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Abstract:
Abstract: Spoken-word recognition requires encoding of
linguistic relevant 'information bearing elements' (Suga, 1994) of
the acoustic speech signal such as formant transients, i.e.,
relatively rapid shifts of spectral energy distribution. Formant
transients cue perception of stop consonants preceeding vowels,
e.g., /ga/ or /da/. Dichotic listening studies indicate higher
proficiency of the left hemisphere in processing these acoustic
features (Tallal, 1993). In order to further delineate the
underlying neurophysiological mechanisms, evoked magnetic fields in
response to stop consonant vowel (CV) syllables (oddball design)
were recorded using a whole-head device (CTF, 151 channels) under
visual distraction vs reaction to prespecified syllables. In the
first experiment, deviants (CV syllables) were applied in a
dichotic manner. The subgroup of individuals exhibiting a right ear
advantage (REA) during a preceeding behavioral test showed, among
other, enlarged mismatch fields to right ear deviant transients. A
second experiment considered the duplex perception paradigm
(Liberman, 1996) in order to investigate specific linguistic
processes. In addition to the pre-attentive REA for transients,
attending to syllables, a distinct left hemispheric mismatch source
with higher ipsilateral sensitivity is localized
posterior-laterally to the primary auditory cortex. The results
suggest parallel mechanisms of transient detection and syllable
fusion at the left hemisphere.
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