Auditory
thresholds and the EEG alpha rhythm excitability cycle
M.Phil. (Master of Philosophy)
University of Hull, Hull, U.K.
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Abstract
This study is an attempt to clarify the relationship between the
hypothesised alpha rhythm excitability cycle and auditory thresholds.
Previous reports have shown awareness enhancement of external stimuli at
certain points of the alpha cycle but some report this effect on the ascending
phase and others on the descending phase of the alpha cycle. These
inconsistencies could be explained in terms of inadequate control of the
presentation of stimuli at particular alpha phases. In this experiment an on-line
computational method was used to identify the EEG alpha rhythm allowing
presentation of stimuli at predetermined phases of the alpha cycle.
Conventional estimation of thresholds, the method of limits, of auditory
10 ms pulses at 1000 Hz on a 50 db auditory mask on the ascending and
descending alpha phases showed that the alpha excitability cycle does affect
auditory thresholds but only when an incrementing series of low amplitude
auditory stimuli are used. Auditory thresholds are lower on the descending than
on the ascending phase of the alpha cycle.
The decrementing auditory stimuli (high to low loudness) did not show
any auditory threshold differences between ascending and descending phases.
This is explained in terms of a defence reflex response. That is, loud stimuli
elicited defence reflex whose function may be to limit the perception of loud
noise.
It was also found that as the subjects become more proficient their
alpha band power increased during the first session but decreased during the
second to a level similar to that one found at the end of the first session.
Suggestions for further research are mentioned to encourage further work
on this almost unknown topic.
The
Whole Project (In Acrobat .PDF
File)