An intriguing question that scientists and psychologists alike have been striving to answer for centuries is how and where memory processes occur in the brain. Many individuals have devoted their knowledge, time, and talents to this study and will be remembered for their research contributions.

Ivan Pavlov was a Russian physiologist who is famous for his numerous studies on classical conditioning. Using dogs as subjects, Pavlov paired a conditioned stimulus (food) that normally elicited a conditioned response (salivation) with an unconditioned stimulus (bell ringing). Eventually, the unconditioned stimulus became associated with the conditioned response. Pavlov further conducted research on classical conditioning. It was determined from research done by Pavlov that learning done through classical conditioning occurred in the cerebral cortex. Work by Pavlov proved that conditioned responses could not be learned by dogs after removal of the cerebral cortex. Thus the cerebral cortex was determined to be critical for the formation and storage of conditioned reflexes.

Karl Lashley was a stimulus-response behaviorist. He theorized that physical memory traces (engrams) must be made in the brain when learning occurs. These new connections of neurons were assumed to involve the cerebral cortex, as proven by studies conducted by Pavlov. In 1929, Karl Lashley wrote his famous monograph, "Brain mechanisms and intelligence." This work consisted of studies with rats and mazes. Lashley removed portions of the cerebral cortex, varying from 10-50% in an effort to study the role the cerebral cortex played in learning.

These studies brought about two important theories.
The first theory, entitled Principles of Mass Action, proved that the amount of cortex removed was critical to the learning abilities of the rats.
The second theory, entitled Equipotentiality, proved that all areas of the cortex are equally important to learning, or no area was proven to be more important than any other area.

Studies with the central nervous system further support the existence of engrams in the brain. All behavior reflects actions of the nervous system and because the nervous system is a physical-chemical system, changes in behavior from learning must cause physical-chemical alterations. Therefore, all learning must involve alterations between input and output of the central nervous system. Engrams must exist. However, Lashley was never able to find the existence of an engram and concluded therefore that "the necessary conclusion is that learning just is not possible." The engram has still never been found, but groundbreaking research has been conducted that has begun to substantiate the theories of Lashley.

Hebb presented the most successful theoretical view of the general nature of the engram in 1949 as his "Theory of Cell Assemblies." It is clear that the engram does not develop at one particular area in the brain. Hebb's theory supports the view that changes that occur during learning develop among interconnections of neurons throughout wide areas of the brain. Particular kinds of learning have been proven to involve the development of particular circuits of neurons. The engram does not appear to be localized, but its existence cannot be questioned.

Studies by James proved the existence of at least two types of memory.
The first type of memory defined by James was primary memory, or what we now call short-term memory. This is defined as material that lasts a matter of seconds. Primary memory consists of successive events in our environment that span all the senses and result in a continuous experience. Material in primary memory has not yet left consciousness.

James defined a second type of memory as secondary memory. This consists of long-term memory, or permanent memories. This material is held indefinitely and does not reside in consciousness, but is available to bring to consciousness if desired.
It is known that memory has several time constants. However, it is unknown if James's two aspects of memory are two different processes or the same process over different time spans.

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