Abstract
A common problem in magnetoencephalographic (MEG) and electroencephalographic (EEG) experimental paradigms relying on the estimation of brain evoked responses is the lengthy time of the experiment, which stems from the need to acquire a large number of repeated recordings. Using a bootstrap approach, we aim at reliably reducing the number of these repeated trials. To this end, we assessed five variants of non-parametric bootstrapping based on the classical signal-plus-noise model constituting the foundation of signal averaging in MEG/EEG. We explain which of these approaches should and which should not be used for the aforementioned purpose, and why. We present results for two advocated bootstrap variants applied to auditory MEG data. The ensuing trial-averaged magnetic fields served as input to the estimation of cortical source generators, with spatio-temporal matching pursuit as an example of an inverse solution technique. We propose, for a wide range of trial numbers, a general framework to evaluate the statistical properties of the parameter estimates for source locations and related time courses. The proposed bootstrap framework offers a systematic approach to reduce the number of trials required to estimate the evoked response. The general validity of our findings is neither bound to any particular type of MEG/EEG data nor to any specific source localization method. Practical implications of this work relate to the optimization of acquisition time of MEG/EEG experiments, thus reducing stress for the subjects (especially for patients) and minimizing related artifacts.
Highlights
M AGNETOENCEPHALOGRAPHY (MEG) [1]–[3] and electroencephalography (EEG) [3], [4] are non-invasive techniques for studying the activity of the human brain with high temporal resolution of the order of milliseconds
Due to the minute amplitude of the brain response evoked by a single stimulus, the signalto-noise ratio (SNR) of a single-trial MEG or EEG recording is poor compared to the background activity, which encompasses all other processes not time or phase locked to the external stimulus
The results are pairwise broken down into rows, one for the right (RH) and one for the left (LH) hemisphere; this order—first RH, LH—reflects the fact that spatio-temporal matching pursuit (STMP) addresses the stronger response first, as it is driven by an energetic criterion [21]
Summary
M AGNETOENCEPHALOGRAPHY (MEG) [1]–[3] and electroencephalography (EEG) [3], [4] are non-invasive techniques for studying the activity of the human brain with high temporal resolution of the order of milliseconds. Due to the minute amplitude of the brain response evoked by a single stimulus, the signalto-noise ratio (SNR) of a single-trial MEG or EEG recording is poor compared to the background activity, which encompasses all other processes not time or phase locked to the external stimulus. In the conventional analysis of stimulus-evoked brain activities, the single-trial responses are arithmetically averaged time-locked to stimulus onset
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