As defined earlier, implicit memory refers to the influence of past experience on current behavior that is not conscious or intentional. In other words, retention of linguistic, visual, or motor information can occur in the absence of explicit retrieval of that information. An example of the manner in which explicit memory and implicit memory can be dissociated is given by the results of a fragment stem completion task (Tulving et al., 1982), which illustrates the priming effect. Priming refers to the increased facilitation of identifying a stimulus that occurs as a result of having had prior exposure to that stimulus (Schacter et al., 1993), even though the individual may have no conscious recollection of the exposure. On the basis of numerous studies with brain-damaged and nor-mal individuals, it is probable that the perceptual representation subsystems for priming are a visual word form system represent-ing the orthographical features of words, a system that represents structural relations among parts of objects, and an auditory word form system that mediates phonological or acoustic information. The likely brain regions associated with each of these subsystems are the extrastriate cortex, inferior temporal region and perisyl-vian cortex.
Although the priming effect is the most extensively studied aspect of implicit memory, it is important to note that the learning of motor skills and habits and the phenomenon of classical condi- tioning fall under the rubric of implicit memory. These processes may be considered as part of implicit memory because behavior changes and learning occur without conscious. This behavioral dissociation between priming and skill learning suggests that the latter may be mediated by the corticostriatal loop that mediates motor programming, namely the basal ganglia, thalamus, and motor, premotor and sensorimotor cortex (Heindel et al., 1991).