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The Journal of Neurophysiology Vol. 87 No. 1 January 2002, pp. 423-433
Copyright ©2002 by the American Physiological Society
Leibniz Institute for Neurobiology, 39118 Magdeburg, Germany
Brechmann, André,
Frank Baumgart, and
Henning Scheich.
Sound-Level-Dependent Representation of Frequency Modulations
in Human Auditory Cortex: A Low-Noise fMRI Study. J. Neurophysiol. 87: 423-433, 2002. Recognition of sound
patterns must be largely independent of level and of masking or jamming
background sounds. Auditory patterns of relevance in numerous
environmental sounds, species-specific vocalizations and speech are
frequency modulations (FM). Level-dependent activation of the human
auditory cortex (AC) in response to a large set of upward and downward
FM tones was studied with low-noise (48 dB) functional magnetic
resonance imaging at 3 Tesla. Separate analysis in four territories of
AC was performed in each individual brain using a combination of
anatomical landmarks and spatial activation criteria for their
distinction. Activation of territory T1b (including primary AC) showed
the most robust level dependence over the large range of 48-102 dB in
terms of activated volume and blood oxygen level dependent contrast
(BOLD) signal intensity. The left nonprimary territory T2 also showed a
good correlation of level with activated volume but, in contrast to
T1b, not with BOLD signal intensity. These findings are compatible with
level coding mechanisms observed in animal AC. A systematic increase of
activation with level was not observed for T1a (anterior of Heschl's
gyrus) and T3 (on the planum temporale). Thus these areas might not be
specifically involved in processing of the overall intensity of FM. The
rostral territory T1a of the left hemisphere exhibited highest
activation when the FM sound level fell 12 dB below scanner noise. This
supports the previously suggested special involvement of this territory
in foreground-background decomposition tasks. Overall, AC of the left
hemisphere showed a stronger level-dependence of signal intensity and
activated volume than the right hemisphere. But any side differences of
signal intensity at given levels were lateralized to right AC. This
might point to an involvement of the right hemisphere in more specific
aspects of FM processing than level coding.
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