How to implement triggers in Python script

Question

I have a psychopy script for a psychophysical experiment in which participants see different shapes and have to react in a specific way. I would like to take physiological measures (ECG) on a different computer and implement event points in my measures. So whenever the participant is shown a stimulus, I would like this to show up on the ECG.

Basically, I would like to add commands for parallel port i/o.

I don't even know where to start, to be honest. I have read quite a bit about this, but I'm still lost.

Answer 1

For parallel port io, you should use pyparallel. Make continuous ECG measurements, store those and store the timestamp and whatever metadata you want per stimulus. Pseudo code to get you started:

while True:
    store_ecg()
    if time_to_show():
        stimulus()
    time.sleep(0.1) # 10 Hz

Answer 2

Here is probably the shortest fully working code to show a shape and send a trigger:

from psychopy import visual, parallel, core
win = visual.Window()
shape = visual.ShapeStim(win)

# Show it
shape.draw()
win.flip()

# Immediately send trigger
parallel.setData(10)  # Trigger code, here 10
core.wait(0.020)  # Number of seconds to send this trigger code (enough for the hardware to send and the receiver to recognize it)
parallel.setData(0)  # Stop sending trigger.

You could, of course, extend this by putting the stimulus presentation and trigger in a loop (running trials), and do various other things alongside it, e.g. collecting responses and saving data. This is just a minimal example of stimulus presentation and sending a trigger.

It is very much on purpose that the trigger code is located immediately after flipping the window. On most hardware, sending a trigger is very fast (voltage on the port changes within 1 ms of the line being run in the script) whereas the monitor only updates its image around every 16.7 ms. and win.flip() waits for the latter. I've made some examples of good and bad practices for timing triggers here.