Lab Streaming Layer and PsychoPy

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Streams and outlets

An LSL outlet is a named source of data: “MyEEG,” eight channels, 250 Hz, float. Your acquisition code creates one and pushes each sample with a timestamp from local_clock(). Any program on the network can find it and pull samples. A marker outlet is the same thing with one string channel and irregular timing: it pushes “target” or “standard” or “flash_10Hz” at the moment the event happens.

from pylsl import StreamInfo, StreamOutlet, local_clock
info = StreamInfo('MyEEG', 'EEG', 8, 250, 'float32', 'myeeg001')
outlet = StreamOutlet(info)
# in the acquisition loop:
outlet.push_sample(list_of_8_floats)          # timestamp taken automatically

If your board already speaks the OpenBCI protocol, the OpenBCI GUI or BrainFlowBrainFlowA library that talks to OpenBCI and most consumer EEG headsets with one API, in Python and other languages. Glossary entry can publish it to LSL for you.

Markers from PsychoPy

PsychoPy shows stimuli with frame-accurate timing and lets you run Python at the moment of each frame. At the flip that shows the stimulus, push a marker.

from psychopy import visual, core
from pylsl import StreamInfo, StreamOutlet
win = visual.Window(fullscr=True, color='black')
markers = StreamOutlet(StreamInfo('Markers', 'Markers', 1, 0, 'string', 'mk001'))
square = visual.Rect(win, width=0.3, height=0.3, fillColor='white')
for trial in range(50):
    core.wait(1.0 + 0.5 * random.random())        # jittered inter-trial interval
    square.draw()
    win.callOnFlip(markers.push_sample, ['flash'])  # marker pushed exactly at the frame that shows it
    win.flip()
    core.wait(0.1); win.flip()

callOnFlip is the important line: the marker’s timestamp is taken when the screen actually changes, not when the code asked it to.

Recording

LabRecorder (a small program from the LSL project) lists every stream on the network; tick the ones you want and press record. It writes an XDF file with every stream and its timestamps, corrected for clock offsets between machines. MNE reads XDF through the pyxdf package. Now you have EEG and markers in one file and one clock, and epoching around markers is one line.

Timing reality

Screens refresh at 60 Hz, so a stimulus appears at some point within a 16.7 ms window after the flip; PsychoPy times the flip, not the pixels. Monitors add their own processing delay, tens of milliseconds on some televisions. LSL’s clock synchronization is good to about a millisecond on one machine and a few across a network. For P300 and SSVEP this is fine. For anything that needs sub-millisecond precision, a photodiode on the screen wired to a spare amplifier channel records the true onset; do this once to measure your setup’s delay and then correct for it.