NeuroGuide exports frequency domain and time domain edits of 19 channel x 256 point digital EEG in microvolts (or uv^2) in the Lexicor electrode order and using the Key Institute T-Matrix. Rows are 256 microvolt time points and the columns are 19 channels at a sample rate of 128 thus producing 0.5 Hz resolution from 1 to 30 Hz. 1 Hz increments in the LORETA viewer are computed as the sum of adjacent 0.5 Hz bins and thus comparisons to the Key Institute programs using the time series of 256 points must also sum adjacent 0.5 Hz current source density in order to obtain approximately the same absolute values (i.e., Key Institute equations and NeuroGuide’s use of the Key Inst. equations). A tutorial example of how exact spatial localization and approximately (differences in windowing, filtering, referencing are undocumented in the Key Institute equations) are approximately the same uA^2/m^2/Hz in 60 increments using the LORETA Viewer.
Time series ASCII files are also exported to the Key Inst. LORETA programs. The time domain exports are especially optimized for import using the “EEG cross-spectra > A1EEGs -> 1Spec(aut)” option, which computes 1 single cross-spectral file for each 256 time point. The exported ASCII values can be used as scalar multipliers of the ‘T’ matrix as computed by the Key Institute software which is free to download from the LORETA-KEY software package.
The step by step procedures to link NeuroGuideâ„¢ to the LORETA-Key software is described in the NeuroGuide Manual. The competent use of LORETA is not easy to learn, it does require advanced training and experience. NeuroGuide and Applied Neuroscience, Inc. are not responsible for the Key Institute equations and descriptions published in the Key Institute’s Help menu, nor is Applied Neuroscience, Inc. and/or NeuroGuide responsible for any errors or imperfections in the Key Institute LORETA equations. NeuroGuideâ„¢ accurately implements the LORETA Key Institute equations which can be verified by exporting the time series to the Key Institute and computing the same values directly. For educational purposes, NeuroGuideâ„¢ provides cross-spectral and time domain ASCII output files from the same samples of EEG as well as calibration sine waves from the NeuroGuide Signal Generator program so that a user can mathematically and physiologically validate LORETA for themselves. The NeuroGuide Signal Generation program is also useful to validate LORETA by mathematical simulation of the EEG. NeuroGuide provides easy access and a gateway to this new world of 3-D electrophysiology so that any user of NeuroGuide by visual examination of the EEG/EP scalp distributions and knowledge of known anatomical and physiological properties can validate LORETA and other inverse solutions using NeuroGuideâ„¢ exports.
Q: How do you do the Artifacting to Patch Together Small Segments of EEG?
A: We achieve stable and reproducible FFTs in three steps:
Permit the user to select a minimum of 600 msec selections of artifact free digital EEG samples and then splice together all of the user’s selections as a series of continuous 256 digital values at 128 samples/second (2 second epoch length) of “artifact free” EEG samples;
Remove splice artifact (maximum = 2 per any 256 sample) by padding zeros as EEG data points at t = 0 and then applying a 5th order Butterworth bandpass filter 1 Hz to 40 Hz to baseline and minimize the “discontinuities” of splicing the EEG samples over the entire selection of “artifact free” EEG; and
Use sliding averages of overlapping FFT windows (75% overlap as Kaiser and Sterman, J. Neurotherapy, 2001) so as to minimize the boundaries of windows = 0.
We experimented with different degrees of overlapping windows and different filters and we computed the cross-validated Z scores for each of these experiments. We essentially replicated Kaiser and Sterman’s method to reduce window effects and we achieved quite reliable results no matter what the splice segment lengths with overlapping of the 2 second epochs of EEG for both the normative EEG samples and patient EEG samples. Tests using the worst case scenario of segment splicing of the troughs and peaks of sine waves revealed side band ringing in the FFT which was maximally about 5% of the signal. This worst case scenario is never obtained in practice and the effects of splicing are < 5% by limiting the minimal segment length to 600 msec and by filtering.
The users of NeuroGuide can compare the effects of windowing in the FFT by comparing the ASCII FFT spectral values for overlapping vs. non-overlapping FFTs.
