Localization accuracy and temporal resolution of MEG: A phantom experiment

Reducing the Number of MEG/EEG Trials Needed for the Estimation of Brain Evoked Responses: A Bootstrap Approach

OBJECTIVE

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.

METHODS

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.

RESULTS

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.

CONCLUSION

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.

SIGNIFICANCE

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.

MEG's ability to localise accurately weak transient neural sources

OBJECTIVE

To investigate the accurate localisation of weak, transient, neural sources under conditions of varying difficulty.

METHODS

Multiple dipolar sources placed within a head-shaped phantom at superficial and deep locations were driven separately or simultaneously by a short-lasting current with varied amplitudes. Artificial MEG signals that were very similar to the human High Frequency Oscillations (HFO) were produced. MEG signals of HFO were also recorded from median nerve stimulation. Different inverse techniques were used to localise the phantom dipoles and the human HFO generators.

RESULTS

The human HFO were measured around 200 and 600Hz by using only 120 trials. The 200Hz HFO were localised to BA3b. The superficial phantom's source was localised with an accuracy of 2-3mm by all inverse techniques (120 trials). The 'subcortical' source was localised with an error of approximately 5mm. Localisation of deeper 'thalamic' sources required more trials.

CONCLUSION

MEG can detect and localise weak transient activations and the human HFO with an accuracy of a few mm at cortical and subcortical regions even when a small number of trials are used.

SIGNIFICANCE

Localizing HFO to specific anatomical structures has high clinical utility, for example in epilepsy, where discrete HFO appears to be generated just before focal epileptic activity.