Event related synthetic aperture magnetometry samerf
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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

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Event-related Synthetic Aperture Magnetometry (SAMerf)

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Event related synthetic aperture magnetometry samerf

Event-related Synthetic Aperture Magnetometry(SAMerf)


Outline

Outline

  • Review of traditional SAM

  • Introduction to SAMerf

  • Cheyne et al. motor experiment

  • Sliding window SAMerf


Review of traditional sam

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


Event related synthetic aperture magnetometry samerf

SAM analysis of an n-back working memory task


How do we increase temporal resolution

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


Event related synthetic aperture magnetometry samerf

Virtual Channels


How can we increase signal to noise in a virtual channel

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

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 experiment

Cheyne et al. motor experiment

Spatiotemporal 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

SAMerf processing stream


Movement related fields

Movement-related fields


Single subject

Single subject


Mean of 8 subjects

Mean of 8 subjects


Sliding window samerf

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


Event related synthetic aperture magnetometry samerf

5 Clicks – 250ms ISI

4-6 seconds between click trains

Low frequency components

Low frequency components

Stimuli

Five Click Auditory Experiment

High frequency component

Low frequency component


Event related synthetic aperture magnetometry samerf

Time-Frequency analysis using the Stockwell Transform


Event related synthetic aperture magnetometry samerf

Sliding window SAM with 50ms windows and 25ms steps Gamma-band: 25-50Hz

left

right


Event related synthetic aperture magnetometry samerf

Right IFG


Summary

Summary

  • Use traditional SAM to find power changes in frequency bands

  • Use SAMerf to localize evoked fields and phase-locked activity


Event related synthetic aperture magnetometry samerf

Thanks!


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