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Every implementation of dual n-back that I've ever seen uses the set of letters 'C', 'H', 'K', 'L', 'Q', 'S', 'R' and 'T'.

Why is that?

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Interesting. Might be to reduce ambiguity, those letters are mostly visually and audibly distinct as opposed to making a test of C D B G P or O G C Q D –  Ben Brocka Feb 10 '12 at 22:49
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I don't know that it always has to use those letters. (see www.brainboffin.com; also, a quick lit search shows it's also been done with numbers and Mandarin phonemes). I suspect it's partly to do with the phonetic properties of the sounds, as Ben Brocka suggests, and partly because the dual n-back has mostly been run by a single lab (Jaeggi). It's easier for them to consistently use the same experimental setup, and increases their chances of reproducing the effect. –  Jeff Feb 11 '12 at 0:16
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up vote 4 down vote accepted

The set is not always the one you suggest. There are many implementations that don't even involve letters, but use random shapes, numbers, spatial locations, or other stimuli.

In Jaeggi et al., 2007, they used spatial locations for the visual task, and

The auditory material consisted of eight German consonants (c, g, h, k, p, q, t, and w) spoken in a female voice and selected on the basis of their distinctiveness.

(emphasis mine).

In Takeuchi et al. 2010, they used numbers and locations:

“Dual” N-back task. In this task, the number stimuli consisted of numbers, 1 to 4, presented in a random sequence in one of four places on a line.

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