Several researchers (Underwood, 1996) draw distinctions between two types of memory. Explicit memory refers to one’s conscious recollection of the past as manifested on tasks like recall and recognition. Implicit memory refers to any changes in experience, thought and action that are attributable to a past event, regardless of whether that event is consciously remembered. Clinical observations of patients following removal of the hippocampus demonstrate related changes in action following exposure to events that cannot be remembered, or perhaps it would it be more correct to say cannot be recalled (Ingram, 1995).
The study of cognitive neuropsychology marks the boundary wherein new evidence is now being gathered about memory loss from brain-damaged patients. The amnesic syndrome results from bilateral damage to the hippocampus and related structures in the medial temporal lobe or alternatively the diencephalon and mamillary bodies (Kihlstrom, 1999). Such patients show dense anterograde amnesia, which means that they cannot remember even the most recent events after only momentarily distraction. Memory for events that occurred pre-trauma is not usually affected and may be recalled after many years as though they had only happened recently. Although amnesiacs cannot recall even the most recent events in their lives an analysis of their subsequent actions and thoughts demonstrates that they have never the less been influenced by these events.
Neuropsychological research has also revealed unconscious influences in the perceptual domain. Blindsight which is documented in some patients with damage to the striate cortex of the occipital lobe (Ingram, 1995) presents a dramatic example of this particular phenomenon. Patients exhibit a scotoma, a portion of the visual field where they have no visual experience. When visually stimulated this area sees’ nothing and yet when later questioned about the stimulus (form, movement, orientation and size) applied to the scotoma and encouraged to guess the answers they give a percentage of correct answers which is better than chance alone. This evidence was of particular interest to the writer in developing the EMW model of consciousness since it could be construed that a non-visual quantum level stimulus is actually received by the scotoma. Although this non-visual stimulus is real, the brain is not conscious of processing the quantum information contained in the stimulus and hence the patient reports seeing nothing. However the information is recorded by the patient in what has previously been referred to as implicit memory, useful for processing but not normally available to introspection.
Lesions in the temporoparietal region of one hemisphere (usually the right one) that do not affect primary sensory or motor cortices give rise to cases of visual neglect (Ingram, 1995). These patients appear to neglect the corresponding portion of the contralateral sensory field (usually the left one). Thus a patient questioned about a series of drawings may ignore items on the left hand side of the page. In one particular series of trials the patient was asked to report on the state of two drawings of houses, one on the left with flames leaping out of the windows and one on the right that appeared quite normal. The patient’s responses ignored the state of the house on the left and reported that all was quite normal, an example that gave rise to the title for Ingram’s popular book, The Burning House’ (1995). As with Blindsight patients when questioned closely about items presented on the left hand side of the page and encouraged to guess the answers they give a percentage of correct answers which is better than chance. Preserved visual functioning in Blindsight and in neglect is referred to by Kihlstrom (1999) as unconscious perception.
Perception without awareness may also be observed in people who have no obvious neurological impairment. This has been termed by Greenwald et al (1996) as unconscious semantic processing. Kihlstrom uses the term subliminal processing, which has unfortunate connotations with discredited methods of advertising that claimed to influence customer choice using subliminal messages projected onto a movie screen. Indeed few topics in psychology have been more controversial over the last fifty years, despite some strong evidence from experiments conducted by Anthony Marcel in the 1980’s some of which have since been successfully replicated (Kihlstrom, 1999). Marcel presented solid evidence of subliminal semantic priming effects on lexical decision; presentation of a word like doctor primed lexical decisions of a semantically related word like nurse even though an intervening mask prevented subjects from consciously perceiving the prime itself. Greenwald et al (1995) repeated and extended these same results. Perhaps it is a seeming lack of any acceptable classical’ explanation that has prompted the negative response to subliminal processing?