How to keep stimulus on screen for certain length of time

Programming Help
1

I am using Psychtoolbox on MATLAB R2020a on Mac OS. I am trying to get the fixation cross to stay on the screen for 10 seconds before displaying the next stimulus. I have tried WaitSec() but this did not work.

% Experimental instructions
line1 = 'Choose the sequence with the highest mean';
line2 = '\n Press any key when you are ready to begin';

% Draw all the text in one go
Screen('TextSize', window, 40);
Screen('TextFont', window, 'Ariel');
DrawFormattedText(window, [line1 line2],...
    'center', 'center', white);
Screen('Flip', window);
KbStrokeWait;

% Fixation cross
Screen('DrawLines', window, allCoords,lineWidthPix, white, [xCenter yCenter], 2);

% trial 1
DrawFormattedText(window, sprintf(seq1), rightX, rightY, [0 1 0]);
DrawFormattedText(window, sprintf(seq2), leftX, leftY, [1 0 1]);
DrawFormattedText(window, sprintf(seq3), upX, upY, [1 1 0]);
2

Use the when argument to Screen(‘Flip’) for precisely timing when things are shown, such as the next thing after your fixation cross.

3

Thanks. Could you please explain how to construct the when argument? I tried:

when(Screen('DrawLines', window, allCoords,lineWidthPix, white, [xCenter yCenter], 2), WaitSecs(10));

But that did not work.

4

Read “Screen Flip?”, Thats how you get help for the PTB functions. One of the inputs is when. See some of Peter Scarfe’s ptb demos as a start, they’ll tell you about basic use and how to do proper timing