Leveraging adaptation to study perceptual weighting of interaural timedifferences
Nima Alamtsaz(1) , Antje Ihlefeld(2)(1) New Jersey Institute of Technology and Rutgers University, USA, nima.alamatsaz@njit.edu(2) New Jersey Institute of Technology, USA, ihlefeld@njit.edu

Abstract
An important question in auditory cognition is how we perceive the location of an object in space. Converging
evidence from animal models and humans suggests that when judging sound direction, the central nervoussystem weighs the anticipated reliability of binaural cues. Here, we used short-term adaptation to bias normalhearing listeners towards source direction favoring either the left or the right frontal quadrant. Listeners rated
perceived laterality of tokens of band-pass filtered noise (300 Hz – 1200 Hz) with interaural time differencesthat were randomly selected from a uniform distribution spanning either -375 to 0 µs or 0 to 375 µs. Using
non-linear mixed effects modeling of behavioral laterality reports, we tested how exposure to source quadrant
affects how listeners weigh the reliability of interaural time differences. The cue reliability hypothesis predicts
that perceived direction should be skewed, such that unreliable frontal source angles are more affected by shortterm adaptation than the more reliable lateral source angles. Alternatively, short-term adaptation may affect all
source angles equally, predicting an overall shift in perceived direction. Results show that frontal angles are
more strongly affected by short-term adaptation than lateral angles, supporting the cue reliability hypothesis.
Keywords: functional near infrared spectroscopy, cochlear implant

1INTRODUCTION

A wide range of species relies on sound localization for both navigation and auditory scene analysis. Forlow-frequency sound, interaural time differences (ITDs) are the dominant cue for determining the direction of asource in the horizontal plane, a phenomenon called sound lateralization. The mechanisms by which the nervous
system maps ITD into perceived sound direction are incompletely understood.
In anechoic spaces, a given source angle in the horizontal plane typically gives rise to the same ITD, across awide range of source distances. However, in everyday environments where background sound and reverberantenergy are often present, ITDs are much less reliable indicators of source direction. This raises the possibility
that when estimating source direction, a listener’s interpretation of ITD changes depending on the context ofthe listening environment. Indeed, previous work shows plasticity in perceived sound direction across a wide
range of conditions, including: 1) Prolonged exposure to constant interaural delays (Javer and Shwarz, 1995),
2) Modifications to shape of the pinnae (Hofman et al., 1998), 3) Long-term procedural learning (Wright andFitzgerald, 2001), 4) Chronic unilateral ear plugging (Kumpik et al., 2010), 5) Short-term adaptation to stimulus
history (Stange et al., 2013), 6) In presence of preceding distractors (Kopčo et al., 2017).
However, we have an incomplete understanding of short-term adaptation of sound lateralization based on theoverall uncertainty of ITDs. The steepest point of the psychometric function linking ITD and perceived laterality is midline (the frontal direction). Thus, a small change in ITD around 0µs baseline ITD causes a muchlarger change in perceived laterality than a small change in ITD around 375µs ITD. Thus, the cue reliability
hypothesis predicts that perceived direction should be skewed, such that unreliable frontal source angles are
more affected by short-term adaptation than the more reliable lateral source angles. Alternatively, short-term
adaptation may affect all source angles equally, predicting an overall shift in perceived direction. Leveraging short-term plasticity based on stimulus context, we set out to test the cue reliability hypothesis in human
psychophysics experiments. We varied context by blocking stimuli such that they would predominantly arise8253

Figure 1. Raw lateralization scores from one representative listener.
from the left, from the right or from both sides of the head. Normal-hearing listeners judged lateralization of
band-limited noise tokens as a function of interaural time difference and context.

2METHODS

Overall methods were similar to previous work (for detailed description see: Ihlefeld et al. 2018). We tested 34
naïve normal-hearing listeners, 16 females, age 20-30. Audiometric thresholds were 20 dB HL or better at alloctave frequencies and did not differ by more than 10 dB across ears at each octave frequency. We divided ourlisteners into four groups. Using a target matching task without response feedback, we initially trained threegroups to be reliable reporters. Listeners trained and tested on the frontal 1) left hemisphere (LEFT HEMI,N=8) or, 2) right hemisphere (RIGHT HEMI, N=8)or, 3) both hemispheres (BOTH HEMIS, N=10). A fourthcontrol group, NAÏVE RIGHT HEMI (N=8) was tested on the right frontal hemisphere without training.
Listeners were seated in a double-walled sound-attenuating booth. Sounds were presented through ER-2 earphones. Stimulus consisted of a band-limited random noise token, generated independently on each trial (300 to1200 Hz, 1 sec duration, 10 ms cos-squared onset/offset ramps, 192 kHz sampling frequency, 24 bit resolution,
0 dB ILD). ITDs covered the range of 0, ±75, ±150, ±225, ±300 or ±375 µs. Sound levels varied from 10to 40 dB sensation level (SL).
Prior to testing all sounds were calibrated using KEMAR manikin and BK 2250 with 2-cc coupler. This study
was approved by the NJIT Institutional Review Board.
All listeners performed three training sessions (except the naive group) and then a single session of testing. Eachgroup were presented with their associated range of ITDs, i.e. RIGHT HEMI: positive ITDs only (including 0),
LEFT HEMI: negative ITDs only (including 0), BOTH HEMI: full range of ITDs.
2.1 Results
Figure 1 shows the results of a representative listener. Results follow the expected sigmoidal trend. Thus, anon-linear mixed-effects model (NLME) capturing sigmoidal sensitivity was developed to analyze the data (fordetails, see Ihlefeld et al. 2018).

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Figure 2. Fitted lateralization curves across all four context groups.
Figure 2A shows the responses of all 34 participants in the lateralization task. Circles show the raw acrosslistener average for each corresponding ITD and listener group, with error bars showing one standard error of
the mean across listeners. Lines are the fitted model value, with ribbons indicating the predicted standard errorof the mean fit. Figure 2B shows the slopes of these fits as a function of ITD.
2.2 Discussion
At the least reliable cue point, 0µs, results show robust response biases in all unilaterally tested listeners towards
the opposite side, with a larger effect for the trained group versus the naïve. In contrast, lateralization at
375µs base ITD is not appreciably affected by context. Moreover, slope of lateralization curves increases forunilateral groups compared to the bilateral, hinting at an expansion in the perceived space and increase inlateralization sensitivity. Thus, despite the fact that humans are much more sensitive to frontal ITD than lateral
ITD, as evidenced by lower just-noticable differences in ITD discrimination (Brughera and Hartmann, 2013),
frontal ITDs are also more susceptible to short-term adaptation than lateral ITDs, supporting the cue reliability
hypothesis.

3CONCLUSIONS

Current results support the idea that short-term plasticity of ITD is more pronounced in front than at the sides.

ACKNOWLEDGEMENTS
This work was supported by start-up funding to AI.

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