Memory-impaired patients express intact implicit perceptual-motor sequence learning, but it has been difficult to obtain a similarly clear dissociation in healthy participants. When explicit memory is intact, participants acquire some explicit knowledge and performance improvements from implicit learning may be subtle. Therefore, it is difficult to determine whether performance exceeds what could be expected on the basis of the concomitant explicit knowledge. Using a challenging new sequence-learning task, robust implicit learning was found in healthy participants with virtually no associated explicit knowledge. Participants trained on a repeating sequence that was selected randomly from a set of five. On a performance test of all five sequences, performance was best on the trained sequence, and two-thirds of the participants exhibited individually reliable improvement (by chi-square analysis). Participants could not reliably indicate which sequence had been trained by either recognition or recall. Only by expressing their knowledge via performance were participants able to indicate which sequence they had learned.
Experts in skilled motor performance are acutely aware of the distinction between the ability to perform a task and the ability to describe the operations involved in its execution. This dissociation leads to such effects as verbal overshadowing (Flegal & Anderson, 2008; following Schooler & Engstler-Schooler, 1990), in which consciously reflecting on motor movements hinders performance. Knowledge supporting skilled performance may sometimes be reportable verbally, but when conscious reflection produces interference, it implies that there may be separate representations for performance and reporting that can be put into competition.
Memory systems theory has provided a sizeable volume of research on the separate brain systems supporting these different memory types. Explicit, declarative memory supports verbal description and depends on the medial-temporal lobe (MTL) memory system, whereas implicit learning is supported by nondeclarative memory systems that improve performance (Milner, Squire, & Kandel, 1998; Squire & Knowlton, 2000). The basis for this research is the observation of preserved implicit memory in amnesic patients with MTL damage. When trying to learn new facts and events, these patients are typically severely impaired. However, certain forms of skill learning remain intact-for example, mirror tracing, mirrored text reading, and perceptual-motor sequence learning (Brooks & Baddeley, 1976; Cohen & Squire, 1980; Gabrieli, Corkin, Mickel, & Growdon, 1993; Nissen & Bullemer, 1987; Reber & Squire, 1994).
Despite the abundance of neuropsychological evidence for multiple memory systems, clear dissociations between knowing "how" and reporting "what" have been difficult to observe in healthy participants. Functional neuroimaging with healthy participants has shown a dissociation between memory systems in some tasks (Reber, Gitelman, Parrish, & Mesulam, 2003) but not in others (Willingham, Salidis, & Gabrieli, 2002). Attempts to show behavioral dissociations often run into two types of problems: First, healthy participants have intact declarative memory capabilities and are able to remember events that occurred during learning, creating difficulties in separating explicit memory from knowledge of the skill being acquired (Willingham, Greeley, & Bardone, 1993). Both types of memory often develop in parallel (Willingham & Goedert-Eschmann, 1999), and the amount of explicit knowledge can range from minimal to extensive enough that participants can describe and cognitively manipulate their newly obtained memories (Smyth & Shanks, 2008; Wilkinson & Shanks, 2004).
Second, most studies of implicit skill learning find small learning effects that are reliable across groups of participants, making it difficult to be certain that individuals with poor explicit memory have significant implicit knowledge. …