A secondary tone-counting task suppresses expression of knowledge in the serial reaction task

T. Curran and S. W. Keele (1993, Experiment 3) reported that if dual-task t raining on the serial reaction task (M. J. Nissen & P. Bullemer, 1987) is f ollowed by 2 tests of learning, the Ist under dual-task conditions and the 2nd under single-task conditions, then participants' learning scores do not differ for the 2 learning tests. This finding argues against the "suppress ion" hypothesis, according to which the presence of a secondary task suppre sses expression of knowledge on the primary task. The authors repeated Curr an and Keele's experiment with a slightly different design and with 2 diffe rent types of sequences. The results did not conceptually replicate the ear lier findings but instead demonstrated larger learning scores under single- than dual-task tests, thus supporting the suppression hypothesis. The find ings imply that existing results demonstrating impaired performance under d ual-task relative to single-task conditions cannot be unequivocally interpr eted as supporting the attention dependency of implicit sequence learning, both in terms of need for resources and lack of interference.