Code check: Unusual reaction times (like less than 100 ms) for an AX-CPT task coded using PTB version 3.0.19

Programming Help
1

I am coding an AX-CPT task using PTB in MATLAB and while doing pilot testing, I am getting unusual RTs (Less than 100ms) when the participant is trying to do the task fast. Below are the details of the environment I am using:

PsychtoolBox version: ‘3.0.19 - Flavor: Manual Install, 16-Jan-2026 16:17:44’

Platform: Windows 11

Matlab Version: Matlab R2025b

Graphics detail: Intel UHD graphics 730

Code (Just the probe section where I am trying to record the RT):

Target letter code snippet: 
%Probe letter
% Clear queue before probe

KbQueueStop;
KbQueueFlush;

% Wait until all keys are released

while KbCheck
end

Screen('TextSize',window,60);
DrawFormattedText(window,probeLetter,'center','center',magenta);


% Flip probe to screen and record TRUE onset time

tOnProbe = Screen('Flip',window,vbl+(waitframes-0.5)*ifi);

KbQueueFlush;

KbQueueStart;

while isnan(respProbe)

 [pressed, firstPress] = KbQueueCheck;

if pressed

% Accept response only if it occurred AFTER probe onset

if firstPress(numpad1) > tOnProbe

 respProbe = 1;

 rtProbe = firstPress(numpad1) - tOnProbe;

elseif firstPress(numpad2) > tOnProbe

 respProbe = 2;

 rtProbe = firstPress(numpad2) - tOnProbe;

end

end

end

KbQueueStop;

Can anyone tell me if it’s an issue with how I have set up the probe timing recording or is it anything else?

2

As you are not using a paid version of Psychtoolbox, but an end of life version, I didn’t read far beyond the headline.

But what will obviously end badly if accurate visual stimulus timing matters, as explained on our websites System Requirements section, and numerous times here, is using Intel graphics on MS-Windows. This is true not only for PTB. Intel graphics under Linux is usually fine, at least with PTB, as opposed to other less advanced toolkits.