How do I keep the target for a specific time period?

I am trying to let my target (*) stay for .5 sec if the key is not pressed by the user and then continue to another trial. If the key is pressed then I want to record the reaction time. The code I have put in doesn't seem to be working.

if input == get(gcf, 'CurrentCharacter')
tic 
toc = RT 
elseif isempty(input)
    pause(.5)  
   
end

This is the full script.

%Lab Assignment 2: Posner Task

ntrials = 20 %number of trials

RT = zeros(2,ntrials) %reaction time for 1 row valid and 2 invalid

%create a matrix for recording valid and invalid trials
Valid = [1]
invalid = [Valid-1]


%Create the figure for trial 
t = figure
xlim([1 5])
ylim([1 5])
lower_l=rectangle('Position', [1.3 2.8 .5 .5]); 
hold on
upper_l=rectangle('Position', [2.3 2.8 .5 .5]);
hold on
lower_r=rectangle('Position', [3.3 2.8 .5 .5]);
hold on
upper_r=rectangle('Position', [4.3 2.8 .5 .5]);
hold on 
cross=text(3, 3, '+');
set(cross, 'fontsize', 20);

%create conditions for trails
for ntrials = 1:20
rnd = rand; %randomizing the cue for trials
 if (rnd < .25)
     set(lower_r, 'EdgeColor', 'r')
     pos = 3
      pause(0.1)
set(lower_r, 'EdgeColor', 'k')
pause(0.3)
 elseif (rnd > .25) && (rnd < .5)
    set(lower_l, 'EdgeColor', 'r')
    pos = 1
pause(0.1)
set(lower_l, 'EdgeColor', 'k')
pause(0.3)
 elseif (rnd > .5) && (rnd < .75)
    set(upper_l, 'EdgeColor', 'r')
    pos = 2
pause(0.1)
set(upper_l, 'EdgeColor', 'k')
pause(0.3)
 else
    set(upper_r, 'EdgeColor', 'r')
    pos = 4 
pause(0.1)
set(upper_r, 'EdgeColor', 'k')
pause(0.3)

 end

 target_rand = rand;
 
if (target_rand > .85) %no target condition
 
elseif (target_rand < .425) %valid trial conditions
    if pos == 4
        Targ3 = plot(4.5, 3, '*') %plot target at 1
        pause (.15)
        
            
        set(Targ3, 'Marker', 'none')
        
        
    elseif pos == 1
        Targ1 = plot(1.5, 3, '*') %target at 2
        pause (.15)
        set(Targ1, 'Marker', 'none')
    elseif pos == 2
        Targ2 = plot(2.5, 3, '*') % target at 3
        pause (.15)
        set(Targ2, 'Marker', 'none')
    else pos == 3
        Targ4 = plot(3.5, 3, '*')
        pause (.15)
        set(Targ4, 'Marker', 'none')
    end
elseif pos == 4  %invalid trial conditions
              rand_invalid = rand;
              if (rand_invalid > .33)
              Targ1 = plot(1.5, 3, '*')  % target at 2
              pause (.15)
              set(Targ1, 'Marker', 'none')
              elseif (rand_invalid < .33)
                Targ2 = plot(2.5, 3, '*') %target at 3
                pause (.15)
                set(Targ2, 'Marker', 'none')
              else
                 Targ4 = plot(3.5, 3, '*') %target at4
                 pause (.15)
                 set(Targ4, 'Marker', 'none')
              end
              
elseif pos == 1
    rand_invalid = rand;
    if (rand_invalid > .33)
       Targ3 = plot(4.5, 3, '*') %targ at 1
       pause (.15)
       set(Targ3, 'Marker', 'None')
    elseif (rand_invalid < .33)
       Targ2  = plot(2.5, 3, '*') %targ at 3
       pause (.15)
       set(Targ2, 'Marker', 'none')
    else
       Targ4 = plot(3.5, 3, '*') %targ at 4
       pause (.15)
       set(Targ4, 'Marker', 'none')
    end
    
elseif pos == 2
    rand_invalid = rand;
  
    if (rand_invalid > .33)
       Targ3= plot(4.5, 3, '*') %targ at 1
       pause (.15)
       set(Targ3, 'Marker', 'none')
    elseif (rand_invalid < .33)
       Targ1 = plot(1.5, 3, '*')%targ at 2
       pause (.15)
       set(Targ1, 'Marker', 'none')
    else
       Targ4 = plot(3.5, 3, '*') %targ at 4
       pause (.15)
       set(Targ4, 'Marker', 'none')
    end
    
elseif pos == 3
    rand_invalid = rand;
  
    if (rand_invalid > .33)
       Targ3 = plot(4.5, 3, '*') %targ at 1
       pause (.15)
       set(Targ3, 'Marker', 'none')
      
    elseif (rand_invalid < .33)
       Targ1 = plot(1.5, 3, '*') %targ at 2
       pause (.15)
       set(Targ1, 'Marker', 'none')
    else
       Targ2 = plot(2.5, 3, '*') %targ at 3
       pause (.15)
       set(Targ2, 'Marker', 'none')
    end
    
 %Press key  if target is present and record RT. If not pressed
 %then go to the next trial after a pasue 



if input == get(gcf, 'CurrentCharacter')
tic 
toc = RT 
elseif isempty(input)
    pause(.5)  
   
end

%mean RT for valid and invalid tials
 %RT_Valid) = mean(RT(Valid))
 %RT_invalid = mean(RT(invalid))

 
  
  
 
end


 %add pause between each trial   
 pause(1)
end    

I am new to MATLAB, any help will be appreciated!

Answer 1

[Deepak]

Hi @Aashita Galundia,

To record reaction times when a key is pressed and ensure a pause when no key is pressed, we can use “waitforbuttonpress” function in MATLAB.

Start timing with “tic” when the target appears. If a key is pressed, capture it with “get(gcf, 'CurrentCharacter')” and compute the reaction time using “toc”, storing it in the “RT” matrix.

If no key is pressed, simply “pause” for 0.5 seconds before proceeding to the next trial. This method ensures accurate reaction time recording and smooth trial transitions.

Below is the specific code changes required in your code:

% Inside the main loop where you display the target
tic; % Start timing

% Wait for a key press or timeout
keyPressed = waitforbuttonpress;
if keyPressed
    % A key was pressed
    key = get(gcf, 'CurrentCharacter');
    reactionTime = toc; % Stop timing and get reaction time
    % Store the reaction time in the RT matrix
    % Use pos to determine valid or invalid trial
    if target_rand < .425
        RT(1, ntrials) = reactionTime; % Valid trial
    else
        RT(2, ntrials) = reactionTime; % Invalid trial
    end
else
    % No key was pressed, wait for .5 seconds
    pause(0.5);
end

Please find attached the documentation of functions used for reference:

waitforbuttonpress: www.mathworks.com/help/matlab/ref/waitforbuttonpress.html

get: www.mathworks.com/help/matlab/ref/get.html

I hope this assists in resolving the issue.

Answer 2

[Walter Roberson]

You need a different approach.

You do not want to pause(0.5) -- you want to wait up to 0.5 for a key press and stop waiting as soon as the button is pressed

If you use pure MATLAB, you need to code it as a "busy loop", along the lines of

fig = gcf;

fig.CurrentCharacter = '';

tic;
reaction_time = nan;
while toc < 0.5
    key = fig.CurrentCharacter;
    if ~isempty(key)
        reaction_time = toc;
        break;
    end
end
reaction_times = reaction_time;

but better is to use facilities such as those provided by Psychtoolbox, https://www.mathworks.com/matlabcentral/answers/143088-real-time-detect-keypress#answer_285124