A neurobiology of learning beyond the declarative non-declarative distinction

INTRODUCTION In neuroscience and psychology it has become common to distinguish between declarative and non-declarative forms of learning (e.g., Squire and Zola, 1996). The perspective (Squire, 2004, p. 173) that declarative learning “refers to the capacity for conscious recollection about facts and events and is the kind of memory that is impaired in amnesia and dependent on structures in the medial temporal lobe” developed when it was observed that patients with hippocampal damage were able to learn and improve on “procedural” tasks, such as hand-eye coordination, without being able to remember the specific learning episodes (e.g., Scoville and Milner, 1957; Squire, 1992). Such evidence led researchers (e.g., Moscovitch, 1995) to suggest that there are two separate learning systems: one involved in declarative, conscious, learning situated in the Medial Temporal Lobe (MTL) comprising the hippocampus, and another involved in non-declarative, implicit, skill/habit learning involving the basal ganglia. An interpretation of the roles of the hippocampus and the basal ganglia in learning independent from the declarative/non-declarative dichotomy may allow understanding the role of neural structures critical in learning without relying on phenomenological categories—difficult to examine from a scientific perspective—such as awareness and consciousness. The very nature of what consciousness is in fact still hotly debated (e.g., Morin, 2006) and grounding an entire taxonomy of learning and memory on consciousness-based criteria might complicate, instead of simplifying, scientific interpretations, and progress. An alternative perspective to the declarative— non-declarative distinction (e.g., Packard and McGaugh, 1992; McDonald and White, 1993) emphasizes the role of the hippocampus in learning stimulusstimulus relations, and of the dorsal striatum in acquiring stimulus-response relations. Such perspective is consistent with a recent review (Henke, 2010) suggesting that neurobiological models of learning based on consciousness might not be adequate to describe the available experimental evidence.