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I'm trying to present a 4Hz flickering stimuli in PsychToolbox for 5 seconds followed by a 500Hz tone. Does anyone have an idea of how to do this? I've been using the vbl or screen refresh rate to calculate the flicker frequency but I'm not sure if I'm on the right track at all. I also have no idea how to present an auditory stimuli in PTB (I tried the sound function already). Any help is greatly appreciated!

Rannie Xu
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1 Answers1

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I'm not sure about sound presentation in PTB (I've never done it), but you seem to be on the right track for the flicker frequency. The way I do it is to determine the screen refresh rate, divide the total length of time you want the stimulus presented by this refresh rate (this will give you the number of frames that will be drawn during this time), and then have a frame counter that increases by 1 after every flip. You can then use this frame counter to switch commands on or off.

A minimal example (randomly changes the background colour at 4Hz for 5 seconds):

[w, wRect]=Screen('OpenWindow', 0);
MaxTime = 5; %Set maximum time for all stimuli to be presented in seconds
Hz = 4; %Set Hz for stimulus flicker
Screen('Flip',w);
Frametime=Screen('GetFlipInterval',w); %Find refresh rate in seconds

FramesPerFull = round(5/Frametime); % Number of frames for all stimuli
FramesPerStim = round((1/Hz)/Frametime); %Number of frames for each stimulus

StartT = GetSecs; %Measure start time of session
Framecounter = 0; %Frame counter begins at 0
while 1

    if Framecounter==FramesPerFull
        break; %End session
    end

    if ~mod(Framecounter,FramesPerStim)
        randomcolour = rand(1, 3)*255; %Change background stimulus colour
    end

    Screen('FillRect', w, randomcolour, wRect);
    Screen('Flip',w);

    Framecounter = Framecounter + 1; %Increase frame counter
end

EndT = GetSecs; %Measure end time of session
Screen('CloseAll');

EndT - StartT %Shows full length of time all stimuli were presented

The timing precision will depend on your particular refresh rate.

Hope this helps!

KerrBer
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