Signal Processing: EEG to ERP

13
Signal Processing: EEG to ERP A Darren Parker Presentation

description

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. - PowerPoint PPT Presentation

Transcript of Signal Processing: EEG to ERP

Page 1: Signal Processing: EEG to ERP

Signal Processing: EEG to ERP

A Darren Parker Presentation

Page 2: Signal Processing: EEG to ERP

EEG to ERP

To get an ERP signal out of the raw EEG, our goal must be to reduce the signal to noise ratio.

Page 3: Signal Processing: EEG to ERP

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.

Page 4: Signal Processing: EEG to ERP

Filtering

Page 5: Signal Processing: EEG to ERP

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.

Page 6: Signal Processing: EEG to ERP

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.

Page 7: Signal Processing: EEG to ERP

RereferencingCz Reference

Rereferenced

Page 8: Signal Processing: EEG to ERP

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.

Page 9: Signal Processing: EEG to ERP

Rejection

Blink

Eye Movment

Garbage

Page 10: Signal Processing: EEG to ERP

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).

Page 11: Signal Processing: EEG to ERP

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.

Page 12: Signal Processing: EEG to ERP

Averaging

Both pictures taken from:

http://neurocog.psy.tufts.edu/images/erp.htm

Page 13: Signal Processing: EEG to ERP

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.