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The unconscious or subconscious mind, according to classical Freudian psychoanalysis, is a "part" of the mind that stores repressed memories. The theory of repression maintains that some experiences are too painful to be reminded of, so the mind stuffs them in the cellar. These painful repressed memories manifest themselves in neurotic or psychotic behavior and in dreams. However, there is no scientific evidence either for the unconscious repression of traumatic experiences or their causal agency in neurotic or psychotic behavior.
It would be absurd to reject the notion of the unconscious mind simply because we reject the Freudian notion of the unconscious as a reservoir of repressed memories of traumatic experiences. We should recognize that it was Freud more than anyone else who forced us to recognize unconscious factors as significant determinants of human behavior. Furthermore, it seems obvious that much, if not most, of one's brain's activity occurs without our awareness. There is no question that we sense many things without being consciously aware of them (see clever Hans phenomenon, for example). There is also no question that unconscious factors can affect behavior or motor action (see ideomotor action, for example). There is little question that many unconscious factors drive such complex phenomena as language ability. Consciousness or self-awareness is obviously the proverbial tip of the iceberg. But much interest in the unconscious mind has been restricted to potentially harmful memories that might be stored or stirring there, memories of bad experiences that influence our conscious behavior even though we are unaware of their impact. Others have shown interest in the unconscious mind as a reservoir of universal truths or a place where the "true self" dwells. Neither of these views seems well supported by the empirical evidence. The modern view of the unconscious has been driven by a number of empirical studies of how the brain works when it is damaged. Many of these studies use fMRI technology, which allows the researchers to peer at brain activity while a person is actively engaged in thinking. For more on the modern view of the unconscious see On Being Certain: Believing You are Right Even When You're Not by Robert A. Burton, M.D.(2008 St. Martin's Griffin) and Subliminal: How Your Unconscious Mind Rules Your Behavior by Leonard Mlodinow (2012 Random House).
It is assumed that the unconscious is distinguished from the conscious by the fact that we are aware of conscious experience, but unaware of the unconscious. However, there is ample scientific data to establish as a fact that most conscious perception goes on without self-consciousness. It is even possible to be unaware of having experienced something and unable to remember the experience, but still give evidence that one has had the experience. Several examples should suffice to establish this point.
1. blindness denial. There are cases of brain-damaged people who are blind but who are unaware of it.
2. jargon aphasia. There are cases of brain-damaged people who speak unintelligibly but aren't aware of it.
3. blindsight. There are cases of brain-damaged people who see things but are unaware of it.
4. oral/verbal dissociation. There are cases of brain-damaged people who cannot orally tell you what you just said, but they can write it down correctly. Furthermore, they can't remember what they wrote down or what it refers to.
5. sensing without seeing. There are many cases of people whose brains are not damaged who give evidence that they have seen or heard something even though they are not conscious of having seen or heard the item in question.
Psychologist Jim Alcock relates two personal anecdotes that illustrate sensing without seeing (Alcock 1981), mentioned above in the list of items exemplifying experiencing something without awareness. In one, he was standing in line at the snack bar of a theater "and was idly recalling a conversation I had once had with the brother of a colleague. I had only met the brother once or twice and had not seen him in months." Alcock turned around and, lo and behold, there was the brother! The man has a very distinctive voice and Alcock had been hearing him without being aware of it. Others might think the experience is an example of ESP. The other story involves driving a friend from the airport and finding that both Alcock and the friend were simultaneously thinking of a college classmate. They retraced their route and discovered that they had passed a store window with a pendulum clock. This made them realize that even though they were not aware of having seen the clock in their earlier drive-by, they had both sensed it. The fellow they remembered was distinctive because he always walked around with a pendulum pedometer or some such device. Again, others might interpret this experience as paranormal.
[revision] Given the history of the term 'unconscious,' many modern researchers find it to be less confusing to abandon talk of the unconscious mind and refer instead to "lost memory," "fragmented memory," "implicit memory" (a term coined by Daniel Schacter and Endel Tulving), subliminal perception, (Mlodinow), or the hidden layer (Burton). It is not repression of traumatic experiences that causes memories to be lost. Memories are lost sometimes because of inattentiveness in the original experience and because the original experience occurred at an age when the brain was not fully developed. Memories are also lost because we have no recognizable need to reference the original experience. (Many fragments of pleasant experiences, such as the name of a place or a product, may be influencing present choices without one's being aware of it.) Memories are lost because of brain damage, loss of consciousness during an experience, neurochemical imbalance, cognitive restructuring, and sensory, emotional, or hormonal overload. In any case, our brains would be taxed beyond use if we remembered every bit of sense data we've experienced. On the other hand, all the empirical evidence indicates that the more traumatic an experience is, the more likely one is to remember it. Novel visual images, which would frequently accompany traumas, stimulate the hippocampus and left inferior prefrontal cortex and will generally become part of long-term memory.
Neuroscience tells us that a memory is a set of connections among groups of neurons that participate in the encoding process. Encoding can take place in several parts of the brain. Neural connections go across various parts of the brain; the stronger the connections, the stronger the memory. Recollection of an event can occur by a stimulus to any of the parts of the brain where a neural connection for the memory occurs. If part of the brain is damaged, access to any neural data that was there is lost. On the other hand, if the brain is healthy and a person is fully conscious when experiencing some trauma, the likelihood that they will forget the event is near zero, unless either they are very young or they later experience a brain injury.
Long-term memory requires elaborative encoding in the inner part of the temporal lobes. If the left inferior prefrontal lobe is damaged or undeveloped, there will be grave difficulty with elaborative encoding. This area of the brain is undeveloped in very young children (under the age of three). Hence, it is very unlikely that any story of having a memory of life in the cradle or in the womb is accurate. The brains of infants and very young children are capable of storing fragmented memories, however. Such memories cannot be explicit or deeply encoded, but they can nevertheless have influence. In fact, there are numerous situations—such as cryptomnesia—where memory can be manifested without awareness of remembering. But such unconscious memories, even though pervasive, are not quite what Freud or Jung meant by the unconscious. "In Freud's vision, unconscious memories are dynamic entities embroiled in a fight against the forces of repression; they result from special experiences that relate to our deepest conflicts and desires. . . .[I]mplicit memories . . . arise as a natural consequence of such everyday activities as perceiving, understanding, and acting." (Schacter 1996, 190-191) Implicit memory may be far more mundane than Freud's dynamic 'unconscious mind', but it is more significant since it reaches into every aspect of our lives. As Daniel Schacter notes: "If we're unaware that something is influencing our behavior, there is little we can do to understand or contradict it." (191) This applies not just to unwanted thoughts, feelings, or behaviors, but to all thoughts, feelings, and behaviors. We are never consciously aware of the potentially thousands or millions of neural connections going on that have some sort of influence on how we feel, think, and behave. Oh, we're aware that they're happening--if we know anything about how the brain works--but they occur hidden from consciousness.
Most lost memories are lost because they were never elaborately encoded. Perception is mostly a filtering and defragmenting process. Our interests, needs, and past experience affect perception, but most of what is available to us as potential sense data will never be processed. And most of what is processed will be forgotten. Amnesia is not rare but the standard condition of the human species. We do not forget in order to avoid being reminded of unpleasant things. We forget either because we did not perceive closely in the first place or we did not encode the experience either in the parietal lobes of the cortical surface (for short-term or working memory) or in the prefrontal lobe (for long-term memory). But, no matter how closely we perceive anything or how much we work at remembering something, the brain has not evolved to be a recorder of every possible percept. The brain is selective and constructive. Models of the brain as recorders of everything one perceives, such as that of Scientology creator L. Ron Hubbard, are not based on reality. They are fantasies. [/revision]
books and articles
Consciousness and Neuroscience by Francis Crick and Christof Koch