N-back function, same letter repeats more than 4 times

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Selen Soylu
Selen Soylu 2023-2-10
回答： Tridib 2025-6-11，13:05
function [trial, trial_letters] = my_N_back(trial_count,hit_count,N)
alphabet = ["B","C","D","F","G","H","J","K","L","M","N","P","Q" "R","S","T","V","Y","W","Z"];
false_alarm = zeros(1,trial_count - hit_count);
hit = ones(1,hit_count);
no_hit = 1; %determine where there shall be no hits (first two indices)
max_no_hit_N = max(N,no_hit);
trial = [false_alarm(1:max_no_hit_N),Shuffle([false_alarm(max_no_hit_N+1:end), hit])];
%When false_alarm(1:max_no_hit_N) is called, [0 0 ] is the ans, so the
%first two index, no hits and this is determined by the max_not_hit_N.
trial_letters = strings(1,trial_count);
shuffle_alph = Shuffle(alphabet);
trial_letters(1:max_no_hit_N) = shuffle_alph(1:max_no_hit_N); %determine the first two.
last_letter = '';
count_last_letter = 0;
for i= N+1:length(trial)
if trial(i) == 0
rand_letter = alphabet(randi(numel(alphabet),1,1));
while rand_letter == last_letter
rand_letter = alphabet(randi(numel(alphabet)));
end
last_letter = rand_letter;
count_last_letter = 1;
trial_letters(i) = rand_letter;
else
if trial_letters(i-N) == last_letter
count_last_letter = count_last_letter + 1;
if count_last_letter >= 3
rand_letter = alphabet(randi(numel(alphabet),1,1));
while rand_letter == last_letter
rand_letter = alphabet(randi(numel(alphabet)));
end
last_letter = rand_letter;
count_last_letter = 1;
trial_letters(i) = rand_letter;
else
trial_letters(i) = trial_letters(i-N);
end
else
last_letter = trial_letters(i-N);
count_last_letter = 1;
trial_letters(i) = trial_letters(i-N);
end
end
end
end
>This function is supposed to generate a string array and a binary vector when given the input of trial numbers, hit ratio and the N-back specification.
i.e. With the input of 10 as trial numbers, 5 as hit trials and N as 1: the output could be [A B B B C D D E E E ] the binary vector trial should be
[ 0 0 1 1 0 0 1 0 1 1].
But, the trials and trial_letters do not match. The trial vector has ones, but the trial_letters indicate no hit trial. The function needs to be used with N = 2 and N = 3 as well, and same issue happens with that too. Mostly the mid one, so there are ones in the trial output, where there are not hits in the string array.
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Voss
Voss 2023-2-12
Please give an example of correct output and how you know it's correct.

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回答（1 个）

Tridib
Tridib 2025-6-11，13:05
Hi @Selen Soylu,
The code does not work as expected because it relies on “last_letter” and “count_last_letter” to manage repetitions which does not align with the core logic of the N-back task. In an N-back task, whether a letter repeats should depend directly on the value of trial(i).
1. For hit trials (trial(i) == 1), “trial_letters(i)” should be set equal to “trial_letters(i-N)”.
2. For non-hit trials (trial(i) == 0), a new letter should be chosen that is different from “trial_letters(i-N)” to avoid accidental matches. Also, avoid repeating the same letter too many times in a row.
3. Remove “last_letter” and “count_last_letter” and replace the current for loop with the following code that checks trial(i) directly and assigns letters accordingly.
% filling letters based on the trial binary vector
for i = max_no_hit_N + 1 : trial_count
if trial(i) == 1
% Hit: should repeat the letter from N-back
trial_letters(i) = trial_letters(i - N);
else
% Non-hit: pick a new random letter different from i-N
rand_letter = alphabet(randi(numel(alphabet), 1, 1));
while any(i > N & rand_letter == trial_letters(i - N)) || ...
(i >= 2 && rand_letter == trial_letters(i - 1)) % avoiding consecutive repeats
rand_letter = alphabet(randi(numel(alphabet), 1, 1));
end
trial_letters(i) = rand_letter;
end
end
end
Hope this helps!

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