[Eeglablist] Downsampling before or after concatenating and averaging

Tarik S Bel-Bahar tarikbelbahar at gmail.com
Tue Nov 28 01:10:01 PST 2017

Hello Joseph,

*probably best to downsample before concatenation of the four datasets from
each subject session. Downsample only if you have a good reason (or try it
both ways - downsampled and not downsampled).

*epoch after downsampling and before concatenation,

*it doesn't matter if you epoch or don't epoch for ICA, ICA in eeglab
shuffles time points anyway, and is only looking for spatial regularities,
not temporal regularities.

However you might find it useful epoch artifically, e.g., using
eeg_regepoch in .5 sec epochs) to make it easier to remove large-artifact
periods semi-automatically or fully automatically before doing ICA. After
you have an ICA solution you can re-apply it to the pre-epoched data and
make your real epochs from there.

Note that you don't need to epoch to do good-enough artifactual period
rejection before ICA, just make sure to not include periods where there are
no trials or no cognitive activity of interest (such as waiting time during
instructions or between conditions).

If you haven't had a chance to please review eeglab tutorial and work with
eeglab tutorial data to get a better handle of best steps. Reviewing
suggestions in the the googlable makoto's pipeline suggestions may also be
helpful to you.

On Sat, Nov 25, 2017 at 3:16 PM, Joseph Nuamah <jknuamah at aggies.ncat.edu>

> Dear All,
> In a recent experiment, I presented each participant with two conditions
> in four randomized blocks consisting of 30 trial each. Thus, I have four
> data sets under each condition for each participant. The experiment was
> carried out in one session; I did not reapply the cap and did not re-gel. I
> intend to downsample from 512Hz to 256 Hz, epoch the data, and perform ICA
> to remove artifactual components (not dipole modeling)
> My questions are:
> 1. Do I downsample before or after concatenation?
> 2. Do I epoch before or after concatenation?
> 2. Do I epoch before or after ICA ? Does it matter which comes first ?
> Many thanks!
> Joseph
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