how to get psychtoolbox to wait for keypress but move on if it hasn't recieved one in a set time

i'm trying to code an experiment, where a participant must make an orientation judgement (more clockwise or more anticlockwise with the keyboard). i want to code it so that if they don't make a response within, say 2 seconds the code continues.

i have tried things like while WaitSecs(2) < 2; [secs, keyCode, deltaSecs] = KbWait(); end

But this doesn't save the variables for use outside of the while loop. Thanks in advance!

Answer 1

[Mihaela Duta]

There are some missing details in the specification given in the question, so I am going to make the following assumptions:

• Item one only one key-press is expected - let's say a press on the left or right arrow key
• Item two immediately after the response is given the waiting for response stops - the time taken to make the response is the reaction time

The follwing code should implement your requirements according to the above assumptions

% improve portability of your code acorss operating systems 
KbName('UnifyKeyNames');
% specify key names of interest in the study
activeKeys = [KbName('LeftArrow') KbName('RightArrow')];
% set value for maximum time to wait for response (in seconds)
t2wait = 2; 
% if the wait for presses is in a loop, 
% then the following two commands should come before the loop starts
% restrict the keys for keyboard input to the keys we want
RestrictKeysForKbCheck(activeKeys);
% suppress echo to the command line for keypresses
ListenChar(2);

% get the time stamp at the start of waiting for key input 
% so we can evaluate timeout and reaction time
% tStart can also be the timestamp for the onset of the stimuli, 
% for example the VBLTimestamp returned by the 'Flip'
tStart = GetSecs;
% repeat until a valid key is pressed or we time out
timedout = false;
% initialise fields for rsp variable 
% that would contain details about the response if given
rsp.RT = NaN; rsp.keyCode = []; rsp.keyName = [];
while ~timedout,
    % check if a key is pressed
    % only keys specified in activeKeys are considered valid
    [ keyIsDown, keyTime, keyCode ] = KbCheck; 

      if(keyIsDown), break; end
      if( (keyTime - tStart) > t2wait), timedout = true; end
  end
  % store code for key pressed and reaction time
  if(~timedout)
      rsp.RT      = keyTime - tStart;
      rsp.keyCode = keyCode;
      rsp.keyName = KbName(rsp.keyCode);
  end

% if the wait for presses is in a loop, 
% then the following two commands should come after the loop finishes
% reset the keyboard input checking for all keys
RestrictKeysForKbCheck;
% re-enable echo to the command line for key presses
% if code crashes before reaching this point 
% CTRL-C will reenable keyboard input
ListenChar(1)

[AMULYA H]

but its coming as undefined function or kbname

[Walter Roberson]

That code relies heavily on functions from the Psychtoolbox http://docs.psychtoolbox.org/KbName

[Sara Santos]

Hi, thank you very much for the information. I used your example and have the following now:

t2wait=2;

while ~timedout

    [ keyIsDown, keyTime, keyCode ] = KbCheck; 

    if (keyIsDown)
          break; 
    end

    if ((keyTime - tStart) > t2wait)
          timedout = true; 
    end

end

if(~timedout)
    rsp.RT      = keyTime - tStart;
    rsp.keyCode = keyCode;
    response = KbName(rsp.keyCode);
    responded = 1;
end

However, I wanted the screen to remain for 2 seconds total (my t2wait), even after the keypress. For e.g., if key is pressed after 1 second, I wanted the screen to remain one more second until next screen appears. I tried to remove the following part:

   if (keyIsDown)

          break; 
    end

but by doing so, after pressing the key the screen moves on immediately and it also doesn't registers my keypress saying I didn't pressed any key.

Do you have any suggestion on how can I solve this?

Thank you!

[kwn500]

If you remove the 'break', (but not the whole if(KeyisDown) loop) it should register your keypress and continue until the end of your specified time.

[Barbara Fiorani]

What rsp.RT = NaN; stand for?

[Edan Daniel]

It's the initialization of the reaction time field (RT) in the response (rsp) structure. That is, setting that until a reaction time is recorded, rsp.RT equals nothing (NaN). If a key press is detected, this field should be updated as follows (as seen a few lines below the line in question in Mihaela's example)

rsp.RT = keyTime - tStart;

That is, replacing the NaN with the recorded reaction time: the difference between the start time, and the time of key press. If no key press is detected, this field will not be updated, and will keep holding a 'NaN' indicating no reaction time was recorded.