Dreams – The Story in the Brain
So, what *really* is a dream, and what is the purpose of dreaming?
Simply stated, dreams are a necessary function of making *temporary* memories *permanent*.
It's long been known that dreams are composed of memories, and that memories can be triggered by specific electrical activity within the brain. Dr. Wilder Penfield was a Canadian neurosurgeon noted for mapping most of what we know as the "topography" of the brain. In the course of brain surgery to treat epilepsy, he would apply electrical stimulation to various brain areas to locate damaged brain cells. This was (and still is) performed with the patient awake to tell the surgeon what they see, feel, taste, etc. In this manner, as well as studying which functions are lost due to brain damage, neuroscientists know which brain areas are responsible for movement, sensation and various memories. In the 90's, Professors Wilson and McNaughton at the University of Arizona determined that brain cells activated in the rat brain while running a maze were activated in the exact same sequence and relationship while the animal slept immediately following the behavior. From studies since that time, we know that the reactivation of electrical patterns during sleep is essential to "consolidation" of memory from short term storage (essentially what you do when you repeat a phone number to remember it long enough to place a call) to long term storage.
Why is consolidation necessary?
Memory comes in two forms "short term" (also known as "working") and "long-term" memory. Short term/working memory is temporary. It will eventually be forgotten, and in fact, it is best if it is forgotten! The perfect example of short-term memory is remembering a parking spot at work, school or while shopping. It is necessary to remember the detail of the location for a short time-span, but if remembered for longer terms, it can interfere with itself (we call this "proactive interference"). The memory of where I parked yesterday, or last week, would interfere with finding my car today! Thus, short-term memory *should* be forgotten once its usefulness is over.
So how do we (humans) move information from short-term to long-term memory? Two ways: repetition, or strong associations. Repetition is the familiar one. Rote learning. Memorization. "Repeat after me… My car is on level 4, second row..."
The key feature of "consolidation" is that repeating information strengthens the connections between brain cells that form the basis of the information "code" for memory. However, such strengthening requires time, and metabolic processes in the brain cells. This is one reason why an accident victim may not remember details of an accident (or up to hours before) – there may not have been enough time to *consolidate* the memory before the brain received a trauma that caused it to stop processing information. Sleep provides the time, the low body activity frees up the metabolic processes, and dreams provide the repetition to enable memory to be stored most efficiently.
The second method mentioned above is strong association: emotion, trauma, stress. This is a topic for another time, and as we move through The LabRat's Guide to the Brain, we'll touch on Post-Traumatic Stress, and abnormal memory.
With consolidated memory, the mind can build a sense of *time* with respect to events in the outside world. As with yesterday's post on time and dreams – time in the waking world is important, too. Tomorrow's post will deal with Time, the Mind, and Storytelling. In posts to come, we'll move on to look more specifically at memory, and delve into the structure of the brain and how various parts are involved in the process of memory, conscious thought and dreaming.