[Eeglablist] Baseline problem

[Andre Bastos]

Dear Neha,

 

I have recently been dealing with exactly the same problems as you describe.
I am running an attention experiment that requires subjects to maintain
fixation at the center of the screen, so I am using the EOG to measure eye
movements in order to throw away trials where central fixation is not
maintained. I've been using a paper on biofeedback systems to assist with
this.

 

Barea et al (2002) , writing in IEEE, state that the the following:

 

The EOG signal changes approximately 20 microvolts for each

degree of eye movement. In our system, the signals are sampled

10 times per second. The EOG signal is a result of a number of

factors, including eyeball rotation and movement, eyelid movement,

different sources of artifact such as EEG, electrode placement,

head movements, influence of the illumination, etc. It is

therefore necessary to eliminate the shifting resting potential

(mean value) because this value changes. To avoid this problem

an ac high-gain differential amplifier (1000-5000) is used, together with a
high pass filter with cutoff frequency at 0.05 Hz

and relatively long time constant and a lowpass filter with cutoff

frequency at 35 Hz. Ag-AgCl floating metal body-surface electrodes

are also used.

 

I am currently  trying to determine how tightly coupled the magnitude of
change in the EOG channels and the actual change in eye fixation, measured
in degrees, and whether I can determine precisely where the eyes are given
the EOG amplitudes.

 

Anyone else have other suggestions or literature recommendations that could
be helpful?


Thanks

 

Andre

 

 

 

From: eeglablist-bounces at sccn.ucsd.edu
[mailto:eeglablist-bounces at sccn.ucsd.edu] On Behalf Of neha bhutani
Sent: Sunday, October 21, 2007 9:00 PM
To: eglablist-bounces at sccn.ucsd.edu; eeglablist at sccn.ucsd.edu
Subject: [Eeglablist] Baseline problem

 





Dear users,

 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? 

 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? 

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? 

I would be happy if any one can give comments and suggestions regarding my
problems. 

Thanks in advance.

-- 
Neha Bhutani
Graduate Student
Cognitive Neuroscience
National Brain Research Centre
Manesar, India 



-- 
Neha Bhutani
Graduate Student 
Cognitive Neuroscience
National Brain Research Centre
Manesar, India 

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