[Eeglablist] Baseline problem

[Arnaud Delorme]

Dear Neha,

>  I am very new to the EEG recordings. I am working on the presaccadic 
> potentials and am not able to understand some of the very basic 
> problems. It would be of great help if any one of you can kindly help 
> me with it. In my recordings after a saccade is over the hEOG channel 
> activity doesnot come to the baseline. When I plotted the whole hEOG 
> channel in MATLAB I found a constant drift in the hEOG channel. Is 
> this normal to happen? Or if not, then what measures should I take to 
> remove this drift?

Yes, this kind of drift sometimes happen on some recording equipment. To 
remove it, you should high pass your data at 0.5 Hz for instance. This 
may be done in EEGLAB using the menu "Tools > Filter the data".

>  Moreover when I try to analyze my data I find most of my channels 
> being contaminated with the very high saccadic potentials. How should 
> I proceed to ensure that I am able to correctly remove this 
> contamination from the other channels and still get the presaccadic 
> potentials?

You should probably use ICA to do that.
http://www.sccn.ucsd.edu/eeglab/quickrej.html

> One more thing which I want to ask is, that since in my recordings, 
> there is no input to the system when the subject is making a response 
> (in the dat file) how should I proceed to calculate the reaction 
> times, as I want to separate my trials based on slow and fast reaction 
> times?

You need to fuse your dat file with the continuous recording. This 
assumes though that some events (stimulus presentation) have been 
registered along with the continuous data. If you are using neuroscan 
data, you can even fuse the dat file from within EEGLAB after extracting 
data epochs from the continuous data.

Best,

Arno