ThalamicAttention · The role of human thalamus in selective attention via novel denoising applied to magnetoencephalography

The role of human thalamus in selective attention via novel denoising applied to magnetoencephalography

Objective 1. Source localization method development through improving volume conduction estimates (forward model) and denoising signal with SVD during beamforming (inverse model). ThalamicAttention set out to achieve MEG source localization methods development via improvements of the inverse model (i.e. algorithms projecting the MEG sensor signal to source space) and the forward model (i.e. modelling the signal from source space to MEG sensors). The forward model is an essential ingredient of the inverse model, thus improvements of both contribute to obtaining more accurate source localization results with MEG. At the heart of the inverse model improvement is the idea of using a modified version of conventional beamforming (i.e. eigenspace beamforming) algorithm that was designed to restrict source localization of the signal to Principal Component Analysis (PCA) components that are not considered noise. An obstacle of using this method was being able to separate actual signal and noise components. To this purpose I used a denoising method that can threshold PCA or SVD components of covariance matrices based on the Marchenko-Pastur distribution (MP-denoising) which in turn can be fed into the modified (i.e. eigenspace) beamformer. To account for the high dimensional nature of typical electrophysiological data, I also set out to implement a version of MP-denoising, tensor SVD MP-denoising whereby the MP-denosing is carried out on a 3D (or higher) dimensional matrix (i.e. tensor). Objective 2. Tapping into a range of thalamic signals (ASSR, VSSR and α-band) through improved SNR via the application of the developed source localization method with MEG data. This objective was aimed at tapping into thalamic signals in recorded experimental MEG data. The experiment was designed specifically to provide data as closely to achieving a ground truth signal as possible. The experiment used a set frequency auditory (43Hz) and visual (24Hz) stimulation which is well known to result in a frequency following response at the same frequency in the respective sensory cortices (thus establishing ground truth of primary signal processing). Importantly, it is also known to have a similar frequency following response in sensory thalamic nuclei. In addition, the visual and the auditory sensory pathways are well mapped out from ear/retina to sensory areas with established time delays across the pathways. As a result, we can have very clear expectations regarding area, frequency and time delay of the signal. Objective 3. Examining cross-modal attention effects in the thalamus on ASSR, VSSR and α-band signal via the application of the developed source localization method with MEG data. The experimental data also served the purpose of assessing if attention directed towards auditory versus visual stimulation results in detectable differences at any particular brain region and especially in the thalamus as measured with MEG. The conceptual aim was to address whether attention filtering across modalities already starts at the level of the thalamus and to find electrophysiological correlates of such cross-modal attention allocation across the brain in general.

Data: CORDIS, © European Union