Introduction

The concept of multiple memory systems has become one of the foundational constructs in cognitive neuroscience. First suggested by dissociations between different memory tasks in neuropsychological patients, the notion that memory is made up of a number of functionally and neurally distinct components has subsequently gained strong support from functional neuroimaging studies. The most prominent framework separates memory into a declarative system, which relies upon the medial temporal lobe (MTL), and a set of nondeclarative or procedural systems that rely upon brain systems other than the MTL (e.g., N. J. Cohen & Eichenbaum, 1993; Gabrieli, 1998; Squire, 1992). This framework has received substantial support from a wide range of techniques across human and nonhuman species, and has influenced much theoretical and empirical work in skill learning.

Some of the earliest evidence of dissociations between memory systems came from demonstrations that the patient H.M., though suffering from a dense amnesia for episodic information, was able to acquire motor skills in a normal manner (Corkin, 1968; Milner et al., 1968). Subsequent studies demonstrated that amnesic patients are also able to normally acquire a range of perceptual skills (N. J. Cohen & Squire, 1980) and cognitive skills (N. J. Cohen et al., 1985). These findings demonstrated that the MTL is not necessary for skill acquisition, but did not provide any clues as to which neural systems are involved in skill acquisition. Studies of patients with other disorders, particularly basal ganglia disorders such as Parkinson's disease (PD) and Huntington's disease (HD), have shown that the basal ganglia are involved in acquisition of a range of perceptual, cognitive, and motor skills (Heindel et al., 1989; Knowlton et al., 1996a). These findings have suggested to some researchers that the basal ganglia may serve as a general system for the acquisition of novel abilities across domains (e.g., Packard & Knowlton, 2002).