Mapping the Human Auditory Pathway: Computational Models and UHF MRI
Since its introduction, functional MRI (fMRI) has been used to investigate sound processing throughout cortical and sub-cortical areas of the human brain. Compared to conventional magnetic field strengths (3 Tesla), ultra-high field scanners (7 and 9.4 Tesla) allow the acquisition of functional responses with a submillimeter spatial resolution and large coverage. In this talk I highlight how we have leveraged these techniques to investigate the organization of the human auditory pathway at an unprecedented level of detail.We reliably measure fMRI responses to natural sounds from the cochlear nucleus, superior olive, inferior colliculus and medial geniculate body in single subjects. Using computational models of sound processing, we map the functional characteristics (e.g., tonotopic, spectrotemporal modulation preferences) throughout these sub-cortical auditory nuclei. Through high spatial resolution diffusion weighted MRI, we were also able to characterise the anatomical connections between these nuclei and their relation to resting state functional connectivity. We combine functional and anatomical information to parcellate the cortex in auditory fields. With submillimeter data we explore layer-specific processing. We show relatively stable “columnar” tuning for frequency and temporal modulations, but not spectral modulations, in human primary auditory cortex (PAC). Columnarity is lower in non-primary auditory regions. These results, in accordance with previous findings, support the idea that a transformation in spectral modulation processing takes place in the PAC. We observe that the frequency tuning in superficial layers of tonotopic columns sharpens depending on task demands, suggesting that the columnar architecture may play a specific role in the processing of task relevant information. Ongoing work is extending the investigation of layer-specific processing outside primary and secondary auditory cortex. Preliminary data in the lateral superior temporal gyrus and sulcus show that, while acoustic content in natural stimuli may be processed similarly for both synthetic and natural stimuli in middle and deep cortical layers, the semantic information of natural stimuli emerges in superficial cortical layers. To summarise by combining computational modeling with ultra-high field anatomical MRI, we investigate the auditory pathway (and its connectivity) from the cochlea to the auditory cortical subfields. Sub-millimeter fMRI allows us to investigate the nature of the tuning to acoustic features throughout subcortical relays, sound transformations in - and the influence of attention on - the cortical microcircuit of the primary auditory cortex, and finally the emergence of categorical responses beyond sound acoustics in superficial layers of higher order auditory regions.