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

T. Curran and S. W. Keek (1993, Experiment 3) reported that if dual-task training on the serial reaction task (M. J. Nissen & P. Bullemer, 1987) is followed by 2 tests of learning, the 1st 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 hypothesis, according to which the presence of a secondary task suppresses expression of knowledge on the primary task. The authors repeated Curran and Keele's experiment with a slightly different design and with 2 different types of sequences. The results did not conceptually replicate the earlier findings but instead demonstrated larger learning scores under singlethan dual-task tests, thus supporting the suppression hypothesis. The findings imply that existing results demonstrating impaired performance under dual-task relative to single-task conditions cannot be unequivocally interpreted as supporting the attention dependency of implicit sequence learning, both in terms of need for resources and lack of interference.