event-related synthetic aperture magnetometry (samerf)
Post on 19-Jan-2016
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Event-related Synthetic Aperture Magnetometry
(SAMerf)
Outline
• Review of traditional SAM
• Introduction to SAMerf
• Cheyne et al. motor experiment
• Sliding window SAMerf
Review of traditional SAM
• Estimates equivalent current dipole source power within specified frequency bands
• Based on sensor covariance in time windows
• Uses optimal spatial filters to estimate source power on a grid of voxels
SAM analysis of an n-back working memory task
How do we increase temporal resolution?
• Sliding window SAM– Calculate SAM images for small overlapping
time windows
• Virtual channels– Use the SAM spatial filters to estimate time
series
Virtual Channels
How can we increase signal-to-noise in a virtual channel?
• Average - either in the temporal or frequency domain
• Averaging in time will produce an evoked response (and ignore induced activity)
Introduction to SAMerf
• Traditional SAM is performed on time windows and frequency bands of interest
• Virtual channels are created for each voxel
• The virtual channels are averaged to generate an event-related response (ERF)
• Amplitudes of the ERFs at small time windows are used to produce 3D maps
Cheyne et al. motor experimentSpatiotemporal mapping of cortical activity accompanying voluntary movements using an event-related beamforming approach
Douglas Cheyne, Leyla Bakhtazad, William Gaetz
Neuromagnetic Imaging Laboratory, Hospital for Sick Children Research Institute, Toronto, Ontario, Canada
Hum Brain Mapp. 2006 Mar;27(3):213-29.
SAMerf processing stream
Movement-related fields
Single subject
Mean of 8 subjects
Sliding window SAMerf
• Activity is averaged over a small time window
• The averaging window is slid to observe temporal changes
• Good for increasing signal-to-noise and characterizing high frequency bursts
Low frequency component High frequency component
5 Clicks – 250ms ISI 4-6 seconds between click trains
Stimuli
Five Click Auditory Experiment
Time-Frequency analysis using the Stockwell Transform
Sliding window SAM with 50ms windows and 25ms steps Gamma-band: 25-50Hz
left
right
Right IFG
Summary
• Use traditional SAM to find power changes in frequency bands
• Use SAMerf to localize evoked fields and phase-locked activity
Thanks!
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