Postgraduate Research Project
Title: Measuring tonotopic magnification in the human auditory cortex.
Area: Auditory neuroscience
Methods: Brain imaging, psychoacoustics and computational modelling.
The auditory cortex is tonotopically organised, with neighbouring parts of the cortex responding to neighbouring frequencies. It has been shown in other sensory systems, that the cortical surface allocated to a given division of sensory space is proportional to its behavioural importance. For example, in the visual system more cortical surface is dedicated to the fovea than to the periphery. This cortical magnification function has been well characterised in visual cortex using functional magnetic resonance imaging (fMRI). Tonotopic magnification in the human auditory cortex (the cortical distance spanned by a given frequency) is still unknown but we expect it to follow behavioural relevance as in the visual system.
We will measure auditory cortical magnification in normal-hearing subjects using structural and functional MRI at 7T. Our first goal will be to determine whether cortical tonotopic magnification follows psychophysical measures frequency acuity. In vision, cortical magnification is proportional to position acuity, a measure which is akin to frequency discrimination acuity in the auditory system. Visual fMRI studies also suggest that the cortical surface representing a receptive field (referred to as the point image) is constant within a given functional cortical area, i.e. the cortical magnification and tuning width function are inversely related. We will derive the auditory point image from our estimates of cortical magnification and population tuning width to determine if the same holds in auditory cortex. Finally, we will quantify how cortical magnification changes in different auditory areas and how this relates the areas’ specialism.
We intend to use the measurements from this study to build a model allowing predictions of auditory cortical properties from cortical morphology. One other possible application of our research will be to measure the tonotopic reorganisation potentially associated with hearing loss and/or tinnitus, as measured in animal models but not yet detected by human imaging studies.