PhD Project 3
Language regions in Interaction: An investigation of directional connectivity in the human language system using laminar fMRI
(last update 2019-06-27)
This ambitious project will examine the interaction between temporal cortex and Broca’s area during language comprehension using event-related fMRI at 7T. For each area data will be acquired at sufficient spatial resolution to measure the fMRI signal as a function of cortical depth. This will allow us to interrogate feed-forward and feedback connections between regions. Four experiments of increasing linguistic complexity will be performed under the hypothesis that as unification demands increase, so will the strengths of feedforward and feedback interactions.
Directed connectivity measured noninvasively during language processing
Team members: Sharoh, Van Mourik (DCCN), Bains (DCCN), Segaert (UoB), Weber (MPI), Hagoort, and Norris
Laminar resolution, functional magnetic resonance imaging (lfMRI) is a non-invasive technique with the potential to distinguish top-down and bottom-up signal contributions on the basis of laminar specific interactions between distal regions. Hitherto, lfMRI could not be demonstrated for either whole-brain distributed networks or for complex cognitive tasks. This project has shown that lfMRI could be used to reveal whole-brain directed networks during word reading. Distinct, language critical regions were identified based on their association with the top-down signal stream and establish lfMRI for the non-invasive assessment of directed connectivity during task performance.
The sensitivity of adjacent depth bins to distinct regions in distributed networks was the most important finding. The unique connectivity profiles associated with each depth bin empirically demonstrated the ability of laminar fMRI to non-invasively identify directed networks. This outcome is attributed to previously unknown characteristics of depth dependent gradient echo BOLD in combination with gPPI analysis. The findings are direct evidence of top-down interaction between language critical regions in the temporal cortex and the iOTS, relative to the iOTS. Information flow between the temporal and occipital regions is therefore best characterized in terms of top-down rather than bottom-up signalling, a finding which provides insight into the functional role of these regions during word reading. Beyond the unique contribution to the neurobiology of language, the ability to measure directed connectivity has far-reaching consequences for expanding our knowledge of brain networks and the processes they implement.
Words against pseudo-words gPPI for the deep bin. Shown in red: the deep bin preferentially targets left lateralized, language critical regions during word reading. p uncorr = 0.001, α = 0.05, n = 21
This project provided a foundation to observe the brain in unprecedented detail during processes critical to language. Previously limited to invasive recordings, it was not possible to simply measure brain function in large populations and conclude the directionality of observed brain networks from the observations alone. This project shows that laminar fMRI may be useful in the study of neurocognitive phenomena throughout the brain, in basic systems neuroscience, and that it has potential applications in neuroanatomy and in merging the foundational assumptions of cognitive neuroscience and neuroanatomical research. With respect to language specifically, it has been learned that a brain network critical to reading relies extensively on bidirectional information transfer and that information related to word meaning dominates the top-down component of the bidirectional signaling. This informs us of the nature of reading processes, and also leaves clues for the study of other language phenomena implemented by multidirectional brain networks.
In 2018 this project was concluded and described in a PhD Thesis currently under review.
This project involves close collaboration between cognitive neuroscientists, language experts and physicists. It will be the first time that layer specific fMRI has been demonstrated outside of the primary cortices, and the first time that measures of directional connectivity will have been obtained with this technique.