Audio triggers for EEG system

I have 3 scripts that generate 3 different audio files. Each 3 seconds long, with 1 containing white noise in the background and a pulse stimulus, 1 containing white noise in the background and a short pause/gap in the noise, and then the 3rd contains both the gap and the pulse.

I then have a 4th script that combines these 3 audio files and randomly repeats them as a continuous 8 minute sound file.

I want to add triggers to this so that if I play this audio file while connected to an EEG system triggers will appear to identify the timepoints that each of these individual files occur within the full 8 minute file. The triggers will be sent via a parallel port.

Does anyone know the best way to do this?

Can matlab distinguish the changes in db? I was going to time-lock a trigger to each file but there is a jitter in the 8-minute file so I don’t think this would work.

[Taylor]

MATLAB can most likely distinguish changes in the audio signal (depending on how large of a spike in amplitude the pulse is). Does your amplifier have and analog or audio in? It might just be easier to split your audio signal into this channel so it is time-synced with the EEG data by your recording system. Then you can process it in post to determine the exact timestamps of the pulses/gaps.

[Nathan]

Hi there,

Just following up on this. I was able to write a script that detects the time there was a change in db levels. I've now saved these time points in a workspace variable (column 1 = time in seconds, column 2 = label of sound stimuli either 1,2 or 3) there are 150 time points. I am now trying to write a script that synchronises playing the audio file and sending the triggers from this variable to an EEG parallel port at the exact same time (i.e. 1.044 seconds in there is stimulus 1 so I want to play the audio and exactly 1.044 seconds later the trigger is sent to the EEG system).

I have a snippet of code that I can use to send triggers but I seem to have really confused myself on how to synch up the audio and variable containing the time points.

Hope this makes some sort of sense.

if par.recordEEG

if par.booth == 1

port = hex2dec('2010');

lptwrite(port,0);

ioObj = 0;

elseif par.booth == 3

addpath('C:\Windows\SysWOW64');

ioObj = io64;

status = io64(ioObj); % Status = 0, is ready to send triggers out

port = hex2dec('3FB8');% windows 10

elseif par.booth == 2

port = BioSemiSerialPort();

ioObj = [];

end

if par.recordEEG==2% just print triggers for now - dummy mode

sendtrigger = @(trg,port,SITE,ioObj) fprintf('TRIGGER: %d\n', trg);

port = 0;

ioObj = 0;

end

end

Answer 1

[Ayush]

Hi Nathan,

I understand that you can successfully transmit trigger signals to the EEG system but are encountering challenges with synchronizing these signals with the audio player.

A potential solution is the "two-pointer" algorithm. In this method, one pointer tracks the audio timestamps while the other tracks the stimulus timestamps. This can help ensure that both processes are aligned effectively. This can be achieved by using “tic-toc” functions.

For further clarification, please refer to the code provided below.

% you can replace it with actual data 
data = [time_points, stimulus_labels]; 
[audioData, fs] = audioread('your_audio_file.wav'); 
audioPlayer = audioplayer(audioData, fs); 
play(audioPlayer); 

% store the start time of the audio 
startTime = tic;  

% you can replace it with your actual trigger sending function 
sendtrigger = @(trg, port) fprintf('Sending trigger %d to port %d\n', trg, port); 

port = BioSemiSerialPort(); 
ioObj=[] 

% if no hardware device is present, alternatively, you can define the port 
% and ioObj in a similar way as you did in your code 
port = hex2dec('3FB8'); % windows 10 
ioObj = io64; %64 bit OS 


% Iterate through the time points and stimuli 
for i = 1:size(data, 1) 
    % Get the time point and stimulus 
    timePoint = data(i, 1); 
    stimulus = data(i, 2); 
    
    % Calculate the delay 
    delay = timePoint - toc(startTime); % Time to wait until the trigger should be sent 
    % Wait until it's time to send the trigger 
    pause(max(0, delay)); 
    % Send the trigger 
    sendtrigger(stimulus, port); 
end 

% Ensure that the audio playback is completed 
while isplaying(audioPlayer) 
    % can do additional checks or operations concurrently using similar 
    % approach as used above 
end 

stop(audioPlayer); 

For further information, you can refer to the following documentation of “tic” and “toc” functions: https://www.mathworks.com/help/matlab/ref/tic.html

Hope it helps!