1 Introduction
4.1 Dynamic Front/Back Sound Localization Performance
4.1.3 Temporal Integration
In the passive and active conditions, we were able to observe that the effects of increasing velocity and increasing spatial window width were almost exactly reciprocal, such that performance was stimulus duration-limited at the higher velocities of 50o/s and 100o/s. To reach 75% correct front/back discrimination (regardless of cue-change), about 100 ms duration was required. This temporal integration is seen in both passive and active conditions at 50o/s and 100o/s, whereas at 25o/s the stimulus duration required was about 150 - 170 ms. It is unknown why this increase in duration is required at the slowest velocity, especially because the temporal integration of 100 ms duration trend is also seen at faster velocities of 200o/s and 400o/s in previous head movement studies (Macpherson & Kerr, 2008; Macpherson, 2013) (Fig. 41). Results in those experiments were derived from proportion of responses yielding “small” azimuthal errors that was computed as a measure of performance, in the same way the basic localization screening measure was computed in the present study (Section 3.1.1).
The increase in stimulus duration threshold during the 25o/s condition may possibly be due to a loss of vestibular sensitivity to slower horizontal rotations of the head. Previous findings (Grabherr et al., 2008; Valko et al., 2012) demonstrate that horizontal motion thresholds in the vestibulo-ocular reflex began to increase at 0.2 Hz, which is equivalent to approximately the oscillation frequency (0.1875 Hz; Fig. 8) required for rotation in the present study for a velocity of 50o/s during the passive and active conditions. This
suggests that the poor sensitivity to dynamic localization cues at 25o/s in the present study might be a consequence of the declining sensitivity of the vestibular system at lower oscillation frequencies required at this velocity. Furthermore, it is also possible that the results at the slower velocity of 25o/s may be an aberration, as it was commonly noted by the participants that they felt fatigued during the task due to the slower nature of the head and/or movement and were subsequently unable to focus as well compared to the faster
velocities. This may also explain why post hoc analyses of the two conditions at 25o/s showed significant differences, which was inconsistent with the hypothesis that no significant would be observed between the passive and active conditions.
Figure 41. Effect of stimulus duration and head velocity on localization performance. Illustration provided by Ewan Macpherson.
During a head rotation (passive or active), a listener must determine whether the sound source moves to the left or to the right and whether it moves in the same direction as the head rotation during dynamic front/back sound location. In order to examine whether these temporal dynamics observed at the higher velocities are specific to auditory- vestibular integration, the L/R-ITD task was performed in which participants were instructed to discriminate the direction of motion of low-frequency stimuli that monotonically increased or decreased in ITD. Results demonstrated that performance improved with greater cue change, while little to no velocity penalty was observed for higher rates of ITD change.
When the same task was run (L/R-Dynastatic task) but with the same HRTF-filtered stimuli used for the dynamic localization tasks in order to simulate a more realistic
acoustical representation, it appeared that performance improved with greater cue change. Unlike the LR-ITD task however, where only ITD was manipulated, there was a greater velocity penalty, which suggests that the difference in stimuli (perhaps more ILD cue
changes from the HRTF-filtered stimuli) might account for this difference in observed velocity penalty. The difference in stimuli may also account for why we unexpectedly observed a difference in left-to-right/right-to-left discrimination performance between the two tasks, as the participants may have used the extra ILD cues derived from the LR- Dynastatic task to assist them.
The finding of a velocity penalty without a vestibular signal suggests that front/back localization performance can be explained based on sensitivity to the changing acoustic cue information and that the temporal dynamics observed in the passive and active conditions are not a signature of auditory-vestibular integration.
4.1.4
Clinical Relevance
A study that examined the role of the vestibular system in a whole-body motion
discrimination task demonstrated that patients with complete bilateral vestibular ablation had a significantly higher average threshold measurement than those without vestibular ablation (Valko et al., 2012). As demonstrated in this present study, when the vestibular system is not influencing the auditory system with information about head movement, sound localization in the front and back dimension is inaccurate for normally hearing listeners. Thus, it is likely that in clinical populations that demonstrate vestibular difficulty, they will not demonstrate accurate sound localization performance since the auditory system is not receiving information about head movement from the vestibular system.
4.2 Limitations
A possible limitation of this study is that the participants’ knowledge of the experimental setup in the counter condition could have informed the participants that their head was not moving in space, which may have reduced the influence of proprioceptive input they were generating. Future work might involve using a robotic apparatus that would allow for the relationship between head-on-body and head-in-space motion to be manipulated without the participant’s knowledge in order to provide less contextual information about what their heads were doing in space.
Another limitation was that participants felt fatigued particularly in the slowest
head/body rotation conditions at 25o/s. Future work may incorporate more break times and/or modify the number of testing sessions.
4.3 Future Work
Recent findings have demonstrated that optokinetic stimulation alters sound lateralization sensitivity during interaural time difference discrimination (Otake, 2006). While it has been postulated that visual stimulation may play a role in dynamic localization, whether they are sufficient are unknown. Future research might incorporate the use of a head- mounted display to create an optokinetic field for a listener whose head will be stationary. If the visual input is sufficient for the auditory system to accurately interpret dynamic localization cues, the perception of the front/back location of a sound stimulus should change if the optical rotation created by the display is reversed in direction.
Future research might also attempt to determine the relative dominance between the vestibular and proprioceptive cues in the interpretation of dynamic localization cues by counter-rotating a listener’s body at twice the speed to that of their head rotation as to produce conflicting vestibular motion cues and proprioceptive input from the neck.
The addition of participants with vestibular impairment in future studies might provide additional information about the influence of cues from the vestibular system during localization during head movement. It has been hypothesized above that vestibular impaired populations will not be able to accurately localize front and back sound sources, particularly in the passive condition where only the vestibular system is involved because the auditory system would not know where the head is and what it is doing in space during movement. If so, this would reflect the necessity of the vestibular system for accurate sound localization during head movement. However, if such participants
demonstrate accurate performance in the active and/or counter condition, this may further indicate plasticity in the relative weighting between the two sensory cues, which may reflect that the proprioceptive system may compensate for the lack of vestibular input. Such results may be helpful in advising vestibular impaired populations on the
importance of manipulating active head movements to aid with accurate sound localization.
4.4 Significance
Dynamic binaural sound localization plays an important role in our ability to make sense of the sounds that occur around us even in noisy or reverberant environments. It is a complex phenomenon, requiring normally hearing listeners to acquire a sense of auditory space by combining multi-modal information from their surroundings to accurately localize a sound source even during head movement. As opposed to previous studies that examined the biasing effects of the vestibular and proprioceptive systems during static sound localization, the present study is the first to successfully demonstrate the roles of these systems in the direct context of dynamic sound localization and may offer more reliable conclusions.
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Curriculum Vitae
Name: Janet Kim
Post-secondary McMaster University Education and Hamilton, Ontario, Canada
Degrees: 2007-2011 B.Sc.
Honours and Awards:
GTA Union Academic Achievement Scholarship 2013
Health and Rehabilitation Sciences Travel Award 2012
Faculty of Health Sciences Travel Award 2012
GTA Union Academic Achievement Scholarship 2012
Western Graduate Research Scholarship 2011
Canadian Stroke Network Summer Studentship – Declined Offer 2011
Baycrest Research Studentship 2011
Related Work Research Assistant
Experience Department of Medical Biophysics Lawson Health Research Institute Western University
London, Ontario, Canada 2011-2012
Research Assistant
Department of Neuroscience Rotman Research Institute University of Toronto Toronto, Ontario, Canada 2011
Research Assistant, MAPLE Lab
Department of Psychology and Integrative Neuroscience Discovery
McMaster University Hamilton, Ontario, Canada 2009-2011
Research Assistant
Department of Psychology York University
Toronto, Ontario, Canada 2010
Research Assistant
Department of Urology and Surgical Oncology Juravinski Cancer Centre
McMaster University Hamilton, Ontario, Canada 2009
Teaching Assistant
Department of Communication Sciences and Disorders Western University
London, Ontario, Canada Fall 2012
Teaching Assistant
Department of Health Sciences Western University
London, Ontario, Canada Fall 2011
Poster Presentations:
Kim, J. & Macpherson, E.A. Integration of Auditory Input with Vestibular and Neck Proprioceptive Information in the Interpretation of Dynamic Sound Localization Cues.