Trying to increment j won't work like that. Just specify a vector that skips every other integer. In this case, label_name has an odd-integer name; control_name has an even-integer name.
for j = 1:2:length(filenames_asl) % every other
label_name = char(filenames_asl(j)); % filename_asl is the string array where the sequential filenames are saved
control_name = char(filenames_asl(j+1)); % Second image (filename)
label = load(strcat(foldername_asl, '/', label_name)).im1; % Loading the first image
control = load(strcat(foldername_asl, '/', control_name)).im1; % Loading the second image
asl_image = label - control; % Getting difference image
imagename_asl = fullfile(dir_asl, strcat(im_name_asl, '_', num2str(j), '.mat')); % Specifying name and location for saving the difference images
save(imagename_asl); %% Saving the difference image
end
Your output files will be numbered with odd integers unless you change it to use something other than j. If you want them to be numbered sequentially 1-15, then:
imagename_asl = fullfile(dir_asl, strcat(im_name_asl, '_', num2str((j+1)/2), '.mat')); % Specifying name and location for saving the difference images
