OBSERVATION
Perceptual Asymmetry Induced by the Auditory Continuity Illusion
Dorea R. Ruggles and Andrew J. Oxenham
University of Minnesota
The challenges of daily communication require listeners to integrate both independent and complementary
auditory information to form holistic auditory scenes. As part of this process listeners are thought to fill in
missing information to create continuous perceptual streams, even when parts of messages are masked o
obscured. One example of this filling-in process—the auditory continuity illusion—has been studied primarily
using stimuli presented in isolation, leaving it unclear whether the illusion occurs in more complex situations
with higher perceptual and attentional demands. In this study, young normal-hearing participants listened fo
long target tones, either real or illusory, in “clouds” of shorter masking tone and noise bursts with pseudo-
andom spectrotemporal locations. Patterns of detection suggest that illusory targets are salient within
mixtures, although they do not produce the same level of performance as the real targets. The results suggest
that the continuity illusion occurs in the presence of competing sounds and can be used to aid in the detection
of partially obscured objects within complex auditory scenes.
Keywords: continuity illusion, auditory object, perceptual search, perceptual asymmetry
The continuity illusion occurs when a masked or obscured portion
of a stimulus is perceptually “filled in” to create the illusion of a
continuous stream of information (Bregman, 1990; Wa
en, 1999).
Conditions that foster this type of filling in have been identified in
tactile (Kitagawa, Igarashi, & Kashino, 2009), visual (Komatsu,
2006), and auditory perception (King, XXXXXXXXXXIn audition, the induc-
tion of missing information can play a role in speech understanding
(Bashford, Riener, & Wa
en, 1992; Shahin, Bishop, & Miller, 2009;
Shinn-Cunningham & Wang, 2008) and has been studied because of
its potential for providing information about the perceptual and neural
mechanisms underlying auditory object formation. Early studies by
Houtgast XXXXXXXXXXand Duifhuis XXXXXXXXXXused the continuity illusion in
the form of pulsation thresholds to demonstrate psychophysical cor-
elates of nonlinear frequency tuning in the auditory periphery, and
later studies have examined the neural co
elates of the continuity
illusion at higher levels of the auditory system by using neuroimaging
(Riecke et al., 2012; Riecke, van Opstal, Goebel, & Formisano, 2007;
Shahin et al., 2009).
The conditions under which the continuity illusion occurs have
een studied since it was initially identified (Miller & Licklider,
1950). It is generally believed that the illusion occurs when the
peripheral auditory response (e.g., auditory-nerve activity) produced
y the interfering sound (or masker) overlaps completely with the
esponse produced by the target sound (e.g., Duifhuis, 1980; Hout-
gast, 1972; Petkov & Sutter, 2011; Wa
en, Obusek, & Ackroff,
1972). Our understanding of the conditions necessary for the illusion
have been refined by recent studies, which have shown that the
illusion can still occur under some circumstances in which the pe-
ipheral auditory response provides evidence of the inte
uption, sug-
gesting that masking of the inte
uption’s onset and offset are more
critical than the ongoing portion (Haywood, Chang, & Ciocca, 2011)
or that global features such as the specific loudness of the interfere
play a more dominant role than the interferer’s fine-grained temporal
structure (Riecke, Micheyl, & Oxenham, 2012).
The physiological basis of the continuity illusion has also been
studied using a wide range of electrophysiological techniques that
have revealed important details of its generation and attentional
equirements. Especially significant for this study is the finding by
Micheyl et al. (2003), who used mismatched negativity (MMN)
methods to show that physiological responses consistent with the
continuity illusion do not seem to depend on focused attention. A
similar finding was presented by Heinrich et al. (2011), who found
that fMRI co
elates of the continuity illusion seem to be indepen-
dent of attention for complex vowel-like stimuli.
These studies provide some neurophysiological evidence that
the continuity illusion is represented neurally for both simple and
complex sounds in a way that may not depend on directed atten-
tion. Such findings would be strengthened through behavioral
evidence that the continuity illusion effectively generates relevant
auditory objects within attentionally demanding and complex
acoustic environments (Gutschalk, Micheyl, & Oxenham, 2008;
Jones, Macken, & Mu
ay, 1993).
To investigate the role of the continuity illusion in auditory
mixtures, we used an auditory perceptual asymmetry identified by
Cusack and Carlyon (2003), analogous to findings in the visual
modality (e.g., Treisman & Gelade, XXXXXXXXXXCusack and Carlyon
This article was published Online First December 23, 2013.
Dorea R. Ruggles and Andrew J. Oxenham, Department of Psychology,
University of Minnesota.
This research was supported by National Institutes of Health Grant R01
DC007657.
Co
espondence concerning this article should be addressed to Dorea R.
Ruggles, Department of Psychology, University of Minnesota, Minneap-
olis, MN XXXXXXXXXXE-mail: XXXXXXXXXX
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Journal of Experimental Psychology:
Human Perception and Performance
© 2013 American Psychological Association
2014, Vol. 40, No. 3, 908–914
XXXXXXXXXX/14/$12.00 DOI: XXXXXXXXXX/a0035411
908
mailto: XXXXXXXXXX
http:
dx.doi.org/10.1037/a0035411
(2003) found that long tones in mixtures of short tones were
detected more easily than short tones in mixtures of long tones, and
they attributed these asymmetries to the existence of feature-
specific neurons tuned to longer rather than shorter durations.
We asked whether an illusory long tone, composed of two short
tones inte
upted by a noise burst, would be detected if it were
embedded in a complex pattern of similar but noncontiguous short
tones and noise bursts. The question of whether illusory long tones
evoke the same feature mapping and detection asymmetries as
actual long tones has the potential to contribute to a deepe
understanding of the continuity illusion and the processes of fea-
ture coding and selection in complex acoustic environments. If the
esults show that the illusion is not detectable in complex mixtures
of tones and noises and produces no perceptual asymmetries, we
may conclude that the continuity illusion, as measured behavior-
ally, stems from processes that are secondary to the feature map-
ping that results in auditory asymmetry and is thus unlikely to play
an important role in object formation in complex acoustic envi-
onments. In contrast, if listeners are able to detect illusory long
tones in mixtures of tones and noise and display a perceptual
asymmetry similar to that found for physical long tones, it would
suggest that the continuity illusion is formed prior to or in con-
junction with the feature mapping associated with perceptual
asymmetries and therefore could play a crucial role in parsing
complex auditory scenes.
Experiment 1
Method
The experiment tested listeners’ ability to detect illusory long
tones elicited by a continuity illusion when the target tones were
embedded in clouds of distracting tones and noises. Five condi-
tions were studied (see Figure 1). In all conditions, short and long
tones had total durations of 100 ms and 300 ms, respectively. All
the noise bursts had total durations of 100 ms. Raised-cosine onset
and offset ramps were applied to the first and last 10 ms of the tone
and noise bursts. Pure-tone frequencies were randomly selected
from 1/3 octave ranges centered at 315, 500, 800, 1250, 2000, and
3150 Hz with uniform distribution, and the noise bursts were
filtered into the same 1/3 octave bands by 26th order Butterworth
filters centered at the same frequencies. Empty 1/3 octave bands
separated each of the bands to reduce spectral interactions between
neighboring tones and noises.
Each cloud was 2 s in duration and was constructed indepen-
dently so that the timing, tone frequencies, and tone levels were
unique for every presentation. The number of tones and noises was
set for the target and nontarget bands in each condition and was
equal in all nontarget bands, resulting in a predefined number of
tones and noises evenly distributed among the frequency bands. In
conditions without noise bursts, there were an equivalent of 36
100-ms tone units (including target constructions), equally distrib-
uted across the six frequency bands. In conditions with noise
ursts, there were an equivalent of XXXXXXXXXXms tone units and seven
100-ms noise units (including target constructions), distributed so
that the five nontarget bands were all equal, but the target band was
slightly different because of the target constructions.
Each cloud was constructed by first randomly selecting the
target frequency band and then randomly determining the different
presentation frequencies for the tones in each band. After gener-
ating the tone and noise bursts for the cloud, a unique initial onset
time was determined for the target band by randomly selecting a
delay between 100 ms and 200 ms. Tones and noises in each band
were uniquely ordered and separated by random lengths of silence.
The longest possible length for within-band inte
urst silences was
calculated as the total silence in the band divided by the number of
events in that band, and the minimum length was one quarter of the
maximum length. The target could occur at any time after the first
100 ms and before the last 100 ms of the interval. The levels of the
distracting tones were set based on a Gaussian distribution with a
mean of 45 dB SPL and standard deviation of 2 dB, and noise
ursts were set at 75 dB SPL. Target tones were presented at 40 dB
SPL.
The first condition required listeners to detect a long tone (LT)
within a cloud of short tones. The second condition was the same,
ut the distracting clouds also included noise bursts (LTn). In the
third condition, listeners were asked to detect a long tone in
mixtures of short tones and noise bursts (ILTn), but in this case, no
physical long tone was present; instead an illusory long tone was
created by concatenating a short tone, a noise burst, and a second
short tone at the same frequency to produce a tone-noise-tone
sequence (onset and offset ramps overlapped at their half-
amplitude points). The fourth condition required listeners to detect
short tones (ST) in clouds of long tones, and the fifth condition
Figure 1. Five conditions were tested. Random-frequency distractors, or “clouds,” in Conditions 1 (long tone [LT])
and 2 (long tone with noise [LTn]) consisted of short tones (ST) and, in the case of Condition 2, short noise bursts;
the target tone in these clouds was a physical long tone. Clouds in Condition 3 (illusory long tone with noise [ILTn])
were made of short tones and noise, and targets were illusory