lateral superior olive (LSO; for reviews see
Irvine 1986; Yin 2002). The elevation pathway has yet to be
identified, but recent evidence suggests that the first stages of
spectral-shape analysis may already occur at the level of the
dorsal cochlear nucleus (Reiss and Young 2005; Young and
Our finding that the percept of sound-source azimuth is
determined by spectral cues, when the binaural cues become
unreliable, could arise from mechanisms that rely entirely on
the acoustic properties of the signal. Because the different
brain stem pathways all converge in the midbrain, the inferior
colliculus (IC) could play a role in such preattentive compu-
tations. Although an explicit map of auditory space has not
been demonstrated in the mammalian IC, unlike in the barn
owl (Knudsen and Konishi 1978), recent evidence suggests
that auditory space may be represented in the IC by space-
specific modulations within a large population of neurons. For
example, the majority of monkey IC cells are sharply tuned to
sound frequency, although their firing rates are also modulated
by sound level, by the location of the sound in azimuth and
elevation, and by nonacoustic signals such as eye position
(Groh et al. 2001, 2003; Zwiers et al. 2004).
On the other hand, our result that the spectral-cue weighting
that determines elevation is influenced by the listener’s per-
ceived azimuth, rather than by the purely acoustic effects of
stimulus azimuth on the spectral cues (Fig. 11A), might suggest
that higher, perhaps cortical, mechanisms are involved. Recent
recordings indicated that populations of cells in the primate
auditory cortex may encode sound locations in a way similar to
that of the IC (Recanzone 2000; Werner-Reiss et al. 2003).
Whether the auditory cortex may be involved in the perceptual
integration of acoustic cues has to be established by future
A C K N O W L E D G M E N T S
We thank G. Van Lingen, H. Kleijnen, G. Windau, H. Versnel and T. Van
Dreumel for technical assistance.
G R A N T S
This work was supported by Radboud University Nijmegen grants to A. J.
Van Opstal and M. M. Van Wanrooij and by Human Frontiers Science
Program Grant RG 0174–1998/B to M. M. Van Wanrooij.
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