PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Acuity of spatial stream segregation along the horizontal azimuth
with non-individualized head-related transfer functions
Martha M. SHIELL1; Elia FORMISANO1
1Maastricht University, Netherlands

ABSTRACT
Auditory spatial cues help the nervous system segregate features of a soundwave into distinct streams. To
study this process, we must allot for changes in auditory spatial acuity along the horizontal azimuth, andsome evidence suggests that this relationship differs for concurrent versus consecutive sounds. Here, wedeveloped a paradigm to measure the change in spatial stream segregation along the horizontal azimuth andvalidate the effectiveness of non-individualized head-related transfer functions, the most easily-accessed
form of auditory spatialization, in this procedure. We tested 18 normal-hearing adults using
anthropometrically-matched non-individualized head-related transfer functions. We applied a spatial
stream segregation task where participants identified the rhythm of a target stream presented with aspatially-separated masker. Spatial separation varied according to an adaptive staircase procedure, andthresholds were calculated both near the midline and in the far left periphery. This work will assist
neuroscientists and others in the design of stimuli for eliciting brain activity related to spatial stream
segregation.
Keywords: Spatial stream segregation, auditory spatial acuity

1. INTRODUCTION
Typically when we think of auditory spatial information we think of how it helps us understandwhere sounds originate from in our environment – that is, the process of localization. But auditory
spatial information has a second, often overlooked role in perception. Concurrent sounds from theenvironment mix together and hit our eardrums as a single soundwave, and the auditory system needs
to dissect this messy signal to make sense of it. Multiple acoustic cues contribute to this scene
analysis, one of which is spatial information: The further apart two sounds are, the more likely they
are to be heard as two separate sources, a process known as spatial stream segregation (SSS).
Research with neuropsychology patients indicates that SSS and localization behaviours may be
disociable: Some brain-lesioned patients show preserved SSS with impaired localization while others
show preserved localization with impaired SSS (1). While the neural mechanisms supporting eachbehaviour are not fully understood, the few neuroscience studies that have examined neural
responses to concurrent spatially-separated sounds confirm that the brain activity patterns elicited bythese stimuli are not merely combinations of the patterns elicited by single sound locations (2-5). A
better understanding of the neural mechanisms may have clinical impact by highlighting potential
factors that can be optimized in hearing aids and cochlear-implants.
To manipulate SSS for experimental study, neuroscientists can vary the spatial separation
between concurrent sounds. However, every change in spatial separation has a corresponding change
in spatial location that may trigger localization processing, and thus raises a potential confound in
the interpretation of experimental measurements. This confound can be controlled for withconditions where the locations of concurrent sounds change but SSS is held constant. To do so
requires careful monitoring though of the relationship between SSS and horizontal azimuth: Since
listeners have better spatial acuity for sounds in front of them than sounds in their periphery, greater
spatial separations are required in the latter to achieve the same behaviour (6-9).
This difference in acuity across the azimuth can largely be explained by physical differences inthe soundwave as it reaches each ear (i.e. binaural cues vary at a faster rate around the midline thanin the periphery). However, there is evidence that our perception of spatial separation between
concurrent sounds does not merely follow the physics of binaural cues, but has a psychophysical6644

component as well. Evidence for this psychophysical component comes from the finding that there isa "sweet-spot" for sensitivity to changes in spatial separation between concurrent sounds: We are
best at judging changes that occur around six degrees of separation, irrespective of the location along
the horizontal azimuth (10). Note that this sensitivity cannot be explained by the physics of binauralcues. This dissociation between SSS and binaural cues justifies measuring the SSS-azimuth
relationship directly, rather than inferring it from measures of binaural cues.
In the current study, we developed a paradigm to characterize the relationship between SSS andhorizontal azimuth for the purpose of stimulus design in neuroscience experiments. To do so, weadopted the spatial rhythmic-masking release task from Middlebrooks and Onsan (2012). This task isa direct proxy of SSS. It uses streams of broadband noise bursts, which are ideal for brain imaging
because they will elicit brain activity that (1) is independent of ecological associations (unlike, forexample, semantic information in language), and (2) avoid interactions with frequency.
Whereas Middlebrooks and Onsan (2012) report on thresholds in highly-practiced participants,
we evaluated performance in typical inexperienced listeners, whom are more readily available forneuroscience experiments. Furthermore, since brain imaging experiments often require sounddelivery through headphones, we generated virtual auditory space through head-related transfer
functions rather than free-field. Because facilities and resources for measuring these functions are
not always available or feasible for neuroscience experiments, we used non-individualizedhead-related transfer functions (NI-HRTF) from a publicly available library with corresponding
anthrompometry data to match with participants. Finally, we selected different locations than thosethat were previously studied: In order to maximize potential differences in corresponding brain
activity measurements, we measured SSS at the most peripheral location available in our library ofNI-HRTFs (80 degrees). It was also important to limit the locations to a single hemifield, in order tosimplify the interpretation of potential hemispheric interactions in brain activity. Because concurrent
spatially-separated sounds can push apart the perceived locations of one another (11), we measured
SSS at a location that was offset from the midline (10 degrees) to mitigate the risk of the perceptionof the target's location being pushed across the midline into the contralateral hemifield.

2. METHODS
2.1 Participants
The study was approved by the Ethical Review Committee for the Faculty of Psychology andNeuroscience at Maastricht University. Participants were recruited from the Maastricht University
undergraduate psychology program. All gave informed written consent and were compensated fortheir time either by course credits or gift vouchers. Eighteen healthy adults with self-reported normal
hearing participated in the experiment (4 males and 14 females, age in years old: mean = 22.5, range= 18 - 33). All participants were right-handed but one female who was ambidextrous.
2.2

Materials
All stimuli were created with Matlab (Mathworks Inc.). NI-HRTFs were taken from CIPIC
repository (12,13). Stimuli locations that were mid-HRTF measurements were interpolated as needed.
Training and measurements were completed with Psychopy (14). All stimuli were presented withSensimetric earbuds (model S14) at a comfortable sound level in a sound-attenduated booth.
2.3 General Procedure
Over two days that were 1-7 days apart, participants completed two 60-90 minute-long sessions oftesting. In addition to the experimental task, the first session included NI-HRTF selection (~5minutes) and brief task training consisting of instructions, stimulus examples, and a practice run ofthe experimental task (~3 minutes).
2.4 Selection of Non-individualized Head-Related Transfer Function (NI-HRTF)
For each participant, we manually measured head width and depth with a carpenter's square and aruler. These two measures were chosen based on their relevance for spatial hearing: head width ishighly-correlated with interaural timing delays (ITD) (12,15), a feature that may be essential to SSS(9), and head-depth is the best predictor of a listener's preferred NI-HRTF (16). ITD predicted fromthese variables share 76% of variance with actual ITD (12).

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Figure 1 - Examples of target and masker streams for rhythms A and B in rhythmic-masking release task.
NI-HRTFs from the CIPIC repository (12,13) were ordered based on the Euclidean distance of
their corresponding anthropometry from the participant's measures of head width and depth. The fiveNI-HRTFs with the closest measures were selected. We then asked participants to subjectivelyevaluate which of these five NI-HRTFs provided the most realistic representation of auditory space,using a procedure adapted from Seeber and Fastl (17). In this procedure, participants were instructed
to listen to five examples of auditory space, in any order and as many times as needed. Each example
consisted of five 200 ms noisebursts, presented every 400 ms sequentially to the left hemifield at -80,-60, -40, -20, and 0 degrees (i.e. at the midline). In each example, these locations were simulated via
one of the NI-HRTFs. The participants were asked to base their evaluation on the following criteria:
The sound (1) moved from the left to the midline, (2) moved in equally-spaced steps, (3) moved at aconstant elevation, (4) and a constant distance, (5) far away from the participant.
2.5 Measurement of Spatial Stream Segregation (SSS)
SSS was measured using the spatial rhythmic masking release task (RMR) (9). This measurement
was conducted under two conditions, termed peripheral and midline, according to where theparticipant was instructed to direct their spatial attention.
In each trial, a target stream of noise bursts was presented at the attended location in one of two
temporal patterns, termed rhythms A and B, illustrated in figure 1.The rhythm was randomized
between trials. The leading stream (the "Target" in the case of Figure 1) was alternated between
target and masker locations in a random order to ensure that its' onset could not be used as a cue forthe task. The noise bursts were 20 ms in duration, with 5 ms ramps on and off. Combined, the two
streams produce a rate of 10 noise-bursts per second. The rhythm was repeated for a maximum of6400 ms. The participant was asked to identify the rhythm by button-press and could respond at any
moment after stream onset, which terminated the trial. Feedback was displayed for 500 ms betweentrials in order to maintain participant motivation.
Concurrent to the target stream, a masker stream was presented (Figure 1). Because both themasker and target streams consisted of unfrozen noise bursts, if the two streams are co-located theparticipant will hear a single stream, and it is impossible to discriminate between rhythms A and B.
Likewise the task becomes easier as the spatial separation between the streams increases. Thus thespatial separation between masker and target was varied according to an adaptive staircase procedurein order to determine a threshold level for effective stream segregation. For each correct response, inthe following trial the spatial separation was decreased by one step, and for each incorrect response,
it was increased by three steps. The masker was always to the right of the distractor such that bothstreams remained within the same hemifield. Hence in the peripheral condition the target was fixedat -80 degrees, and in the midline condition the masker was fixed at -10 degrees (to the left, where
midline is 0 degrees). Although this introduced a confound of a static versus changing location of thetarget in the peripheral versus midline conditions, participants reported during pilot testing that thiswas easier than switching attention between the left and right streams in the two conditions.
Futhermore, it allowed the target to maintain the maximum potential benefit from the head-shadow
effects relative to the masker in both conditions.

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Figure 2 – Spatial separation thresholds between target and masker in the peripheral and midline conditionsof the rhythmic-masking release task: Individual (circles and diagonal lines), group median (middlehorizontal lines), and interquartile range (rectangles).
The one-up one-down staircase with 3:1 ratio in step size provides two advantages: (1) It allows
the staircase to vary at a faster rate than a typical transformed staircase (e.g. following a
1-up-3-down rule, (18)), thereby sampling space more efficiently, and (2) it quickly removes
participants from below threshold levels, which may be presented by chance (19). Although thethreshold where this staircase procedure converges is not established (20), we were not primarily
concerned with the threshold detection level, but rather with its replicability across sessions (seeanalysis section below), which is essential for neuroimaging experiments involving measurements ondifferent days.
Additionally, we were concerned that the convergence of the staircase was consistent between themidline and peripheral conditions. The step size of a staircase can affect the calculated threshold,which presents a challenge in the current paradigm because the resolution of spatial cues is not equal
between the peripheral and midline conditions. Thus, a step size in the midline condition willpotentially have more binaural change than the same absolute step size in the periphery, leading todifferences in thresholds between the two conditions that are merely due to the sampling of testing
points. Short of measuring binaural cues for each participant across the azimuth, the required
difference in step size between the periphery and midline can only be estimated. In the original RMRtask, Middlebrooks and Onsan (2012) used step size of 2.5 and 5 degrees at 0 and 45 degree locations,respectively. We used the same values in our paradigm, and evaluated this choice post hoc (seeanalysis section below).
The staircase terminated after 15 reversals, which took four minutes on average. Participants weretold to take short rests as needed between runs, and forced to exit the testing booth for a longer breakafter every three runs. Runs of the midline and peripheral conditions were alternated, andparticipants completed 6 runs of each in each session, for a total of 12 runs in each condition.

3. ANALYSIS AND RESULTS
We calculated thresholds for each run as the mean spatial separation between target and masker
across all fifteen reversals. We calculated thresholds for each participant as their median threshold
across twelve runs for each of the peripheral and midline conditions. Participants' individualthresholds, group median and interquartile range are shown in figure 2. In the peripheral condition,
the median threshold (26.4 degrees) was approximately 1.9 times that of the midline (14.3 degrees),but participants varied in this relationship.
To assess the correspondence of the chosen step sizes in the peripheral and midline conditions, we6647Figure 3 – Reliability of spatial separation thresholds in the rhythmic-masking release task across two
sessions, in the peripherial and midline conditions.
used a Wilcoxon signed-rank test to compare participants actual accuracy scores on the staircase
testing level nearest to their calculated threshold. If the 5 and 2.5 degree step sizes in the peripheral
and midline conditions respectively are not sufficiently matched, then these accuracy scores will
differ. Nearest-threshold accuracy scores were similar between peripheral and midline conditions(group median degrees: peripheral = 0.81, midline = 0.80) and we found no evidence for a difference
between them (W = 77, p = 0.64).
To assess the replicability of the staircase over sessions, we measured the Pearson correlation
between thresholds from the first and second sessions, shown in Figure 3. Thresholds correlated
across sessions (periphery: R = 0.82, p = 0.0002; midline: R = 0.76, p = 0.0002), indicating that thetask was replicable. To test if there were effects of learning, we compared group median scores
across sessions with a Wilcoxon-signed rank test. Thresholds in the first session were higher thanthose in the second session (group medians in degrees for session 2, 1: periphery = 25.9, 30.4;
midline = 13.7, 15.5; Wilcoxon-signed rank: peripheral: W = 118, p = 0.049; midline: W = 144, p =0.011) indicating that there may be a learning effect.

4. SUMMARY
We describe here a reliable paradigm for measuring the relationship between SSS and horizontal
azimuth to be used for the design of stimuli in the study of the neural correlates of SSS. We presentdata from a typical group of healthy inexperienced participants. The staircase procedure produces athreshold for 0.805 accuracy, which is consistent across the tested locations in the midline and theperiphery. Participants showed improved performance on the task on the second day of testing,indicating a learning effect that should be taken into account in future applications of this paradigm.

ACKNOWLEDGEMENTS
This research was supported by grant NWO VICI 453-12-002 to EF. MS was supported by a
postdoctoral fellowship from the Canadian Institutes of Health Research.

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