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Abstract:
Magnetoencephalography (MEG) is a measure of the magnetic
fields produced by neural activity in the head. Tonotopic mapping
of primary auditory cortex, based on localization of evoked fields
in response to tones, is a well established use of MEG. Recently,
this lab has developed a dichotic listening task, which, in
addition to the 50 millisecond and 100 millisecond responses
typically seen with pure tones, also evokes later responses arising
from areas in close proximity to primary auditory cortex. It has
been established that the later responses, while bilaterally
present, show greater amplitude and localizability in the dominant
language processing hemisphere. One theory for the mechanism of
this activity proposes that the dichotic input is stored very
briefly in a nearby auditory storage area and is subsequently
replayed in auditory cortex until the words from the dichotic input
are recognized. To test this idea we have developed a non-verbal
task designed to activate the same auditory playback mechanism. The
task requires subjects to listen to a series of five randomly
ordered tones, of different frequencies. After a brief delay, a
target tone from the original set is presented. Subjects must then
identify the position of the target tone in the sequence.
Preliminary analyses of data from several pilot subjects indicate
an evoked response to tonal stimulation with waveform
characteristics similar to those seen in the dichotic language
task. We believe this evoked response to be related to the proposed
auditory playback mechanism.
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