[Eeglablist] Baseline power when comparing ERSP in different conditions

[Shumpei Yano]

Dear Experts.

This is Yano. I am a graduate school student in Japan and studying EEG
during walking. I just started working with EEGLAB.

I have a trouble and 3 questions about common power baseline when comparing
ERSP in different conditions.

1. Can I compare ERSPs directly ?
   I know it is better to use common baseline power. However, in my
opinion, I can distinguish the degree of power increase (decrease) in a
gait cycle even if I compare ERSPs directly (with each baseline power).

2. Did I conduct a bad pre-process ?
     I pre-processed the data according to Snyder, 2015, in which movement
artifact is removed by rejection of Independent Components (ICs) residual
variance (RV) larger than 15% or dipoles outside the brain. Then, the
number of ICs is different between conditions.
Is this a cause of power difference between conditions and my problem?
Should I align the number of ICs for conditions?

3. Is it necessary to normalize powers ?
    Should I normalize the power of each condition? (For instance, dividing
power spectrum by the total power of each condition)

A detail is as follows.  I wonder if you could tell me the solution.

I am comparing Event-Related Spectral Perturbation (ERSP) in different
conditions.
I divided EEG signals into epochs of -1 to 2 seconds from each right heel
strike.
When calculating ERSP, epochs were time-locked to single gait cycles (two
consecutive right heel strikes) with "timewarp" option of "newtimef"
function.
Then, I have a trouble in common baseline power.

I read EEGLAB list and learned it is necessary to calculate common baseline
power (CBP) when calculating ERSP difference in conditions. In my
understanding, in the case of comparing condition A with B, CBP is
calculated as CBP = 10log A + 10log B (Q1). I calculated ERSP of one
subject with a CBP. However, the output figures seems strange. Many
frequency bands of a condition is red (strong ERS) and those of the other
is blue (strong ERD).
I also calculated the power spectrum for each condition and confirmed they
have different total power (Q2, Q3).


Thanks, Best Regards
Yano
-- 
Shunpei Yano
Osaka University, Toyonaka, Osaka, Japan
E-mail : yano at bpe.es.osaka-u.ac.jp