[Eeglablist] simple question about epoch and event

[Mikołaj Magnuski]

Hi Min,

Epoch is a selection of data in some proximity to a given event
while event is (usually) a digital marker that informs about time and type
of some experimental situation (reports given by subjects through button
presses or stimulus presentation).

During epoching with respect to some event EEGlab does not remove all other
events from the data (for example we still want to have information on
reactions even if we epoch with respect to stimulus appearance) so it may
happen that one epoch is home to multiple events.

You may check this by plotting the data or inspecting the EEG.epoch field.

Last thing - you have one less epoch than number of events that you epoch
with. This may be because that one event is either at the end or beginning
of your recording and is not surrounded with enough data poins that your
epoching requires (for example if you specify epoching if [-1, 2] and the
event occurs one second before the end of the recording).
28 paź 2013 22:03 "Min Sheng" <Min.Sheng at utsouthwestern.edu> napisał(a):

>  Dear all,
>
> I am new to EEG process, I am a little confused about the epoch and the
> event. When I use eeglab to extract epochs from raw data, I thought the
> number of   event and the epoch should be the same, but it didn't. For
> example,  if I extracted 'S32' epochs and generated new data set, it showed
> that the number of epochs is 755 while the number of events is 758.
> However, the VMRK file shows the number of 'S32' should be 756. It happened
> when there were some  other markers mixed (like 'buffer overflow') during
> the recording.
>
>
>
> Best regards,
>
> Min sheng
>
>
>
> ------------------------------
>
> UT Southwestern Medical Center
> The future of medicine, today.
>
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