Activation vs. baseline

Tom Holroyd (NIH/NIMH) [E] tomh at KURAGE.NIMH.NIH.GOV
Mon Feb 27 16:57:31 CET 2006

Pascal Fries wrote:
> OK. I had been asking, because coherence (and also power) have a bias
> that is sample size dependent. For those measures, particularly for
> coherence, I would suggest to use equal sample sizes.

Correct me if I am wrong, but this bias is due to the slow convergence of the covariance; with more samples, the covariance converges more (assuming it converges at all).  Furthermore it probably converges 'up', so if you have more samples, you'll see (apparently) more power.

I have sometimes dealt with this problem in the past by simply z-scoring the volumes afterwards, thus shifting the means back to zero.  But perhaps there are skewness issues as well?  I've mostly done that when playing around with things like overlapping windows; for example, using an active window of 0 to 300 ms and a control window of 0 to 250 ms, you expect a slightly higher power in the active state because of the increase convergence, but then normalizing that spatially can give you an estimate of active areas for that 250-300 ms window.  There may also be problematic beamformer overlap in this case, though, but that can be dealt with too.  Just a thought.

Dr. Tom Holroyd
The 9th Amendment of the U.S. Constitution:
"The enumeration in the Constitution, of certain rights, shall not be
construed to deny or disparage others retained by the people."

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