MATLAB: How to ensure equal probability of stim appearing on L or R of screen without hard-coding? Description inside.

psychtoolbox

Hi,

I am working on a task in Psychtoolbox where I'd like to display a stimulus 50% of the time on the left side of the screen, and 50% on the right side. Is there a way to ensure an equal balance of where the stimulus will appear without hard-coding? I would like it to be random (but always balanced) each time I run the task, but am not sure how to set this up. Thanks!

Best Answer

50% turns out to be surprisingly tricky

Related Solutions

MATLAB: Create a pseudo-random array with a set probability of similarity to an existing random array

With such constrained stimulus pairs it might be more straight-forward to generate exactly the pairs you want systematically and then randomize their order at the end, maybe like this:

matches4 = [1 1; 2 2; 3 3; 4 4];
matches16 = repmat(matches4,4,1); % Here are the 16 match trials.
nonmatches12 = [1 2; 1 3; 1 4; 2 1; 2 3; 2 4; 3 1; 3 2; 3 4; 4 1; 4 2; 4 3]; % 12 possible nonmatches
% Now make 4 extra nonmatches so that you will have sixteen total:
nonmatches4 = [1 2; 3 4; 4 1; 2 3];  % Not entirely sure which nonmatches you want to repeat.
stimuli = [matches16; nonmatches12; nonmatches4];  % stimulus pairs for 32 trials
stimuli = stimuli(randperm(size(stimuli, 1)), :);  % randomize the order of the stimulus pairs

MATLAB: Random number between 0 and 1

>> V = [zeros(1,100),ones(1,312)];
>> V = V(randperm(numel(V)));

Checking:

>> nnz(V==0)
ans =  100
>> nnz(V==1)
ans =  312
>> V(:)
ans =
   1
   0
   1
   1
   1
   1
   1
... lots of rows here
   1
   1
   1
   1
   1
   0
   1
   0
   1
   1
   1

Best Answer

Related Solutions

MATLAB: Create a pseudo-random array with a set probability of similarity to an existing random array

MATLAB: Random number between 0 and 1

Related Question