Implicit sub-unit learning in a serial reaction time task in humans

In the present serial reaction time task experiment (SRT), a fixed 12-item sequence was practiced in order to evaluate the effect on response times to 3-item sub sequences (triplets) in a subsequent random sequence. Subjects were visually cued to press one out of four keys with a corresponding right -hand finger. The occurrence of implicit sequence knowledge was evidenced b y the increase in mean response time when the transition was made from the final 12-item sequence block to the subsequent random block. In the stimulu s-set applied, a total of 36 triplets could be constructed, of which 24 tri plets were encountered only during the random blocks (random-only triplet s et) (RO-set), whereas 12 triplets were also part of the sequence used in th e sequence blocks (sequence-also triplet set) (SA-set). Approximately 35% o f the triplets that comprised the two random blocks were also presented in the sequence blocks. There was no difference in mean response times between the triplet sets in the random block that preceded the sequence blocks. In the final random block, however, the SA-set induced significantly faster r esponses as compared with the RO-set. We argue that stimulus response assoc iations within the SA-set are responsible for the difference in response ti mes between the two triplet sets in the final random block. (C) 2001 Publis hed by Elsevier Science Ireland Ltd.