NBACK has 15 difficulty levels and adapts to the client’s ability.
The difficulty is varied by changing the number of stimuli that the client must remember. At the lower levels the current stimulus needs only to be compared with the immediately preceding one. At the highest levels the current stimulus must be compared with the one that was displayed three places back.The semantic similarity of the pictures represents an additional difficulty parameter. At higher levels the pictures become more similar.The picture content becomes more abstract and hence more difficult to verbalise.The picture presentation time becomes shorter as the difficulty increases.