The auditory system needs to be able to analyse complex acoustic waveforms. Many ecologically relevant sounds, for example speech and animal calls, vary over time. This thesis investigates how the auditory system processes sounds that occur sequentially. The focus is on how the responses of neurons in the primary auditory cortex ‘adapt’ when there are two or more tones. When two sounds are presented in quick succession, the neural response to the second sound can decrease relative to when it is presented alone. Previous two-tone experiments have not determined whether the frequency tuning of cortical suppression was determined by the receptive field of the neuron or the exact relationship between the frequencies of the two tones. In the first experiment, it is shown that forward suppression does depend on the relationship between the two tones. This confirmed that cortical forward suppression is ‘frequency specific’ at the shortest possible timescale. Sequences of interleaved tones with two different frequencies have been used to investigate the perceptual grouping of sequential sounds. A neural correlate of this auditory streaming has been demonstrated in awake monkeys, birds and bats. The second experiment investigates the responses of neurons in the primary auditory cortex of anaesthetised guinea pigs to alternating tone sequences. The responses are generally consistent with awake recordings, though adaptation was more rapid and at fast rates, responses were often poorly synchronised to the tones. In the third experiment, the way in which responses to tone sequences build up is investigated by varying the number of tones that are presented before a probe tone. The suppression that is observed is again strongest when the frequency of the two tones is similar. However, the frequencies to which a neuron preferentially responds remain irrespective of the frequency and number of preceding tones. This implies that through frequency specific adaptation neurons become more selective to their preferred stimuli in the presence of a preceding stimulus.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:514805 |
| Date | January 2009 |
| Creators | Scholes, Chris D. |
| Publisher | University of Nottingham |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://eprints.nottingham.ac.uk/10814/ |
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