[Eeglablist] About stimulus presentation delay

[Yamil Vidal Dos Santos]

Dear Chao,
Consider that the amount of jitter that would be acceptable depends on your
ERPs of interest. Long lasting components like the P3b are more resilient
to jitter, and more transient components like the MMN suffer more. The same
applies to frequency analysis. Low frequencies suffer less, ad high
frequencies suffer more.

Some time ago I tested this empirically by artificially adding jitter to a
dataset to see the effect in different ERPs. I think that jitter in the
range of 50ms is tolerable, but take this as a very informal rule of thumb,
as it depends on the specific shape of components in your data.

Regarding how to correct it. Ideally you should measure the real time of
presentation of the stimuli during the experiment and segment your data
locking to that time instead of the intended time. If this is not possible,
subtracting the mean jitter will only remove delay, but as you will be
removing a constant number from all trials, this won't fix the shape of
your waveform.

How is the distribution of your delays? And what type of analysis do you
intend?
I hope you find this useful.
Best,
Yamil

On Wed, Sep 21, 2016 at 3:39 AM, chao wang <hnchaowang at gmail.com> wrote:

> Hi, everyone,
>
> We're recording EEG data and we use E-Prime to present stimuli. During
> recording, stimuli are presented to subjects via a projector. The projector
> induces a delay between stimulus presentation on E-Prime computer and in
> front of subjects. We measured this delay and found that the delay was not
> constant across trials but instead jitters within certain range around the
> average. I would like to know is there a typical range of the jitter that
> can be accepted in ERP experiments? And in this situation, what should I do
> to segment EEG data with respect to stimulus onset? I mean, use the average
> delay to determine the real instant of stimulus presentation, or any other
> ways exist? Forgive me if my words is not clear :)
>
> Thank you in advance!
>
> Chao
>
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