Stim Artifact Observed with Digital IO Triggers

Hi all,
I am facing this issue with Digital IO pins for Trigger Insertion with Cyton Board.
It was working fine previously. From December, it has started showing an artifact of 20 to 28 ms in averaged epochs.

I have tried different auditory/visual stimuli and various workarounds of signal processing and trigger insertion. I still see this artifact shown in the image attached.

The same is reflected in generated ERPs.
When I remove triggers from Digital IO pins, I can clearly see noiseless data, but I need the trigger timings to record ERPs with exact timings.

Any kind of help would be appreciated.

Comments

  • wjcroftwjcroft Mount Shasta, CA

    Hi Rathin,

    Can you clarify? What do you mean by "stim artifact"? What is the source of the stimulation? Something your equipment is generating, then feeding to the Cyton? Generally external trigger sources are not thought of as 'stim'.

    If you are indeed supplying external trigger signals, then these signals can bleed into EEG inputs via capacitive or inductive coupling from the trigger wires into nearby electrode wires. One way to reduce such coupling, is to limit the amount of current flowing in the trigger wires. I don't know what circuit components you are using, but the use of appropriate resistors in the circuit (outside / away from Cyton, located at your trigger source equipment), could possibly reduce the coupling. Shielding the trigger wires is another approach.

    Yet another way to go is to use the 'analog read mode' for the Aux trigger pins. This input is a 10 bit ADC operating from 0V to 3.3V. So you could provide a much lower voltage than the 3.3V required for digital mode. Voltage divider (2 resistors), can reduce an input voltage.

    Regards, William

  • Hello William,
    Thank you for your response!
    Stim artifact, stimulus artifact is the artifact generated immediately after the onset of the stimulus (Auditory or Visual or Somatosensory, etc.)
    We are using Arduino Nano to generate auditory triggers and Raspberry Pi 3 to generate Visual Triggers, feeding them to D11, D12. However, the artifact was evident in all digital IO Pins.
    We have shielded wires with Al foil.
    As suggested, We have tried placing different resistors between Arduino Nano and Cyton Digita Pin.
    Artifact magnitude of reduced.
    However, **the same artifact is observed. ** As human EEG averaged values is less than 15 uV.

    We obtained artifact-free responses with the same setup. We will try the analog option and update you.

    Kindly suggest any other workarounds, if possible.

    Many Thanks,

    Rathin

  • if you don't know what resistor values work, and you have a breadboard circuit setup for the trigger wires, consider putting a 'pot' type variable volume control type resistor on one or both sides of the voltage divider, crank down the voltage til there is no spike signal, then move it up just a bit.

  • retiututretiutut Louisiana, USA

    I tested using OptoIsolation with success. Simply sending digital signals from an Arduino/RPi directly the the Cyton pins will likely cause noise.

    OpenBCI Docs Example:
    https://docs.openbci.com/docs/02Cyton/CytonExternal#optoisolation

    Here is the version I made, which includes an LED light to help with visual confirmation of a trigger:

  • @Billh: Already tried with Pot. Artifacts were evident.

    Previously it worked with 330 Ohm Resistor.

    @retiutut: I Will try will optoisolators once and get back to you. Thank you.

    @wjcroft: Will check with Analog mode. Thank you.

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