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Abstract

Introduction: The spatial progression of neuronal best frequency sensitivity across auditory cortex is known as tonotopy, i.e. a topographical representation of sound frequency that arises from mechanical properties of the cochlea and is preserved along the auditory neuraxis. Tonotopy is a fundamental organizing principle of the auditory cortex, and previous studies in primates and lower
mammals successfully relied on tonotopic gradient maps to identify individual fields of primary and secondary auditory cortices. In humans, however, relatively little is known about the detailed tonotopic organization of auditory cortex, which is in contrast to the sophisticated knowledge about the functional architecture of the visual system.
This is mainly due to methodological shortcomings of auditory functional magnetic resonance imaging (fMRI) that arise from the high intensity acoustic noise produced by gradient switching during data acquisition, and which interferes with experimental acoustic stimulation.
Methods: Here, we used fMRI at 3T together with a novel echo planar imaging (EPI) sequence optimized for examination of the auditory system to identify progression of frequency sensitivity in the auditory cortex of humans. This fMRI sequence - by modulating the temporal structure
of the scanner noise - increases the blood oxygenation level-dependent (BOLD) signal-tonoise ratio in auditory regions but still preserves temporal resolution. Subjects were exposed to sound stimuli with varying frequency content
(sine wave tones or bandpassed noise, frequency range 500 - 8000 Hz) that were presented as quasi-continuous frequency sweeps to allow for phase-encoded mapping.
Results: We found a tonotopic representation of sound frequency in primary and secondary auditory areas, which suggested multiple mirrorsymmetric progressions of frequency sensitivity in core and belt areas of human auditory cortex. The individual analysis of each subject demonstrated the great interindividual variability of
tonotopic maps, and, in addition, showed a good
intraindividual reproducibility across sessions.
Conclusion: The results show that phase-encoded mapping of the progression of frequency sensitivity along primary and secondary auditory areas in humans is feasible using dedicated MR sequences. This constitutes a prerequisite for
the delineation of tonotopic map borders (analogous to the segregation of visual fields based on retinotopic maps), which might greatly aid in the characterization of the functions of individual auditory fields in humans.