Memories are stored in the brain by changing the basic sensitivity of synaptic transmission between neurons as a result of previous neural activity. The new or facilitated pathways are called memory traces. They are important because once the traces are established they can be selectively activated by the thinking mind to reproduce the memories.
Experiments in lower animals have demonstrated that memory traces can occur at all levels of the nervous system. Even spinal cord reflexes can change at least slightly in response to repetitive cord activation, and these reflex changes are part of the memory process. Also, long term memories result from changed synaptic conduction in lower brain centers. However, most memory that we associate with intellectual processes is based on memory traces in the cerebral cortex.
Positive and Negative Memory—“Sensitization” or “Habituation” of Synaptic Transmission. Although we often think of memories as being positive recollections of previous thoughts or experiences, probably the greater share of our memories is negative, not positive. That is, our brain is inundated with sensory information from all our senses. If our minds attempted to remember all this information, the memory capacity of the brain would be rapidly exceeded. Fortunately, the brain has the capability to ignore information that is of no consequence. This capability results from inhibition of the synaptic pathways for this type of information; the resulting effect is called habituation, which is a type of negative memory.
Conversely, for incoming information that causes important consequences such as pain or pleasure, the brain has a different automatic capability of enhancing and storing the memory traces, which is positive memory. It results from facilitation of the synaptic pathways, and the process is called memory sensitization. As we discuss later, special areas in the basal limbic regions of the brain determine whether information is important or unimportant and make the subconscious decision about whether to store the thought as a sensitized memory trace or to suppress it.
Classification of Memories. We know that some memories last for only a few seconds, whereas others last for hours, days, months, or years. For the purpose of discussing these types of memories, let us use a common classification that divides memories into (1) short-term memory, which includes memories that last for seconds or at most minutes unless they are converted into longer term memories; (2) intermediate long-term memories, which last for days to weeks but then fade away; and (3) long-term memory, which, once stored, can be recalled up to years or even a lifetime later.
In addition to this general classification of memories, we also discussed earlier (in connection with the prefrontal lobes) another type of memory, called “working memory,” which includes mainly short-term memory that is used during the course of intellectual reasoning but is terminated as each stage of the problem is resolved.
Memories are frequently classified according to the type of information that is stored. One of these classifications divides memory into declarative memory and skill memory, as follows:
1. Declarative memory basically means memory of the various details of an integrated thought, such as memory of an important experience that includes (1) memory of the surroundings, (2) memory of time relationships, (3) memory of causes of the experience, (4) memory of the meaning of the experience, and (5) memory of one’s deductions that were left in the person’s mind.
2. Skill memory is frequently associated with motor activities of the person’s body, such as all the skills developed for hitting a tennis ball, including automatic memories to (1) sight the ball, (2) calculate the relationship and speed of the ball to the racquet, and (3) deduce rapidly the motions of the body, the arms, and the racquet required to hit the ball as desired—with all of these skills activated instantly based on previous learning of the game—and then moving on to the next stroke of the game while forgetting the details of the previous stroke.
