[Eeglablist] baselining using 500ms from the inter-trial interval

[Stephen Politzer-Ahles]

Hi Hannah,

I doubt that your baseline is being contaminated by the letter prime--it
starts 900 ms after the presentation of the letter (and 400 ms after the
letter disappears), so the low-level visual responses to the letter should
have been finished by then. It's likely that the issue is just the length
of the baseline (100 ms is pretty short, even though it's used a lot in
language research).

In any case, doing what you are looking for would be easy. When you epoch
the data, just use the onset of the phoneme target as your time-locking
event, and let the epoch limits be [-1.5 x] (where x is however many
seconds you want the epoch go go on after the onset of the target). The
fact that the target's duration is variable is not a problem, since the
duration of the parts of the trial before the target are not variable.
Having the prime and ISI inside the epoch is not a problem, either, if
you're just going to be subtracting the baseline and then analyzing the ERP
after the onset of the target (i.e., the prime and the ISI will get ignored
anyway). I guess it could be a problem if for example you can to do an ICA
decomposition and you don't want the ICA to be contaminated by the
responses to the prime; in that case, you could run the ICA on smaller
epochs that don't include the prime and then apply the resulting ICA
weights to the larger epochs.

Best,
Steve

On Tue, Feb 26, 2013 at 7:48 AM, Nash, Hannah <hannah.nash at ucl.ac.uk> wrote:

>  Hi,****
>
> ** **
>
> I’ve read the tutorials and other post on the topic of baselining and I
> can’t quite seem to find the solution to my problem.****
>
> ** **
>
> In our experiment a letter prime (presented for 500ms) precedes (ISI
> 500ms) a phoneme target (variable duration 282ms – 428ms)****
>
> ** **
>
> There is an inter-trial interval of 1000ms.****
>
> ** **
>
> To summarise a trial****
>
> ITI (1000ms) – letter prime (500ms) – ISI (500ms) – sound target
> (282-428ms)****
>
> ** **
>
> Both the primes and targets send triggers and so are events.****
>
> ** **
>
> At the moment I am looking at ERPS to the targets (period of 500ms after
> event onset) and using the default of -100ms before event onset as the
> baseline period.****
>
> ** **
>
> This is not resulting in a very good baseline at all and I’m wondering if
> this is related to both the length of the period (too short) and the fact
> that it’s only 400ms after the offset of the letter prime.****
>
> ** **
>
> Therefore I’d like to use a period of 500ms but from the inter-trial
> interval.****
>
> ** **
>
> The problem is that I don’t know how program this in eeglab.  Do the epoch
> limits have to be extended to include this period this would be variable
> from 2282ms – 2428ms?    I know that the baseline period would then be
> -1500 to -1000 but the epoch would include the prime and the ISI as well….
> ****
>
> ** **
>
> Perhaps this is straightforward for someone more familiar with eeglab.
> Any help will be appreciated.****
>
> ** **
>
> Thanks****
>
> Hannah****
>
> ** **
>
> Dr Hannah Nash****
>
> Research Fellow, UCL – Division of Psychology & Language Sciences****
>
> Chandler House (room 115)****
>
> 2 Wakefield Street****
>
> London****
>
> WC1N 1PF****
>
> ** **
>
> Email: hannah.nash at ucl.ac.uk****
>
> ** **
>
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-- 
Stephen Politzer-Ahles
University of Kansas
Linguistics Department
http://people.ku.edu/~sjpa/
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