[Eeglablist] difficulties in re-epoching data

[Arnaud Delorme]

Dear Clayton,

you are doing the right thing. You could also run ICA on the continuous 
data before extracting data epochs and then you would have this problem.

First, it seems normal that you will lose some epochs because most of 
them correspond to different conditions than the one you are interested in.

Second if you have a large window for the original epochs (let say 3 
seconds from -1 to 2 seconds) and your time-locking events are every 1 
second for instance, then over the course of a 3-second epoch you will 
have several events (for instance, one at -1s, one at 0s obviously, one 
at 1 second and one at 2 seconds). When you re-epoch the data, not only 
the events at 0s are considered but also all events at -1s,1s, and 2s. 
However, for these non-0 events, your new epoching window (let say still 
-1 to 2 seconds) will not allow to extract a full epoch and EEGLAB will 
return the "out of boundary" message you mentionned.

Hope this helps,

Arno

> Hello all,
>
> I'm having some difficulty in re-epoching a dataset.
>
> I'm doing vision research, and want to correct for blink artifacts but 
> discard trials in which eye movements took place. I'm currently 
> starting analysis by modifying event codes such that relevant 
> conditions are identified. I then epoch the data such that all 
> relevant trials are selected, and run infomax ICA on this 'grand' 
> dataset. Later, I identify components associated with blinks, and 
> components associated with eye movements. I remove the components 
> associated with blinks, but discard epochs associated with eye 
> movements. I then re-epoch the clean dataset such that each condition 
> is stored in its own datafile.
>
> When I re-epoch, I lose a number of trials, sometimes the majority. 
> For example, here's the output I receive on one try:
>
> [*code*]
>  >> test = pop_epoch(EEG, {'223'}, [-0.5 1.0]);
> pop_epoch():212 epochs selected
> Epoching...
> Warning: event 7 out of data boundary
> Warning: event 12 out of data boundary
> Warning: event 53 out of data boundary
> Warning: event 66 out of data boundary
> Warning: event 69 out of data boundary
> Warning: event 84 out of data boundary
> Warning: event 87 out of data boundary
> Warning: event 104 out of data boundary
> Warning: event 107 out of data boundary
> Warning: event 124 out of data boundary
> Warning: event 137 out of data boundary
> Warning: event 140 out of data boundary
> Warning: event 147 out of data boundary
> Warning: event 152 out of data boundary
> Warning: event 157 out of data boundary
> Warning: event 162 out of data boundary
> Warning: event 165 out of data boundary
> Warning: event 168 out of data boundary
> Warning: event 193 out of data boundary
> Warning: event 208 out of data boundary
> pop_epoch():192 epochs generated
> pop_epoch(): time limits have been adjusted to [-0.500 1.000] to fit 
> data points limits
> eeg_checkset: recomputing the ICA activation matrix ...
> pop_epoch(): checking epochs for data discontinuity
> [*end code*]
>
> I'm not sure what it means that the events are out of the data 
> boundary, as I'm epoching based on a single event code. I have traced 
> this error in epoch.m, but the code is reasonably complicated and I 
> can't quite figure out what's going wrong. Has anyone else had similar 
> troubles? Or perhaps does anyone have any insight?
>
> Thanks very much,
>
> Clayton Hickey
> Vrije Universiteit Amsterdam 


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