When task exposure facilitates performance without producing corresponding changes in verbalizable knowledge, learning is said to be implicit. In Experiment 1, event-related potentials (ERPs) were recorded as individuals practiced an implicit structured sequence learning (ISSL) task wherein only some target events required a response. With practice, the ERPs to targets that obeyed the underlying grammar diverged from those that did not at around 200 ms; grammatical targets appeared to be more positive between 200 and 500 ms because a similar positivity for the ungrammatical targets was delayed. In Experiment 2, the grammar was simplified allowing a direct comparison to be made between an implicit learning group and an explicit group, who were taught the grammar prior to recording. The results of the comparison revealed a remarkable similarity but did implicate at least partially nonidentical neural mechanisms in implicit and explicit structured sequence learning.Descriptors: Implicit structured sequence learning, P300, Implicit learning, Awareness ERP There is mounting evidence from neuropsychological studies of amnesia for the existence of multiple memory systems. Despite being severely impaired on conventional memory tests such as recall, cued recall, and recognition, amnesic patients are unimpaired on indirect memory measures, like priming, which do not require any conscious recollection (for reviews, see RichardsonKlavehn & Bjork, 1988;Schacter, 1987). Squire and his colleagues have argued that explicit recollection is a property of a declarative hippocampal memory system, which is damaged in amnesia, whereas the heterogeneous indirect memory phenomena can be attributed to a procedural (or nondeclarative) memory system (or systems), mediated by spared structures such as the neocortex, striatum, cerebellum, and amygdala (Cohen & Squire, 1980;Shimamura & Squire, 1989;Squire, 1992; but see Roediger & Blaxton, 1987, for a contrasting view).It is a logically independent question, however, whether there exist dissociable learning systems that supply these memory systems with information. There is some weak evidence for this dis- 74sociation from the amnesia literature. Milner and colleagues demonstrated in the 1960s that the profoundly amnesic patient H.M. could acquire motor skills such as pursuit rotor and mirror tracing, despite being unable to remember explicitly that he had previously performed the task (Milner, Corkin, & Teuber, 1968). Since that time, amnesic patients have been shown to exhibit normal or near-normal learning in a variety of motor, perceptual, or cognitive tasks, again without any conscious recollection of having practiced them (for review, see Schacter, 1987). Although these data are suggestive, they offer no direct evidence indicating that the original learning was not accompanied by awareness; these data merely demonstrate that the acquired knowledge is implicit.Recently, robust implicit learning has been claimed for normal individuals in three different experimental paradigms: a...