Interaural Timing Differences (ITDs) are a cue for sound localisation. In response to low-frequency sounds, sensitivity to ITDs can be conveyed by the fine-structure of the sound waveform. In response to high-frequency sounds, sensitivity to ITDs can only be conveyed by the amplitude modulated envelope of the sound waveform. Sensitivity to ITDs within high-frequency sounds has classically been described as poorer than in response to low-frequency sounds. However, using a "transposed" sound stimulus, it has been shown that human sensitivity to ITDs in high-frequency sounds can be equivalent to sensitivity to ITDs in low-frequency sounds. In the present study, sensitivity to ITDs was investigated in the responses of neurons from the Inferior Colliculus of the guinea pig using transposed, and conventional, stimuli. A neural correlate of the improvement in sensitivity to ITDs provided by transposed tones was found. ITD-tuning functions had greater depths of modulation in response to transposed tones as compared to conventional stimuli, and neural discrimination thresholds for ITDs in transposed tones were similar to those obtained in response to low-frequency tones. Neural coding of ITDs at low frequencies has been shown to depend on a neuron's frequency tuning. Therefore, the responses of neurons were examined for evidence of frequency-dependent tuning to ITDs in the envelope of high-frequency stimuli. The frequency-dependent ITD-tuning that was found contradicts a model of ITD coding proposed in 1948 by Jeffress. ITD-coding at high-frequencies, similarly to at low- frequencies, may use a population of neurons which are broadly tuned to ITDs. It is suggested that sensitivity to ITDs in the envelope of high-frequency sounds is restricted both by peripheral processing and also by an upper fm above which sensitivity to ITDs does not occur. For these reasons, the physiological relevance of sensitivity to ITDs in the envelope of high-frequency may be limited.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:718999 |
| Date | January 2006 |
| Creators | Griffin, S. J. |
| Publisher | University College London (University of London) |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://discovery.ucl.ac.uk/1445561/ |
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