How to randomly present stimuli a specific number of times

Hi, I am currently trying to randomly present a series of consonents as part of an N-back task. I have a folder in which I have stored the consonents as jpg. images. There are 21 consonents in the folder. For the task, there should be 48 letter presentations in one sequence. However, there are only 21 consonents in the alphabet. So I was wondering if there was a way in which I could ensure that my code allows me to present the stimuli 48 times. Currently my code only allows me to present each letter once, so I only have 21 trials in the sequence.

I have the following code;

d=21; %Number of images to load in is 21. 
if d<1 || d>21 %A check to ensure that the correct number of images will be displayed. 
    disp('Error, the number of images loaded in out of range')
end

rts= (zeros(21, 1)); %rts is a 21 by 1 array of zeros 
response=(zeros(21, 1)); % response is a 21 by 1 array of zeros
dirname= 'C:\Users\User\Documents\MATLAB\stimuli\'; %Location of stimuli  

count= 0;

d=dir([dirname '*.jpg']); 

for a =randperm(numel(d))% return scalar count of elements in matrix
    %And randomly present the images under the variable of d. 
    a= imread(d(a).name); %read in images in desired folder
    imshow(a) %show selected images
    pause(2); %allow a 2 second pause between successive stimuli.
    b= imread('FixationDot.jpg');% read in the fixation dot 
    imshow(b) %show the fixation dot between every letter
    pause(0.5); %allow the fixation dot to remain there for 0.5 seconds 
    tic
    count= count+1;
    response(count)= (getkey()); %Gain user input and convert ASCII to char format
    rts(count)= toc; %Record the time taken to produce a response from user 
    
end
CH= char(response); %conver ASCII to underlying char elements

I appreciate any suggestions that anyone has, thank you.

Answer 1

[Jeff Miller]

If you want to present 48 stimuli, you need to replace your line

for a =randperm(numel(d))

with something like this

for a =numstoshow

where numstoshow is a vector of 48 numbers, each of which is between 1 and 21. You'll have to work out exactly how you want to construct that sequence of 48 numbers, depending on how often and how closely you want the stimuli to repeat within the sequence. There would be presumably be some constraints on that for an N-back task, depending on exactly how you want that task to work.

[Walter Roberson]

You should also consider reading in all of the images ahead of time, since you know you are going to need to show them all and the setup is not time-sensitive but the presentation is time sensitive.

You should also have a look at the third party Psychtools (PTB) which has developed functions for precise presentation times and precise response timing.