# KbQueueXXX functions: which to use and where do they go in relation to the trials' loop?

**URL:** https://psychtoolbox.discourse.group/t/kbqueuexxx-functions-which-to-use-and-where-do-they-go-in-relation-to-the-trials-loop/4832
**Category:** Programming Help
**Tags:** experiment-design
**Created:** [February 6, 2023, 11:54am UTC](https://psychtoolbox.discourse.group/t/kbqueuexxx-functions-which-to-use-and-where-do-they-go-in-relation-to-the-trials-loop/4832 "2023-02-06T11:54:56Z")
**Posts on this page:** 1
**Page:** 1

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### Author: ![PHJT003](https://avatars.discourse-cdn.com/v4/letter/p/a87d85/32.png) [@PHJT003](https://psychtoolbox.discourse.group/u/PHJT003)
#### Post date: [February 6, 2023, 11:54am UTC](https://psychtoolbox.discourse.group/t/kbqueuexxx-functions-which-to-use-and-where-do-they-go-in-relation-to-the-trials-loop/4832/1 "2023-02-06T11:54:56Z")

</div>

Dear Psychtoolbox users,  
I wrote a script to run a 2AFC task where participants must respond within a fixed deadline of 2 seconds. If participants respond earlier, then they move to the subsequent trial straight away. To collect their responses (i.e., the first key they pressed and their RT), I coded a custom function that incorporates `KbQueueCheck`. I am now reviewing and debugging my script but I have some doubts about the _KbQueueXXX_ functions. I would be grateful if you could give me some feedback.

# Task script

In a mixture of code and pseudocode, here is what I am doing.

```auto
KbQueueCreate(KEYBOARD, KEYS_OF_INTEREST);
KbQueueStart(KEYBOARD);

% TRIALS' LOOP STARTS
for iTrial = 1:nTrials
    KbQueueFlush(KEYBOARD);
    % show stimulus and record its onset and offset times

    respDeadline = stimulusOffset + 2;
    collectResponse(KEYBOARD, respDeadline, stimulusOnset); % <- KbQueueCheck() here!
end
% TRIALS' LOOP ENDS

KbQueueStop(KEYBOARD);
KbQueueRelease(KEYBOARD);

```

## Custom function

Below is my custom function `collectResponse()` where I embedded `KbQueueCheck`.

```auto
function [choice, rt, choiceTime, firstPress, keyCodes, pressed] = collectResponse(deviceIndex, untilTime, targetOnset)
%% LOOK FOR KEYPRESSES
pressed = false;

while pressed == false && GetSecs <= untilTime
    [pressed, firstPress] = KbQueueCheck(deviceIndex);
end

%% PROCESS KEYPRESSES
if pressed == false % NO KEYS WERE PRESSED
    keyCodes = NaN;
    firstPress = NaN;
    choiceTime = NaN;
    choice = NaN;
    rt = NaN;
else % ONE OR MORE KEYS WERE PRESSED
    keyCodes = find(firstPress > 0); % all keypresses
    choiceTime = min(firstPress(keyCodes)); % ts of first keypress
    choice = find(firstPress == choiceTime); % first keypress
    if length(choice) > 1
        % handle simultaneous keypresses
        choice = -999;
    end
    rt = choiceTime - targetOnset; % reaction time
end

end

```

# My questions

I am not sure whether I am calling _KbQueueXXX_ functions correctly and whether they are in the expected position.

1. Shall I keep `KbQueueCreate()`/`KbQueueStart()` and `KbQueueStop()`/`KbQueueRelease()` respectively before and after the trials’ loop?
2. Shall I rather just `KbQueueStart()`, `KbQueueCheck()`, and `KbQueueStop()` at each trial iteratively?
3. Is it OK to `KbQueueFlush()` at the beginning of each trial, before the presentation of a new stimulus?
4. Is my custom function `collectResponse()` fit for the purpose I described at the top of this post?

Thank you very much for your time, I look forward to knowing your thoughts.  
Kind regards,  
Valerio

* * *

> **OS:** Windows 10  
> **MATLAB:** 9.10.0.1851785 (R2021a) Update 6  
> **PTB:** 3.0.18 - Flavor: beta - Corresponds to SVN Revision 13009