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Hi, all:

I am very confused about the definition of P300 amplitude these days. I have seen several definitions, one is from Polich (Polich, 1997): Amplitude (µv) is defined as the voltage difference between the largest positive peak from the baseline of the EEG waveform within the latency window (e.g., 200 ~ 700 ms). I have the following questions:

1. What is the baseline here? Is it 0 or the average of the non-target stimuli amplitude, or the most negative peak?

2. How can I get the amplitude and latency value of non-target stimuli from the topology plot?

3. How does the classifier classify the P300 brain signals between target stimuli and non-target stimuli? Does it use the difference between target stimuli amplitude and non-target stimuli amplitude or just the target stimuli amplitude?

4. Is there any threshold in the classifying process? Can I say that there is no difference between the target stimuli if the difference or the amplitude itself is above the threshold?

Thanks a lot.

Yueqing,

I am very confused about the definition of P300 amplitude these days. I have seen several definitions, one is from Polich (Polich, 1997): Amplitude (µv) is defined as the voltage difference between the largest positive peak from the baseline of the EEG waveform within the latency window (e.g., 200 ~ 700 ms). I have the following questions:

1. What is the baseline here? Is it 0 or the average of the non-target stimuli amplitude, or the most negative peak?

As you described, the P300 amplitude is usually measured as the amplitude of the highest peak in relation to the, say, average amplitude in some baseline period. The baseline period can be whatever you define it as. Usually, you would pick a period prior to the stimulus to rule out some artifact or other signal evoked by the stimulus. Thus, you may take the average from 100ms prior to the stimulus to stimulus onset as the baseline period.

2. How can I get the amplitude and latency value of non-target stimuli from the topology plot?

Try using only one condition in the OfflineAnalysis program:

(StimulusCode > 0) && (StimulusType == 0)

3. How does the classifier classify the P300 brain signals between target stimuli and non-target stimuli? Does it use the difference between target stimuli amplitude and non-target stimuli amplitude or just the target stimuli amplitude?

It cannot do what you suggest because it - at least in free spelling - does not know what the target is.

What it does is very simple. The system measures the P300 amplitude using the entries in the Classifier. This can simply be the amplitude at one location at one time, or can be more complex, depending on what you enter. Then, the system produces one measurement of P300 amplitude for each flash. It simply picks the row with the highest measurement, and the column with the highest measurement. This determines the selected character.

4. Is there any threshold in the classifying process? Can I say that there is no difference between the target stimuli if the difference or the amplitude itself is above the threshold?

There is no threshold. See above.

I hope this helps.

Gerv[/quote]

Thanks, Dr. Schalk. It helps me a lot.
Thank you very much as always.