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Binaural masking release is a psychophysical phenomenon whereby the binaural properties of a sound signal (ie., the relationship of the sound reaching the two ears) can alleviate the masking effect of background noise. The largest release from masking occurs when the waveform of either the signal or the masker is inverted at one ear. The signal detection threshold for this so-called "antiphasic" condition can be as much as 12- 15 dB lower than the threshold for the condition in which both the signal and the masker are identical at the two ears (referred to as "homophasic" condition). The difference between the homophasic and antiphasic thresholds is known as the binaural masking level difference (BMLD). The aim of this thesis is to explore the neural mechanisms of binaural masking release in both humans and other mammals (guinea pigs). In the first experiment, electroencephalographic (EEG) techniques were used to develop an accurate and objective neural correlate of binaural masking release. A linear relationship was observed between EEG response size and signal level for stimuli presented in the homophasic and antiphasic configurations. Backward extrapolation of a linear regression function to zero response size gave a remarkably accurate estimate of the psychophysical detection thresholds. This held true not only for the average data, but also on an individual basis for most participants. This measure was then used to test the underlying assumption of models of binaural masking release. The Equalisation-Cancellation (EC) model is the dominant psychophysical model of this effect and assumes that the underlying processing is mediated by the responses of neurons tuned to the binaural properties of the noise. An 2 interaural time difference of ± 500 liS was imposed on the entire stimulus so that the signal was perceived as clearly lateralised towards one hemifield or the other. The results showed that the hemispheric distribution of the neural responses reflected the perceived lateralisation of the signal (Le., greater activity in the hemisphere contralateral to the signal's lateralisation). In contrast to the EC model, these data suggest the global neural response to binaural masking release conditions is dominated by the activity of neurons sensitive to the signal properties. The final experiment was designed investigate whether the neuronal responses measured globally in the EEG experiments reflect the coincidence-detecting behaviour observed in binaurally sensitive neurons in the midbrain. Single unit responses were measured in the primary auditory cortex of urethane anaesthetised guinea pigs to homophasic and antiphasic binaural masking release conditions. Individual auditory cortex neurons could signal the presence of the tone in the masking noise by either an increase or a decrease in discharge rate, in a manner generally consistent with their interaural time difference sensitivity. Across the sample of auditory cortex neurons the magnitude of the BMLD was of the order of that shown psychophysically in humans. A simple cross-correlation function was capable of explaining much of the data collected here. This physiological data suggests the coincidence-detecting behaviour of midbrain neurons is faithfully projected to the cortex. The EEG data showed that neurons sensitive to the ITD of the signal are recruited in binaural masking release processing. The combination of these results is not compatible with the current assumptions of the EC model. 3 |
