Research group: Auditory System and Multisensory Integration.
Dept. Medical Physics and Biophysics, Geert Grooteplein 21.
Supervisor:
prof. dr. John van Opstal
Donders Neurophysics Unit - Dept. Biophysics, HG00.830
Tel: 024-361 4251
Brief description of the Project:

Research question: Which mechanisms allow the auditory system to use the complex spectral shape cues for sound localization?

Background:
Human sound localization relies on the use of implicit cues in the sound waves that enter the ears: (for a detailed treatment, see my web pages (in Dutch) on Localization).

In this project, we will concentrate on the mechanisms underlying the analysis within the auditory system of the spectral shape cues for sound-source elevation. The important point here is, that the sensory spectrum (= frequency content) at the eardrum consists of two, inseparable, contributions:
Sens(ω, ε) = Stim(ω) x Ear(ω, ε), where are both a priori unknown to the auditory system. In mathematical terms, the problem for the auditory system to extract sound-source elevation, ε, from the sensory spectrum is an ill-posed problem!
Yet, human sound localization is quite accurate, and therefore seems to follow a strategy that somehow gets around this fundamental problem. The question, to be studied in this project, is how?

We hypothesize that the auditory system relies on the following two assumptions: The second assumption is easily verified and has been shown to be satisfied for the human ear. The first assumption, if true, leads to an interesting prediction, that will be fully exploited in our project, namely: if a sound in the free field does resemble one of the pinna filters, subjects will perceive the sound-source direction to come from the elevation belonging to that filter, rather than to the actual sound-source direction.

So, if the speaker is at a straight-ahead direction (elevation zero), and the stimulus spectrum is designed, for example, to be:
Stim(ω)=Ear(ω, ε=+30 deg)/Ear(ω, ε=0 deg), the theory predicts that the subject will hear the stimulus from the 'virtual' +30 deg direction, and not from the free-field 0 deg direction.

In this project, we will fully dissociate the free-field source locations from the simulated 'virtual' locations. Our hypothesis predicts that subjects will orient toward the virtual sound source locations, rather than to the free-field locations. Once we have established this, we will corrupt the virtual stimulus by independent noise, to measure the effect on the localization percept. This experiment will allow us to test that (1) broad-band stimuli can be fully mislocalized, and (2) that the entire spectral filter (rather than a particular local spectral feature, like a spectral slope) determines the localization percept.
In a third series of experiments, we will then add more virtual sound-source locations (with different weighting factors), and determine how the subject's localization percept depends on these weighting factors. This experiment will yield important insights into the neural computations that underly the elevation percept. For example, it is expected that when the number of added sources is large, the subject's localization will be more and more determined by the actual free-field location, as the signal will resemble more and more gaussian noise.

Aim: The study will lead to a full peer-reviewed publication in a high-rated journal (e.g. the Journal of Neuroscience) that will be submitted by the end of the project.
Preferred start: any time

Background knowledge:
Master CNSc: The obligatory courses of the Perception and Action program. The elective course on Auditory Perception.
Science (Natuurwetenschappen): IPsychofysica 1, Neurofysica 1.
If you are seriously interested in this project, please contact John van Opstal (adress/tel. see above).
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