P300 is negative time scale / negative polarity

RafiaRafia USA
edited October 2019 in Cyton

Hi,
Can anybody tell me Why P300 giving peak at negative time range ?

Comments

  • wjcroftwjcroft Mount Shasta, CA

    Most likely your trigger is being detected / placed wrong.

  • I have filtered on 0.5 Hz.
    Why P300 giving negative peak ? Why peak is at around 120 ms ?
    By pic24 microcontroller generating auditory stimulus and getting event on digital mode on D17 nad D18. Hearingtone by head phone . When tried with speaker, then becomes weired signal.

  • wjcroftwjcroft Mount Shasta, CA
    edited October 2019

    @Rafia, hi.

    I already replied to your question on your previous thread. Please don't create duplicate threads, I merged the two threads.

    Please examine your trigger creation / detection, as to why timing differs from standard P300.

    As far as polarity, the Cyton pins on the board are setup as the INxN pins for scalp connection, (IN1N, IN2N, ... IN8N). And the INxP pins are ganged together on SRB2. Thus pulses like P300 will appear to be inverted.

    William

  • Hi,
    I am a little bit confused with sampling frequency .
    (1) When it is 250 hz, P300 giving peak around 200 ms for a data set. But while the sampling frequency is decreasing at the same data set ( EEGlab), P300 giving peak around 350 ms. Why this is happening?

    (2) while measuring only eeg, no stimulus, on (fp1 and fp2, ) , ( c3, c4), (p7, p8), what should be the values in microvolt? Also, all the pairs should give same range of micro volt? Or there might be differences? Like,
    Say, c3 is 4 microvolt range, then what should be the range of c4?

    (3) also I am getting high peak on p7 and p8 comparing to fp and c. Is it usual?

    N.B. my enquiries are for without external trigger. So that I could be sure that my measurements are right.

  • wjcroftwjcroft Mount Shasta, CA

    (1) "...while the sampling frequency is decreasing..." How are you doing this? I understood you were using Cyton at 250 Hz.

    (2) Here is Jeremy's P300 comparison between Cyton and g.tec device,

    https://arxiv.org/pdf/1606.02438.pdf

    How can you do P300 at all without some kind of trigger to align your recordings for averaging? Why are you not using the trigger?

    https://www.google.com/search?q=p300+tutorial
    https://backyardbrains.com/experiments/p300

  • (2) Well for P300 I must use trigger. My question was for confirming that I am getting the right eeg range value. Because eeg are so sensitive. For any movement sometimes peak changes. I wanted to know the normal rsnge of eeg in elecrodes.
    (1) after recording by cyton, post processing steps to find P300, I am using Eeglab. There you can edit sampling frequency.

  • wjcroftwjcroft Mount Shasta, CA
    edited October 2019

    I don't believe that you need to "edit sampling frequency" via re-sampling.

    The P300 is a weak signal at the scalp, as shown in Jeremy's paper. This is why multiple trials must be averaged to improve signal to noise ratio. (Separate averages for normal and odd-ball cases.) I still don't understand how you can be doing "post processing steps to find P300", without using a trigger and averaging. Please use the trigger and signal averaging as mentioned in the tutorials.

  • wjcroftwjcroft Mount Shasta, CA

    If you don't use the signal averaging, aligned by the trigger, your P300 pulse will be subject to all the random noise that accompanies EEG. This is how (your quote) "the normal range of eeg in electrodes" noise (or other irrelevant activity), is tamed.

  • RafiaRafia USA
    edited December 2019

    I still don't understand how you can be doing "post processing steps to find P300", without using a trigger and averaging.

    I am using trigger for getting P300.

  • Hi,
    Why thereare continuus oscillation in p300 plot ?

  • wjcroftwjcroft Mount Shasta, CA

    Did you see the graphs in:

    https://backyardbrains.com/experiments/p300

    Even when you average the oddball events to reduce the noise, there will still be some residual randomness in the averaged signal.

    What is important is that the oddball P300 does seem to stand out in your graph.

  • Hi,
    Can anyone tell me why positive peak is at 200 ms as it should be 300 ms. Also, why there is a sharp negative amplitude around 0-0.04 ms ?

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