Issue with accurate timing of keyboard response

Programming Help
1

I’m trying to show a visual stimulus in 1s (i.e., 60 frames on my computer) and record keypress and response time. The stimulus would be disappeared after 1s if there is no keypress, or it would vanish immediately once the response is happening. For the former condition, I record the response time as offset - onset and which is equal to none response in my script. But for the latter one, I’m quite confused. Is the response time should be regarded as secs - onset, or it should be offset - onset too? I found that the results of these two approaches only have minimum differences (for example, 1.0003s verse 1.0164s). Does secs - onset is the more accurate one?

Screen('Preference', 'SkipSyncTests', 0);

[w, wrect] = Screen('OpenWindow', 0, [0 0 0]);

topPriorityLevel = MaxPriority(w);
Priority(topPriorityLevel);

numSecs = 1;
waitframes = 1;
ifi=Screen('GetFlipInterval', w);
frame = round(numSecs / ifi);

    
vbl = Screen('Flip', w);  % onset of stimuli
onset = vbl; 

for i = 1:frame

    Screen('DrawDots', w, [wrect(3)/2; wrect(4)/2], 10, [255,255,255], [], 1);  % stimuli
    vbl = Screen('Flip', w, vbl + (waitframes - 0.5) * ifi);

    [keyisdown, secs, keycode] = KbCheck;  % record response time
    if keyisdown == 1
        break
    end

end

offset = Screen('Flip', w, onset + (waitframes - 0.5) * ifi);

Screen('close', w);
2

they’re different things, one is latency from stimulus onset to key press, other is how long the stimulus was shown on the screen. Just choose the one most appropriate for you

3

it’s clear to me now, thanks

4

Btw.: Your onset_time is not your actual onset time. That would be the vbl returned from the flip in the first iteration of the for-loop. Current code would introduce an error of at least 1 video frame duration, more if frames are skipped due to high machine load.

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Best,
-mario