E. Aiba

Proceedings of Meetings on Acoustics
Volume 19, 2013

http://acousticalsociety.org/ICA 2013 Montreal
Montreal, Canada
2 - 7 June 2013
Psychological and Physiological Acoustics
Session 3aPP: Auditory Physiology and Modeling (Poster Session)
3aPP28. Accuracy of synchrony judgment between two pulses: effects of variations incochlear delay amount
Eriko Aiba*​​ *Corresponding author's address: Health Research Institute, National Institute of Advanced Industrial Science and Technology(AIST), 1-8-31, Ikeda, 5638577, Osaka, Japan, aiba.eriko@aist.go.jp
The cochlear delay shifts the arrival of lower-frequency components of an auditory signal slightly but systematically behind that of higherfrequency components. Therefore, even if all of the components of a complex tone physically begin simultaneously, their temporal relation is
not preserved at the cochlear level. In our previous study, the accuracy of synchrony judgment was measured using two types of chirps(compensated and enhanced chirps) and a pulse. The compensated chirp had an increasing frequency pattern to cancel out the cochlear delay.
An enhanced chirp had a delay pattern that enhances the assumed cochlear delay. The pulse had a usual cochlear delay at the auditory peripheral.
As a result, the accuracy of synchrony judgment was the highest in the pulse and higher in the enhanced chirp than the compensated chirp,
implying that there is an asymmetric aspect. The purpose of this study is to investigate how our auditory system processes this asymmetricaspect, and to investigate the extent of the amount of temporal collapse was tolerated. We also measured the accuracy of synchrony judgmentusing stimuli that reverse the cochlear delay (the higher-frequency components arrive behind the lower-frequency components), or enhance thedelay of lower-frequency components up to 8 times.
Published by the Acoustical Society of America through the American Institute of Physics© 2013 Acoustical Society of America [DOI: 10.1121/1.4799728]
Received 22 Jan 2013; published 2 Jun 2013
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INTRODUCTION
Onset synchrony is widely assumed to be an important cue for perceptual uniﬁcation ofmultiple sinusoidal sound as a single complex tone [1]. However, even if all of the components
physically begin exactly simultaneously, their temporal relation might not be preserved at thecochlear level because of the "cochlear delays." There is the possibility that this phenomenon has
some effect on perceptions of temporal information of sound.
In our previous psychoacoustic studies [2, 3], experiments were performed to measure theaccuracy of synchrony judgment using stimuli that controlled the cochlear delays. The results ofthe experiment showed that the synchrony judgment accuracy was highest for stimuli thatevoke an intrinsic cochlear delay (intrinsic delay chirp). Furthermore, the synchrony judgment
accuracy was higher for stimuli that evoke the enhanced cochlear delay (enhanced delay chirp)
than for stimuli that cancelled out the cochlear delay (compensated delay chirp), which implies
that there is an asymmetric aspect of temporal processing in the human auditory system.
The purpose of this study is to investigate how our auditory system processes thisasymmetric aspect, and also to investigate the extent of the amount of temporal collapse thathas no effects on our perceptions of temporal information of sound.

Cochlear Delay
The cochlear delay was caused by the stiffness of the cochlear Basilar Membrane (BM)gradually decreases from the basal side to the apical side (Fig. 1)[4]. Therefore, the higher
components of an input wave excite the basal side, while the lower components excite the apicalside. Due to the time delay required for wave transmission, the vibration caused by
low-frequency components reaches its associated location later than that caused byhigh-frequency components. This delay largely occurs for components below 1000 Hz, and thevibration in the lowest-frequency associated location is delayed by about 10 ms relative to thevibration in the high-frequency associated location [5]. As a result, the cochlear delay shifts thearrival of lower-frequency components of an auditory signal slightly but systematically behindthat of higher-frequency components.
In previous physiological studies [6, 7, 8], the auditory brainstem response (ABR) wasobserved, when participants listened to the compensated delay chirp, the enhanced delay chirp
and the pulse respectively. As a result, the ABR wave-Vs that evoked by the compensated delay
chirp appeared to be the most salient, and the enhanced delay chirp had the gentlest peak. It is
generally assumed that the ABR is an electrophysiological event evoked by the onset of an
acoustic stimulus. It seems that the ABR wave-V reﬂects the synchrony of excitation along theentire BM partition. If this physiological saliency directly correlates to the perceptual accuracyof the onset detection, the synchrony judgment could become more accurate by compensating
rather than by enhancing the cochlear delay
As mentioned above, however, the synchrony judgment accuracy that evoked by each chirp
was decreased in order of intrinsic delay chirps, enhanced delay chirps and compensated delaychirps. There is the possibility that the human auditory system performs some compensation for
temporal information of sounds, or otherwise it appears to be more tolerant to the delay of thelower-frequency, i.e., the cochlear delay.
In this study, to investigate the extent of the amount of temporal collapse that has no effects
on our perceptions of temporal information of sound, we measured the accuracy of synchrony
judgment using stimuli that reverse the cochlear delay (the higher-frequency components arrive
behind the lower-frequency components), and, on the other hand, enhance the delay oflower-frequency components up to 8 times.

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Rela ve
Amplitude

1.0

1600

800 400 200 100 50 25 Hz

0.5010Ves bular
Window

20

Distance from Stapes

30(mm)

StapesCochlear
Window

Basilar Membrane

BaseSTIFF
THICK
NARROW

chainging
Elastic Coefficient

Apex
SOFTTHINWIDEF IGURE 1: The lower part shows cochlea that was straightened and its characteristic features. The Upper part
indicates a place of cochlear basilar membrane (BM) where the amplitude of different frequencies reaches the peak
respectively.

EXPERIMENT
Estimation of the Accuracy of Synchrony Judgment
To observe the accuracy of synchrony judgment, temporal thresholds to detect synchronous
between two pulses were estimated.
Apparatus
The experimental sequences were carried out with a PC (Let’s note CF-S10, Panasonic). The
stimuli were generated digitally (48 kHz, 24 bit) and controlled by matrix operation software(Matlab, MathWorks) and diotically presented, i.e., the same sound was presented to both earsof participants through insert earphones (IE8, Sennheiser) driven by an audio interface(UA-101, Roland). The participants were seated in a soundproof room and responded to thestimuli by pressing a button on a GUI presented on the PC screen.
Stimuli
Three types of "delay direction" were employed as the experimental stimuli. The second top
panel in Fig. 2 shows the (C×1) compensated delay chirp that cancelled out cochlear delay (Thebroken line in the left panel of Fig. 2). The forth top panel in Fig. 2 shows the (E×1) enhanced
delay chirp that enhances cochlear delay. The frequency of the (C×1) chirp increased from 0.1 to10.4 kHz within 10 ms. In the (E×2) chirp, the frequency decreased from 10.4 to 0.1 kHz within
10 ms. The third top panel shows the (I) intrinsic delay chirp without physical delay imposed on
any frequency component was used. The (I) chirp also passed through a low-pass ﬁlter with acut-off frequency of 10.4 kHz.
In addition to the condition of delay direction, we prepared four types of "delay amount" forthe (C×1) and the (E×1) chirps. For example, the (C×2) chirp is two times longer than the (C×1)
chirp in time to increase the frequency. It means that the frequency of the (C×2) chirp increased
from 0.1 to 10.4 kHz within 20 ms. As a result, the (C×2) chirp (The ﬁrst top panels in Fig. 2)that reverse the cochlear delay (the higher-frequency components arrive behind thelower-frequency components). In a similar way, the (C×4) and the (C×8) chirps were generated.

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(I)

0.2
0-0.2
0.2

(E×1)

0-0.2

(E×2)

0.2
0-0.2
00.01
0.02
Time (sec)

0.03

Freq (kHz)Freq (kHz)

(C×1)

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Freq (kHz)0-0.2

Freq (Hz)

(C×2)

0.2

Freq (Hz)Amplitude

Amplitude

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Amplitude

E. Aiba

105010
5010
5010
5010
500(a) waveform

0.01
0.02
Time (sec)

0.03(b) function

F IGURE 2: The left panels (a) show the waveforms of each chirp. The right panels show the functions of each chirp.
The solid line of the right panels (b) shows the frequency pattern as a function of time used under each delay condition.
The broken line shows the time required for all frequencies to reach maximum amplitude on the BM.

The (E×2) chirp (The bottom panels in Fig. 2) that evokes two times longer delay than the (E×1)
chirp was also prepared. It means that the frequency of the (E×2) chirp decreased from 10.4 to0.1 kHz within 20 ms, and the (E×4) and the (E×8) chirps were also generated. All chirps except
the (I) chirp were tapered transients at both ends with a raised cosine wave of 0.1 kHz.
The level of each stimulus presented was approximately 60 dB SPL. They varied from +6 to-6 dB to prevent participants from applying loudness differences as cues for the experimental
task.
Procedure
The experimental task was to detect a synchronous pair in the two-interval, two-alternative
forced choice (2I2AFC) procedure. Two pairs of sounds were presented to the participant in onetrial (Fig. 3). In each trial, one interval contained a synchronous pair and the other interval
contained an asynchronous pair. The asynchronous pairs consisted of 7 variations of a temporalgap (0.2, 0.4, 1.0, 2.3, 5.1, 11.4, or 25.6 ms), spaced roughly logarithmically. The order of thesynchronous and asynchronous pairs was randomized across trials to prevent prediction by theparticipants. The two pairs were separated by a 500-700 ms inter-stimulus interval. The type ofthe sound was the same within each trial.
Each trial was categorized as one of the following nine types of chirp: I, C×1, C×2, C×4,C×8, E×1, E×2, E×4, or E×8. The total number of stimulus type combinations was 126 (ninetypes of chirp, seven temporal gaps, and two patterns of order of the synchronous pair). The
participants repeated each combination 10 times, which brought the total number of trials to1260. All factors (sound type, temporal gap, and presentation order) were randomized andexecuted as a within participant factor.
The participants were instructed that both intervals contained two sounds and that the twosounds were synchronous in one interval but asynchronous in the other interval. They wererequired to choose the interval containing the synchronous pair. They were able to take breaks
at any time. Thresholds were estimated from seven points of psychometric function by ﬁtting asigmoid function on the results of each participant and computing the temporal gap value

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Gap

Amplitude

500-700 ms

Synchronous Pair
Asynchronous Pair
TimeF IGURE 3: Example of stimulus presentation by pulse. In this case, the ﬁrst interval contains the "synchronous" pairand should be chosen as "correct."ɹ Each vertical line represents one of the nine types of chirp.

corresponding to 75% correct responses.
Participants
Seven people (four females and three males) with normal hearing and no history of hearing
problems participated.

RESULTS
Figure 4 shows the average estimated thresholds and the standard deviations (SDs) overseven participants for each sound type. A two-way analysis of variance (ANOVA) was performed
where the conditions of delay direction and delay amount were treated as the main factors andthe participants were treated as a random effect. Delay amount was signiﬁcant as a main factor(F(3, 18) = 3.84, p < .05). Delay direction was also signiﬁcant as a main factor (F(1, 6) = 6.58,
p < .05). (I) chirp was excluded from this analysis, because it does not have any variations in thecondition of delay amount. The interaction between delay amount and delay direction was
marginally signiﬁcant (F(3, 18) = 2.55, p < .10).
To investigate detailed differences in delay direction, we performed one sample t-test foreach delay time. Among the (I), the (C×1ʣ and the (E×1), the Tukey-Kramer HSD test wasperformed. As a result, for the delay amount of one time (×1) and two times (×2), there weresigniﬁcant differences between the delay direction conditions of compensated delay (C) andenhanced delay (E). Thresholds increased as delay amount increased. It means that theaccuracy of synchrony judgment decreased as delay amount increased. For delay direction, thethresholds of compensated delay (C) were higher than those of enhanced delay (E). It meansthat the accuracy of synchrony judgment on compensated delay (C) was lower than that ofenhanced delay (E) as previous study reported [3, 9, 7, 8].

DISCUSSION
For delay time, the accuracy of synchrony judgment decreased as delay amount increased.
This result suggests that the silent interval between two chirps is one of the important cues tojudge onset synchrony. Because of the presence of the shorter duration of a chirp, the longer
silent interval arises between two chirps. For example, the duration of the (E×8) chirp is about80 ms. It means that an asynchronous pair of 25.6 ms gap that consists of two (E×8) chirps has
no silent interval. Also, in the case of the (E×4) chirp, there is no silent interval. Actually,

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Intrinsic

8Compensated6

Enhanced*

4** *20×1×2×4
Delay Amount×8

F IGURE 4: The average estimated thresholds in ms and the SDs for the conditions of each sound type (∗ p < .05,∗∗ p < .01).

thresholds for the (E×4) and the (E×8) chirps increase rapidly compared to the (E×1) and the(E×2) chirps. There might be big differences in the task between the (E×4) and the (E×8) chirps,
and the (E×1) and the (E×2) chirps.
For delay direction, the accuracy of synchrony judgment on compensated delay (C) was lower
than that of enhanced delay (E). It suggests that enhanced delay (E) was perceived as moresimilar to intrinsic delay (I) than to compensated delay (C). In this study, it was also observedthat there is an asymmetric aspect of temporal processing in the human auditory system. Thethresholds of the (I), the (E×1) and the (E×2) chirps appear to be almost the same. This isbecause under the (I), the (E×1) and the (E×2) chirps the lower frequencies reach the associatedlocations on the BM later than the higher frequencies. This temporal relationship is similar tocochlear delay. In the compensated delay (C), however, the lower frequencies reach theassociated locations earlier than the higher frequencies, or otherwise reach almost the sametime in the higher frequencies. As we mentioned in Section of Cochlear Delay, the human
auditory system appears to be more tolerant to the delay of the lower-frequency than that of thehigher-frequency.
As the thresholds of the (E×1) and the (E×2) chirps are almost the same, it is difﬁcult todistinguish the delay of lower frequencies on the BM even if it delays three times more of
intrinsic cochlear delay. There is the possibility that the human auditory system is tolerant to
the delay of the lower-frequency up to about three times more of cochlear delay.

ACKNOWLEDGMENTS
This work was supported by Grant-in-Aid for the Japan Society for the Promotion of Science(JSPS) Fellows No.24-10633, Grant-in-Aid for Scientiﬁc Research (A) No. 24243070 and the
JSPS through the "Funding Program for Next Generation World-Leading Researchers (NEXTProgram)," initiated by the Council for Science and Technology Policy (CSTP).

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R EFERENCES[1] A. S. Bregman, Auditory scene analysis (MIT Press) (1990).
[2] E. Aiba and M. Tsuzaki, “Perceptual judgment in synchronization of two complex tones :Relation to the cochlear delays”, Acoustical science and technology 28, 357–359 (2007).
[3] E. Aiba, M. Tsuzaki, S. Tanaka, and M. Unoki, “Judgment of perceptual synchrony between
two pulses and veriﬁcation of its relation to cochlear delay by an auditory model”, Japanese
Psychological Research 50, 204–213 (2008).
[4] G. v. Bekesy, Experiments in hearing (McGraw-Hill, New York) (1960).
[5] S. Uppenkamp, S. Fobel, and R. D. Patterson, “The effects of temporal asymmetry on thedetection and perception of short chirps”, Hearing Research 158, 71–83 (2001).
[6] T. Dau, O. Wegner, V. Mellert, and B. Kollmeier, “Auditory brainstem responses (abr) withoptimized chirp signals compensating basilar membrane dispersion”, J Acoust Soc Am 107,
1530–1540 (2000).
[7] E. Aiba, T. Shimotomai, K. Kazai, N. Nagata, and M. Tsuzaki, “Judgment of perceptual
synchrony between two pulses and its relation to the auditory brainstem response”,
Proceedings of the 40th Annual Meeting of Society for Neuroscience (Neuroscience2010)170.5 (2010).
[8] E. Aiba, K. Kazai, T. Shimotomai, T. Matsui, M. Tsuzaki, and N. Noriko, “Accuracy ofsynchrony judgment and its relation to the auditory brainstem response: the difference
between pianists and non-pianists”, Journal of Advanced Computational Intelligence andIntelligent Informatics 15, 962–971 (2011).
[9] E. Aiba, “"uniﬁcation" and "separation" of overlapped sounds.”, Doctral thesis, Graduate
School of Kyoto City University of Arts (2009).

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