Tonotopy

Summary

In physiology, tonotopy (from Greek tono = frequency and topos = place) is the spatial arrangement of where sounds of different frequency are processed in the brain. Tones close to each other in terms of frequency are represented in topologically neighbouring regions in the brain. Tonotopic maps are a particular case of topographic organization, similar to retinotopy in the visual system. Tonotopy in the auditory system begins at the cochlea, the small snail-like structure in the inner ear that sends information about sound to the brain. Different regions of the basilar membrane in the organ of Corti, the sound-sensitive portion of the cochlea, vibrate at different sinusoidal frequencies due to variations in thickness and width along the length of the membrane. Nerves that transmit information from different regions of the basilar membrane therefore encode frequency tonotopically. This tonotopy then projects through the vestibulocochlear nerve and associated midbrain structures to t

Official source

https://en.wikipedia.org/wiki/Tonotopy

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Tonotopic mapping of human auditory cortex

Melissa Saenz

Since the early days of functional magnetic resonance imaging (fMRI), retinotopic mapping emerged as a powerful and widely-accepted tool, allowing the identification of individual visual cortical fields and furthering the study of visual processing. In contrast, tonotopic mapping in auditory cortex proved more challenging primarily because of the smaller size of auditory cortical fields. The spatial resolution capabilities of fMRI have since advanced, and recent reports from our labs and several others demonstrate the reliability of tonotopic mapping in human auditory cortex. Here we review the wide range of stimulus procedures and analysis methods that have been used to successfully map tonotopy in human auditory cortex. We point out that recent studies provide a remarkably consistent view of human tonotopic organisation, although the interpretation of the maps continues to vary. In particular, there remains controversy over the exact orientation of the primary gradients with respect to Heschl's gyrus, which leads to different predictions about the location of human A1, R, and surrounding fields. We discuss the development of this debate and argue that literature is converging towards an interpretation that core fields A1 and R fold across the rostral and caudal banks of Heschl's gyrus, with tonotopic gradients laid out in a distinctive V-shaped manner. This suggests an organisation that is largely homologous with non-human primates. This article is part of a Special Issue entitled . (C) 2013 Elsevier B.V. All rights reserved.

Elsevier2014

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Tonotopic organization is the main topographic feature of the primary auditory cortex. Importantly, tonotopy allows to define borders of cortical areas as reversals of the frequency gradient (i.e the spatial derivative of the preferred frequency map). Very different tonotopic organisations have been described with respect to Heschl’s gyrus in humans. Furthermore, human tonotopy remains difficult to relate to the organization in nonhuman primates and a comparison would require the same measurement, design and analysis in both species. Here, we measured blood oxygen leveldependent signal with functional MRI (fMRI) in humans and monkeys. We applied phase-encoded designs ('travelling wave') and performed surface-based analyses using high-resolution individual anatomical MRIs. Thirteen humans and three rhesus monkeys were scanned during passive listening. Monkeys were scanned at 4.7 Tesla and humans were scanned at 3T and 7T. Two run types with either low-to-high or high-to-low progression of tone bursts were alternated. Crosscorrelation between time-series from both run types were computed and time delay between the two signals revealed the preferred frequency. In monkeys, we found a tonotopic pattern with a posteroanterior axis of high-low-high preference located at the coordinates of A1-R according to monkey atlas. In humans, we also observed a tonotopic pattern of high-low-high preference along an overall postero-anterior axis, running across Heschl’s gyrus. Our findings suggest that the precise 3D representation of the individual cortical surface is critical to better appreciate the organization in both species. This first comparative fMRI study of the tonotopy would contribute to the definition of a unified primate model of core and belt fields (Baumann et al., 2013).

2013

Study of tonotopic brain changes with functional MRI and FDG-PET in a patient with unilateral objective cochlear tinnitus

Melissa Saenz

We studied possible brain changes with functional MRI (fMRI) and fluorodeoxyglucose positron emission tomography (FDG-PET) in a patient with a rare, high-intensity "objective tinnitus" (high-level SOAEs) in the left ear of 10 years duration, with no associated hearing loss. This is the first case of objective cochlear tinnitus to be investigated with functional neuroimaging. The objective cochlear tinnitus was measured by Spontaneous Otoacoustic Emissions (SOAE) equipment (frequency 9689 Hz, intensity 57 dB SPL) and is clearly audible to anyone standing near the patient. Functional modifications in primary auditory areas and other brain regions were evaluated using 3T and 7T fMRI and FDG-PET. In the fMRI evaluations, a saturation of the auditory cortex at the tinnitus frequency was observed, but the global cortical tonotopic organization remained intact when compared to the results of fMRI of healthy subjects. The FDG-PET showed no evidence of an increase or decrease of activity in the auditory cortices or in the limbic system as compared to normal subjects. In this patient with high-intensity objective cochlear tinnitus, fMRI and FDG-PET showed no significant brain reorganization in auditory areas and/or in the limbic system, as reported in the literature in patients with chronic subjective tinnitus.

Elsevier2016