SensorSpm - Meg Wiki

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3D Sensor SPMs

3D Topography x time sensor SPMs can help you identify the latency and location of effects in sensor space. For example, here's a word-pseudoword effect (N=19) on magnetometers and gradiometers (RMS), thresholded for height (p<.001 unc) and extent (p<.05 corr):

attachment:WdNw3DSensorSpm.jpg

Creating 3D Sensor SPMs

1. Select an averaged file (m*.mat), split it into separate files for magnetometers, gradiometers, and gradiometer-magnitude (m*-mags.mat, m*-grds.mat, and m*-grms.mat, respectively). (Note you can do a similar analysis within a single subject by writing epochs to images).

clear S
S.D='mae_mydata.mat';
spm_eeg_splitFIF_grms(S);

2. Write image volumes.

% Select options (see help for spm_eeg_convertmat2ana3D):
clear S
S.interpolate_bad = 0;
S.n = 32;
S.pixsize = 3;
% Select trial types:
S.trialtypes = [1 2];
% Mags:
S.Fname = 'mae_mydata-mags.mat';
spm_eeg_convertmat2ana3D(S);
% Grad magnitude:
S.Fname = 'mae_mydata-grms.mat';
spm_eeg_convertmat2ana3D(S);

3. Smooth image volumes (optional, but good idea for grms, since distribution of RMS of gradiometer values [over voxels] is not Gaussian, even if by Central Limit Theorem, the distribution of the mean over enough subjects will be).

% Select files (or pass from stage above)
P = spm_select(Inf,'image','Select image file(s)',[],pwd,'.*img');
% Smoothness in x (mm), y (mm) and z (ms)
SmoothKernel = [5 5 10];
for n=1:size(P,1);
           [pth,nam,ext] = fileparts(P(n,:));
           Pout{n}       = fullfile(pth,['s' nam ext]);
           spm_smooth(spm_vol(P(n,:)),Pout{n},SmoothKernel);
           Pin = strvcat(P(n,:),Pout{n});
           spm_imcalc_ui(Pin,Pout{n},'((i1+eps).*i2)./(i1+eps)',{[],[],'float32',0});
end
%The final imcalc step above is just to reinsert NaN's for voxels outside space-time volume into which data were smoothed

4. Then compute an ANOVA over subjects. Either use SPM's GUI, or a script like 'meg_batch_anova.m' which can be found in /imaging/local/meg_misc (or here http://imaging.mrc-cbu.cam.ac.uk/svn/meg_misc/devel/), eg:

% wd is your working directory (eg pwd)
outdir = fullfile(wd,'SensorTimeSPMs');
meg_batch_anova(Pout,outdir);

Viewing 3D Sensor SPMs

To view the results, navigate to the directory that contains the SPM.mat for your ANOVA. In an SPM window, click 'Results' (1), select SPM.mat (2), select contrast Unwhitened EOI (3), and select options: (4) don't mask, whatever title, choose your p-value and extent thresholds (I usually start with .001 and 0).

attachment:Demo3dSpm1Scaled.jpg

Click 'whole brain' (5) to view whole-volume results. The coordinates on the far right (6) show x (mm), y (mm), and time (ms). Ignore the glass brain images (7).

attachment:Demo3dSpm2Scaled.jpg

To view the cloud-in-a-box images, click 'overlays'->'sections', (8) and select mask.img (9). Lovely (10). If you want to write out the image volume, click 'save' and give it a name. You can then view it using 'Display' or 'Check Reg'.

attachment:Demo3dSpm3Scaled.jpg

If you want to correct for extent, rather than clicking 'Results', click 'Toolbox'->'ns'. As above, select SPM.mat, Unwhitened EOI, options: don't mask, whatever title, no p-value adjustment, threshold {p} .001 (or your choice), extent threshold 0. The leftmost column ('cluster-level p-corrected') shows p-values corrected for cluster extent (11). Note that this information was absent when you clicked 'Results'.

attachment:Demo3dSpm4Scaled.jpg

If you want to view or write out an image thresholded for cluster extent, determine what cluster size ('cluster-level k-voxel') needed to achieve your desired p-value ('cluster-level p-corrected'; in the example, 214 gets p=0.052 (12)). Then follow the above 'Toolbox'->'ns' directions, but this time enter the k-voxel extent threshold when prompted (eg., 215 (13)). Now the results are thresholded for both height and extent (14). View and save as above.

attachment:Demo3dSpm5Scaled.jpg

A priori timewindows/sites of interest

Finally, if you have an a priori timewindow and/or an a priori set of sensors where you expect an effect, you can use Small Volume Correction (the SVC button). With this, you can specify a mask image saved from an orthogonal contrast in the same SPM analysis, or from a previous SPM analysis of independent data. Or perhaps more commonly, you can select a 3D "box". For example, say you had an a priori hypothesis that a condition effect would occur from 100 to 200ms poststimulus, then type [0 0 150] into the coordinates on the Input window, press SVC, select "box" and enter [80 80 100] as its dimensions (corresponding to an +/-40mm range around x, +/-40mm range around y, and +/-50ms range around 150ms). The corrected (but not uncorrected) p-values in the Graphics window will now change (get smaller) owing to the reduced search volume. Hopefully some will be <.05 corrected for height or extent!

A note on Gradiometer RMS

Sensor-level SPMs of gradiometer data are normally performed on a scalar value representing the root-mean-square (RMS) - i.e, vector length - of the two planar gradiometers at each location. This is because the gradiometer data reflect in-plane gradients (x,y) in two orthogonal directions, so interpolating raw values for each location does not make any sense. Even if one performed SPMs on each gradient direction separately, this direction varies across locations with respect to the brain, so is not particularly meaningful (and if signal were present in both x and y directions, separate tests may be less sensitive than a combined test).

However taking the RMS is a nonlinear operation so care must be taken. This is because noise will not cancel when averaging the RMS (similar issues apply of course to tests of Global Field Power, GFP, or any rectification). To see this, consider that each measurement, y, is a mixture of signal (s) and noise (n), i.e, y=s+n (under additive noise assumptions). Let's assume, as typical, that the noise is zero-mean and Gaussian. Therefore, when averaging across many trials (observations of y), <n>=0, and <y> approaches the true signal, s (in fact, the Signal-to-Noise ratio, SNR, increases with the square root of the number of trials). However, when you take the RMS (or simply the square, ie power), then the noise no longer cancels across trials (more formally, y² = (s+n)² = s² + s×n + n², and though <n>=0 (and <s×n>=0), n² is not zero). This means that even if there were no signal, the distribution of y across trials would be non-central (i.e, mean greater than zero). Thus one cannot test the null hypothesis that there is no signal (e.g, tests of an evoked peak versus baseline, when original data have been baseline-corrected); likewise, the RMS during the baseline period will be always be greater than zero. One might suggest "re-baseline-correcting" after taking the RMS, but this still makes assumptions about stationary noise, as explained below.

Because the RMS of the noise does not tend to zero with averaging, when comparing the RMS of two observations, where y₁=s₁+n₁ and y₂=s₂+n₂, one has to assume that the noises, n₁ and n₂, are equivalent. When comparing the same peristimulus timepoints across two trial-types, and assuming those trial-types are not confounded by some other variable such as time during experiment (unlikely if randomly intermixed), then the assumption that n1=n2 would seem reasonable. However, when comparing two different peristimulus timepoints within the same trial, e.g, a pre-stimulus (baseline) time (where it is assumed s₁=0 and y₁=n₁) vs a post-stimulus time (y₂=s₂+n₂), the assumption that n₁=n₂ is arguably less tenable, because the noise is unlikley to be stationary, e.g, because of random drifts through the trial.

Even if you think the assumption that n₁=n₂ is valid, it is also important to remember that the distribution of RMS across any observations will be positively-skewed (non-Gaussian). In theory, this would invalidate parametric statistical tests (though some are quite robust, and the skewness depends on the ratio of fixed signal to random noise). Fortunately however, the distribution of the *mean* of any distribution, with enough observations, will approach a Gaussian (by the Central Limit Theorem). Therefore if your statistical tests are over enough subjects, and each subject has multiple trials, then performing parametric tests on the mean-across-trials of the RMS can be valid. Note that this requires that, within each subject, you take the RMS of each trial and then average; not that you calculate the average across trials and then take the RMS (the latter will not produce a Gaussian distribution across subjects).

For further information about this issue, see SensorStats.