KbCheck (?) unreliable in Psychtoolbox RT paradigm - matlab

Question

I am trying to create a simple RT experiment in MATLAB R2014b, using Psychtoolbox. Participants will have to categorize faces, pressing one of two buttons as fast as they can. I created the paradigm on one computer, and it is working fine on that, but when I moved it to another (the one I want to be testing on), there was a weird error: even though the program seemed to be logging keypresses on the majority of trials, sometimes it wouldn't respond, and I had to press the key a number of times before it proceeded to the next trial. I am unsure what's going in, but I assume there could be something wrong with the computer itself (what might that be?), or with this particular bit of the code:
Screen('Flip', mainwin);
timeStart = GetSecs;keyIsDown=0; correct=0; rt=0;
while 1
bf = 0; %this variable is irrelevant here, I use it later to break
out of a loop
while (GetSecs - timeStart) < 0.2 %faces are presented briefly, but
%I'm recording responses here anyway, just in case there are some
%fast anticipatory responses - after this loop is over, I keep
%recording RT and button press the exact same way, but with no
%stimulus present
[keyIsDown, secs, keyCode] = KbCheck;
FlushEvents('keyDown');
if keyIsDown
nKeys = sum(keyCode);
if nKeys==1
if keyCode(Key1)||keyCode(Key2)
rt = 1000.*(GetSecs-timeStart);
keypressed=find(keyCode);
Screen('Flip', mainwin);
type = 'T';
bf = 1;
if keyCode(Key1) & targ_pic == 1
correct = 1;
elseif keyCode(Key2) & targ_pic == 0
correct = 1;
end
break;
elseif keyCode(escKey)
ShowCursor; fclose(outfile); Screen('CloseAll');
return
end
keyIsDown=0; keyCode=0;
end
else
keypressed = 0;
end
end
Can anyone maybe spot something that may be wrong with this?
Incidentally: is this the correct way to get RT out of PTB? I found that bit of code online, but I'm a little uncertain about why the "secs" variable is not used.
Both computers run Windows 10.

Answer 1

A few suggestions:
The Flip command will return the estimate of stimulus onset time, currently you are calling GetSecs after the Flip command, which is not necessary and will always return a value that is slightly later than the actual screen Flip. Likewise, you can use the time of key press returned by KbCheck, rather than calling GetSecs after identifying a key press.
I don't think you need the FlushEvents command, and it might be causing some timing variability.
It is also sometimes useful to pause for a short amount of time (for example 1 millisecond) between KbCheck events.
Below is a version of your code snippet with a few of these changes. It may also be more concise to have a single response checking loop, in which you Flip off the stimulus after 200 ms, rather than separate pre 200 ms and post 200 ms response checking loops, though I haven't made that change here.
keyIsDown=0; correct=0; rt=0;
[~, timeStart] = Screen('Flip', mainwin);
while 1
bf = 0; %this variable is irrelevant here, I use it later to break
%out of a loop
while (GetSecs - timeStart) < 0.2 %faces are presented briefly, but
%I'm recording responses here anyway, just in case there are some
%fast anticipatory responses - after this loop is over, I keep
%recording RT and button press the exact same way, but with no
%stimulus present
[keyIsDown, secs, keyCode] = KbCheck;
if keyIsDown
nKeys = sum(keyCode);
if nKeys==1
if keyCode(Key1)||keyCode(Key2)
rt = 1000.*(secs-timeStart);
keypressed=find(keyCode);
Screen('Flip', mainwin);
type = 'T';
bf = 1;
if keyCode(Key1) & targ_pic == 1
correct = 1;
elseif keyCode(Key2) & targ_pic == 0
correct = 1;
end
break;
elseif keyCode(escKey)
ShowCursor; fclose(outfile); Screen('CloseAll');
return
end
keyIsDown=0; keyCode=0;
end
else
keypressed = 0;
end
WaitSecs(.001);
end
end