Transcript Signal Processing: EEG to ERP

Signal Processing: EEG to ERP
A Darren Parker Presentation
EEG to ERP
To get an ERP signal out of
the raw EEG, our goal must
be to reduce the signal to
noise ratio.
How can we reduce noise?
• Averaging trials in the same
condition together.
• Filter out frequencies that are not of
any concern. (i.e. 60Hz noise)
• Rereference data so channels
become more uniformly emphasized
with relation to the reference
channel.
• Reject “bad” trials.
• Correct for artifacts in the data, like
blinks.
Filtering
Filtering
•Waveforms can be broken down into a
series of sine waves at different
frequencies.
•Our filters separate the signal into the
individual sine components, then
remove the components we aren’t
interested in.
Rereferencing
• Need a reference for each electrode
to be measured relative to.
• Electrodes near the reference will
show little signal, since there is not a
great deal of difference between
them.
• Electrodes far away will be
overrepresented since they are far
away and much more different.
Rereferencing
Cz Reference
Rereferenced
Rejecting Bad Trials
• Sometimes parts of the waveform are
just messy, due to subjects moving
around in one way or another.
• Some artifacts are common and
stereotyped, such as blinks, and
sometimes eye movements.
• Potential for correction factors.
• Others are more random with no
predictable pattern.
Rejection
Blink
Eye Movment
Garbage
Eye Blink Correction
•Blinks and (sometimes) eye movements are
common, and an individual’s blinks and eye
movements may be fairly constant.
•We can try to build a prototype for a
subject, so we can correct for blinks (and
eye movements).
Averaging
• The EEG records all changes in
potentials at all electrodes, not just
the signals we are interested in.
There will always be additional noise
since our subjects aren’t dead.
• Assuming that other processes are
random, and not time-locked to the
stimulus, averaging trials together in
any given condition will increase the
signal and decrease any random
noise.
Averaging
Both pictures taken from:
http://neurocog.psy.tufts.edu/images/erp.htm
The Plot Thickens
• By now you should have your perfect
little ERP averages… but is
everything really that perfect?
• Bad channels affect averaging and
rereferencing.
• Filtering is not perfect.
• What reference is best?
• Latency jitter.
• Artifact correction woes.