[Eeglablist] ICA Misinformation

Ramesh Srinivasan r.srinivasan at uci.edu
Wed Jun 14 16:31:12 PDT 2017


Hi Bob

I certainly think your suggestion to "delete the one second of data" is 
preferable, if thats all there were to it.  But there are two additional 
things to consider -

(1) Many EEG studies are in populations, or use experimental paradigms, 
where inevitably there will be an eye-blink/eye-movement on a large 
fraction of trials.

(2) I believe manual editing as you describe is also highly subjective, 
as is the selection of ICA components to remove.  We usually only remove 
the eye-movement component.

In my opinion, there really is no such thing as artifact-free data, 
except for narrow-band signals like SSVEPs.

ramesh


On 06/14/2017 03:15 PM, Robert Thatcher wrote:
> Ramesh,
>     Thank you for your post and I agree that artifact is broad-band 
> and superimposed on many if not all of the EEG channels. 
>  Reconstruction therefore will necessarily change relative phase which 
> can be seen in the waves themselves and is accumulative in the average 
> phase differences between channels.  As for your concern "It's not 
> obvious to me to prefer the original relative phase with the artifact 
> components." I believe that you should have no concern because the 
> original phase differences that are artifact free are real and 
> produced by the underlying physiology and represent the summation of 
> LFP due to synaptic rise times and synaptic integration times and 
> conduction velocities between groups of neurons in networks of the 
> brain.  The original phase differences must be preserved and not 
> altered in any manner if one wants to study brain networks and dynamics.
>
> The simple solution is to not use ICA for artifact rejection and 
> instead use algorhythms to delete the parts of the record that have 
> artifact and retain the parts of the original record with no artifact. 
>  Because of the stochastic and nonstationarity of the EEG as one 
> increases the sample size then one converges toward the stable and 
> reproducabe average of the instantaneous phase differences between 
> channels that is not corrupted by artifact.  ICA reconstruction alters 
> an entire 5 minute EEG recording even if there is ony a single 1 
> second of eye movement artifact.   Why not simply delete the one 
> second artifact and then work with the remaining 4 minutes and 59 seconds?
>
> ICA is excellent for feature detection and can serve as "seeds" to 
> guide further cross-spectral analyses only if the phase differences in 
> the original recording are preserved.
>
> Bob
>
>
>
>
> On Wednesday, June 14, 2017, 4:46:34 PM EDT, Ramesh Srinivasan 
> <r.srinivasan at uci.edu> wrote:
>
>
> Hi All -
>
> I think Bob is right that the relative phase will be changed by 
> deleting 1 or 2 artifact components. Any artifact is broad-band and 
> hence has components in each frequency bin.  When reconstructing the 
> (in this example, 19) channels, the relative phases will change 
> because some of the signal in each frequency bin has been removed when 
> using only 17 or 18 components.
>
> The open question is whether the original relative phase or the 
> ICA-corrected relative phase is the better estimate of the relative 
> phase between the populations that contributed to each electrode. It's 
> not obvious to me to prefer the original relative phase with the 
> artifact components.
>
> Part of the problem for me (and I do use EEGLAB's ica) about 
> identifying components as artifact in the ICA is that I don't think 
> they contain just the artifact, they also contain some genuine brain 
> activity that we are removing.  This bothers me, but I don't know a 
> better solution.  Even the case of the eye-movement artifact 
> components is likely a mixture.
>
> I'd like to see this discussion move away from algorithm to this 
> harder question about artifact removal.
>
> ramesh
>
>
> On 06/14/2017 10:43 AM, Robert Thatcher wrote:
>
> Iman,
>
> Thank you for the information.  I could only find a power point 
> attachment of a simulation in your post.  I did not find a scientific 
> publication where you compared the phase differences changes between 
> an original EEG recording and a ICA reconstruction after removing one 
> or more components.   Please re-send your study. Also please give the 
> citation to any of your publications or other’s publications where 
> phase differences were compared between the original EEG recording and 
> post ICA reconstruction.  It will be interesting to see if you found 
> similar changes like in the study by Montefusco-Siegmund et al or by 
> Georges Otte or even in the example pre vs post data files that you 
> can download from the internet.  I am assuming that you have 
> downloaded the EEG data and then used a JTFA like the Hilbert 
> transform or even the FFT cross-spectrum to prove to yourself that the 
> phase differences between the original and the ICA reconstruction have 
> not been preserved.
>
> As for the mathematics concerning reconstruction from a lower 
> dimensional matrix to a higher dimensional matrix where there are no 
> simple linear transforms I refer you to Taken’s theorem where “The 
> reconstruction preserves the properties of the dynamical system that 
> do not change under smooth coordinate changes, but it does not 
> preserve the geometric shape of structures in phase space.”  Also, in 
> standard differential geometry math courses the issue of lower 
> dimensional manifold mapping to higher dimensional manifolds shows a 
> loss of information in all cases.  Also, commonsense operates here 
> where one tries to reconstruct 19 channels of EEG using only 15 or 16 
> or 17 ICA components hence a loss of information.
>
> Finally, the brain is not a total chaotic organ.   As demonstrated by 
> many scientists (e.g., Nunez; Walter Freeman; Roberto-Pascual Marqui; 
> E. Roy John; Joel Lubar; etc) coherence and phase differences are well 
> behaved and highly reproducible within and between subjects.  
> Coherence and phase are dependent on the number and strength of 
> connections between groups of neurons.  Here is a URL to a study that 
> tested Paul Nunez’s two-compartmental model of Coherence and Phase 
> Differences and found that these measures vary as a function of 
> distance and packing density:
>
> http://www.appliedneuroscience.com/TWO-COMPARTMENTAL_MODEL_EEG_COHERENCE.pdf
>
> Here is a url to a study that used EEG LORETA correlations to 
> replicate Diffusion Tensor Imaging measures of connectivity in the brain:
>
> http://www.appliedneuroscience.com/DTI-ThatcherHumanBrainmapping.pdf
>
> Here is a url to a study that measured phase lock and phase shift 
> duration from birth to about 16 years of age in 458 and where phase 
> differences were stable and well behaved: 
> http://www.appliedneuroscience.com/PhaseresetDevelopment.pdf
>
> If you do a search of the National Library of Medicine database 
> (Pubmed) using the search terms “EEG coherence” you will find 2,874 
> citations.  There is huge consistency in this vast literature which 
> would be impossible if the brain was totally chaotic.
>
> Best regards,
>
> Robert
>
>
>
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