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Abstract:
Auditory temporal processing deficits, associated with the
ordering of brief stimuli presented in rapid succession, have
recently been linked to dyslexia. Here, we provide evidence to
support the hypothesis that such temporal ordering deficits occur
due to an abnormal representation of rapidly sequential and
successive stimuli. We measured evoked magnetic fields from seven
adult dyslexics exhibiting such auditory temporal ordering
deficits. Subjects were selected based on the criterion that their
inter-stimulus interval (ISI) threshold for consistent ordering of
brief tone pairs was greater than 50ms (normal adult thresholds are
0-5 ms). Subjects were asked to indicate the order of tone (20ms
duration, 0.8 or 1.2 KHz) pairs presented at ISIs above and below
their discrimination threshold. While performing this ordering
task, magnetic fields were measured from primary auditory cortical
areas using dual 37-channel magnetometers (BTI Magnes system).
Evoked responses were determined by averaging responses to tones
presented at identical ISIs. All subjects had a normal response to
isolated tones (400ms duration). However, the response to the
second tone of a pair, at an ISI below their discrimination
threshold, was weaker than those observed in normals.
Interestingly, in three subjects, the response to the second tone
of a pair was absent at an ISI below their discrimination threshold
and only emerged at longer ISIs.
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