The biasing experiments with H. M. show a rather different kind of dichotomy; explicit vs implicit memory. Explicit memories can be accessed to guide multiple types of behavior and can rise to consciousness, whereas implicit memories are tied to specific contexts and cannot be accessed deliberately or detected except in special circumstances. Schacter [Schacter, 1987], gives a nice review of the implicit memory research. Savings during relearning, effects of subliminally encoded stimuli, learning and conditioning without awareness, repetition priming effects and findings on amnesic patients like H. M. are some of the evidence for implicit memory.
Shimamura and colleagues studied the effects of word priming in dementia and amnesia [Shimamura et al., 1987]. They tested both word completion priming and verbal memory ability in patients with Alzheimer's disease, patients with Korsakoff's syndrome, and patients with Huntington's disease. Alzheimer's disease causes amnesia because of the degeneration of corticolimbic connections which isolate the hippocampal formation from the rest of the neocortex. Korsakoff's disease have memory impairment due to damage to the diencephalic midline. Huntington's disease prominently affects the basal ganglia and impairs both motor and cognitive functions. All three patient groups exhibited impaired verbal memory on both recall and recognition tests, although Huntington's patients were significantly better than the other two groups. In the word priming task, the Alzheimer's patients were significantly impaired relative to the other two groups. Korsakoff's and Huntington's patients did not show a significant difference from their respective control groups. This probably shows that damage to brain regions in addition to those damaged in amnesia must occur at relatively early stages of the Alzheimer's disease.
Buckner and colleagues analyzed PET scans of subjects during explicit and implicit memory retrieval tasks [Buckner, 1995]. The three different tasks they gave the subjects were explicit recall of previously presented study words, implicit priming effects without intentional recall and a baseline task of retrieval of information from a general knowledge store. Many activations were found to be consistent across experiments. The recall task activated regions in anterior right prefrontal cortex, in addition to areas activated in the baseline condition. In the priming task there was a blood flow reduction in occipitotemporal regions. These experiments suggest that areas of frontal cortex play a role in explicit recall and that an effect of priming may be to require less activation of perceptual regions for the processing of recently presented information.