Abstract
The spatial topology of the human motor cortex has been well studied, particularly using functional Magnetic Resonance Imaging (fMRI) which allows spatial separation of haemodynamic responses arising from stimulation of different body parts, individual digits and even spatially separate areas of the same digit. However, the spatial organisation of electrophysiological responses, particularly neural oscillations (rhythmic changes in electrical potential across cellular assemblies) has been less well studied. Mapping the spatial signature of neural oscillations is possible using magnetoencephalography (MEG), however spatial differentiation of responses induced by movement of separate digits is a challenge, because the brain regions involved are separated by only a few millimetres. In this paper we first show, in simulation, how to optimise experimental design and beamformer spatial filtering techniques to increase the spatial specificity of MEG derived functional images. Combining this result with experimental data, we then capture the organisation of the post-movement beta band (13–30 Hz) oscillatory response to movement of digits 2 and 5 of the dominant hand, in individual subjects. By comparing these MEG results to ultra-high field (7T) fMRI, we also show significant spatial agreement between beta modulation and the blood oxygenation level dependent (BOLD) response. Our results show that, when using an optimised inverse solution and controlling subject movement (using custom fitted foam padding) the spatial resolution of MEG can be of order 3–5 mm. The method described offers exciting potential to understand better the cortical organisation of oscillations, and to probe such organisation in patient populations where those oscillations are known to be abnormal.
Highlights
We develop methodology to optimise the spatial resolution of MEG, and use it to investigate the motortopic organisation of neural oscillations
In what follows we show, both analytically and in simulation, that the spatial resolution of MEG can be improved in cases where experimental design is optimised such that MEG signals, generated at separate cortical locations, can be separated in time, facilitating a MEG spatial filtering methodology with enhanced spatial resolution
In method 3, where sources are segmented in time so that weights are based only on a time window in which Source 1 is active, Source 2 is completely invisible and is not represented in the covariance matrix, it doesn't require minimisation. This means that the beamformer is able to better optimise spatial specificity around the source that is active in the time window selected (Source 1), improving spatial specificity
Summary
The topographic organisation of the sensorimotor cortices is well established (Penfield and Boldrey, 1937), and in recent years functional magnetic resonance imaging (fMRI) has allowed a non-invasive means to map the brain's response to stimulation of different body parts (Maillard et al, 2000; Sakai et al, 1995; Stippich et al, 2002), individual digits of the hand (Francis et al, 2000; Gelnar et al, 1998; Sanchez-Panchuelo et al, 2010), and the within digit functional parcellation of Brodmann areas (Sanchez-Panchuelo et al, 2012) This mapping, which separates functionally specialised brain regions lying only millimetres apart, has been made possible by the high spatial resolution of high- and ultra-high field fMRI. We develop methodology to optimise the spatial resolution of MEG, and use it to investigate the motortopic organisation of neural oscillations
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
