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Detection of binaural processing in the human brain

Azam, Sami (2016). Detection of binaural processing in the human brain. PhD Thesis, Charles Darwin University.

Document type: Thesis
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Author Azam, Sami
Title Detection of binaural processing in the human brain
Institution Charles Darwin University
Publication Date 2016-03
Thesis Type PhD
Supervisor De Boer, Friso George
Jonkman, Mirjam Elisabeth
Abstract Binaural hearing is the ability to combine the information from both ears in order to detect the location of a sound source or to distinguish a sound from background noise. It may be impaired in people who have suffered from prolonged hearing loss as children, for example due to otitis media. Currently, there is no objective method to detect binaural hearing. Auditory evoked potentials (AEPs), the brain’s response to auditory stimuli, may be used as an objective measure of hearing and auditory processing. Auditory evoked potentials can be recorded using electroencephalography (EEG), but are difficult to detect and distinguish from other brain activity due to their small amplitudes.

The aim of this research is therefore to develop, test and evaluate an approach towards building an objective methodology to detect binaural processing in the human brain using auditory evoked potentials.

To achieve this, the time averaged EEG responses of normal hearing subjects to repeated auditory stimuli were analysed. The stimuli, 500 Hz or 1000 Hz Blackman windowed pure tones, were presented as homo-phasic (the same phase in both ears) or anti-phasic (180 degree phase difference between the two ears) and mixed with various noise conditions. Auditory evoked potentials for homo-phasic and anti-phasic conditions were obtained by averaging 500 trials of in-phase and 500 trials of out-phases of each EEG epoch.

When the auditory evoked potentials were analysed, it was found that the amplitude of the dominant frequency component in the 20 - 50 Hz range of the MLR (middle latency response) of the brain was larger for the anti-phasic condition than for the homo-phasic condition for both 500 Hz and 1000 Hz stimuli. It was also noted that the normalised amplitude differences were larger when phase shifts occurred every few seconds than when phase shift only occurred once per epoch. The effect was more pronounced when the stimuli were embedded in noise, resulting in a higher mean value of the Normalised amplitude difference than for stimuli of pure tones without noise. These results are likely to relate to the psychoacoustic phenomenon known as binaural masking level difference which finds that the detection of a signal in a background of noise is easier when the signal has a different inter-aural phase difference than the noise.

The overall findings of this research indicate that the amplitude of the dominant peak in the 20 - 50 Hz frequency range of the MLR second peak may be used as a marker for binaural processing in the human brain.
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