Research focusing on human memory suggests that memory is not a single or unitary faculty of the mind. Instead, it can be conceived of as a variety of distinct and dissociable processes and systems that are subserved by particular constellations of neural networks that mediate different forms of learning (e.g., Gabrieli et al., 1995; Tulving, 1995; McDonald, Ergis, and Winocur, 1999).

One categorical distinction within human memory is that between declarative memory and non-declarative memory (Tulving, 1995). According to this division, non-declarative memory is characterized by unintentional learning, or learning without awareness, and by inability to access conscious recall. This form of memory is manifested in multiple dissociable processes and is measured in terms of changes in performance (produced by conditioning and priming) in the learning of motor skills (e.g., Schacter, 1992). In contrast, declarative memory can be conceptualized as learning with awareness and refers to the acquisition and retention of information about events and facts, and is typically assessed by accuracy in tests of recall and recognition. The available evidence suggests that medial temporal/diencephalic structures are critical for the integrity of declarative memory, whereas non-declarative forms of memory rely on other brain areas, such as occipital structures (visual priming) (Gabrieli et al., 1995) and basal ganglia (procedural memory) (Heindel et al., 1989).

Although based on hypothetical constructs and still under considerable debate, the view that human memory is composed of at least five different systems has been highly influential (e.g., Nyberg and Tulving, 1996; Roediger, Buckner, and McDermott, 1999).