Serial reaction time (SRT) task studies have established that people can implicitly learn sequential contingencies as complex as fourth-order probabilities. The present study examined people's ability to learn fifth-order (Experiment 1) and sixth-order (Experiment 2) probabilities. Remarkably, people learned fifth- and sixth-order probabilities. This suggests that the implicit sequence learning mechanism can operate over a range of at least seven sequence elements.
Implicit sequence learning is sequence learning that is not the result of conscious, intentional processes and has been studied using the serial reaction time (SRT) task. On each trial, a target appears at one of a number of locations on a monitor and the key corresponding to the location of the target is pressed. In many cases, the sequence of target locations is deterministic. Sequence learning occurs when the repeating sequence of target locations elicits shorter reaction times (RTs) than does a random or newly introduced sequence of target locations. In other cases, the sequence of target locations is probabilistic. Sequence learning occurs when, given previous target locations, more probable succeeding locations elicit shorter RTs than do less probable succeeding locations.
Most SRT task studies establish implicit sequence learning by assessing awareness of the sequence of target locations. Sequence learning that is explicit (i.e., the result of conscious, intentional processes) would presumably lead to an awareness of the sequence of target locations. Thus, a lack of awareness of the sequence of target locations would suggest that sequence learning was implicit. In many studies, RTs reveal learning of the sequence of target locations, and free recall, cued recall, or recognition tasks reveal no awareness of the sequence (e.g., Curran & Keele, 1993; Lewicki, Hill, & Bizot, 1988; McDowall, Lustig, & Parkin, 1995; Reed & Johnson, 1994; Remillard, 2008a; Remillard & Clark, 2001; Stadler, 1989, 1993, 1995).
Implicit sequence learning involves the extraction of sequential contingencies. Surprisingly, there has been very little systematic investigation into the kinds of sequential contingencies that people can learn implicitly. The overwhelming majority of studies have employed sequences in which the next element in the sequence is predictable from elements one or two trials back, with only a handful of studies having used sequences in which the next element is predictable from elements three trials back. One reason for this state of affairs may be the results of a highly influential study by Cleeremans and McClelland (1991) suggesting that people could not learn to use sequence elements four trials back to anticipate the next element in the sequence.
There are two reasons to believe that people are capable of implicitly learning much more complex sequential contingencies. First, implicit sequence learning, unlike explicit sequence learning, is not bound by the capacity limits of working memory. Implicit sequence learning proceeds independently of individual differences in working memory capacity, whereas explicit sequence learning does not (Frensch & Miner, 1994, Experiment 1; Song, Marks, Howard, & Howard, 2009; Unsworth & Engle, 2005). Also, implicit sequence learning is unaffected by the amount of working memory resources devoted to learning the sequence. This is suggested by studies showing that explicit learning of a sequence is accompanied by parallel implicit learning of the sequence, and the extent of implicit learning is equivalent to that when there is no explicit learning (Song et al., 2009; Song, Howard, & Howard, 2007; Willingham & Goedert-Eschmann, 1999; Willingham, Salidis, & Gabrieli, 2002). Finally, sequence knowledge acquired through explicit learning can be used in a controlled and flexible manner (suggesting the involvement of working memory), whereas sequence knowledge acquired through implicit learning cannot (Jiménez, Vaquero, & Lupiáñez, 2006). …