Tonotopic Mapping with Automated Characterization (ToMAC)

Chief investigator

Dr Adam McNamara

Study team

Professor Deborah Hall
Dr Julien Besle
K Krumbholz
Susan Francis
Rosa Sánchez-Panchuelo
Professor Richard Bowtell

Funder

NIHR Nottingham Hearing Biomedical Research Unit

Study period

2015-2017

The subdivisions of the human brain that are involved in hearing are organised in such a way that the various frequencies that are picked up by the ear are processed more or less in parallel fashion. Because this separation of sound frequencies occurs in the cochlea of the inner ear already, this organisation is referred to as cochleotopy. The organization persists further into the system and is called tonotopy for example when it is observed in the brain.

It is one of the most prominent features of the auditory system, and it forms a guiding principle in auditory brain research. Nevertheless, the precise layout of the cortical sites where low versus high frequencies are processed has only recently been unravelled and remains poorly understood in humans.

The tonotopic organisation of the brain is also thought to play an important role in various hearing disorders. For instance, following age-related high-frequency hearing loss, those areas of the brain that are tuned to the higher end of the frequency spectrum receive little input from the affected ears. In a process known as plastic reorganisation, the neurons in these areas may shift their response properties to the lower frequencies that have remained relatively intact. Thus, the tonotopic maps in the brain are rearranged, and certain sound frequencies at the edge of the hearing loss can become overrepresented in the brain. This may result in abnormal levels of neural activity or synchronicity in these regions, a mechanism that is thought to underlie tinnitus.

Recent successful work by Nottingham Hearing BRU has developed high-resolution functional magnetic resonance imaging (fMRI) methods that are able to reveal the tonotopic organisation in the auditory cortex of individual subjects (see Langers and colleagues 2014 and Langers and colleagues 2014 (2)). This project builds on that early work. The project attempts to develop a standard, automated characterization of the maps. Specifically an automated process that is useful in two ways.

  1. Be robust enough to permit application of the methods to those with partial hearing loss.
  2. Used to rapidly assess changes in the character of the tonotopic maps over time and between groups in a clearly replicable and hypothesis driven manner.

These techniques will be used in the future to further study the neural changes that take place following hearing loss and to identify the mechanisms that result in the perception of phantom sounds in tinnitus.