Biology 202
1998 First Web Reports
On Serendip

Learning and Memory

Ruchi Rohatgi

Learning and memory are not unitary processes. Learning is the process by which new information is acquired; memory is the process by which that knowledge is retained. Learning can be divided into two types: 1)Explicit memory is the conscious acquisition of knowledge about people, places and things. It occurs in the highly developed vertebrate brain ,mainly in the diencephalic structure (1). 2) Implicit memory is the non conscious learning of motor skills and other tasks. It does not depend on the temporal lobe, but involves the sensory, motor associated pathways in the expression of learning process. This type of memory can be studied in higher invertebrates whereas explicit learning is only studied in mammals (2).

After reviewing the enormous amount of information pertaining to memory and learning, I decided to focus on two areas of interest: the actual mechanism of learning and the process of consolidation which relates to how something that is learned is then stored as memory. The first area of interest involving learning has been elucidated by the concept of long term potentiation (LTP) in mammals and vertebrates and the second related area of consolidation has been elucidated by an invertebrate model of Aplysia.

Long term potentiation can be defined as a long lasting increase in synaptic effectiveness which follows from high frequency of stimulation of afferent fibers. Although LTP occurs throughout the nervous system, it's focus has mainly been in the hippocampus which is involved in the formation of certain memories (1). Furthermore, LTP has been found to occur in not only the mammalian brain but also in other vertebrates such as the goldfish, bullfrog, bird and lizard. Characteristics of LTP commonly associates it with memory storage, especially in the Hebbian descriptions of memory formation. The idea behind this is consistent with what a memory mechanism should and can possess. Donald Hebb wrote, "When an axon of cell A...excites cell B and repeatedly takes part in firing it, some growth process or metabolic change takes place in one or more cells so that A 's efficiency as one of the cells firing B is increased (4)" Thus, memory is likely to involve a strengthening of synaptic activity. This "Hebb's rule closely resembles the definition of LTP in that learning is accompanied by an increase in the efficiency of communication between neurons.

After reviewing information, it seems that LTP implication in learning has been accepted as a model because it makes sense and nothing better has been proposed. It is a long standing theory that has come under various controversy. However, conflicting experimental results have lead some to believe that LTP is involved in only some learning. Furthermore, many scientists argue that statements such as "learning underlies learning and memory" should be replaced by "enhanced synaptic efficacy underlies memory storage (3)" Shors and Matzel believe that LTP mainly acts by "altering the organisms responsiveness, or perception of environmental stimuli." They proposed the argument that certain environmental cues are represented in the brain as a pattern of synaptic responses and LTP acts to magnify these responses in order to allow a more rapid detection of stimuli (3).

Many studies have been done with LTP in learning. Certain drugs or other genetic manipulations that block hippocampal LTP facilitate performance in some tasks but impair performance in other tasks. However, a number of studies have found evidence that LTP -like increases in synaptic potency occur in the hippocampus during learning of tasks. Some tasks include spatial learning, associative eye blink conditioning (as shown in rabbits) and olfactory discriminations. Nevertheless, it has been shown that artificial induction of LTP has no clear effect on learning new tasks. Thus, maybe the induction of LTP is not a sufficient enough condition for the storage of new memories and other mechanisms besides LTP are involved in this storage (4).

Models of the invertebrate Aplysia have demonstrated that memory storage are represented at the level of individual neurons by changes in the strength and structure of synaptic connections. Long term memory, lasts for days, weeks and is associated with the growth of new synaptic connections activated by altered gene expression and new protein synthesis (5).

Sensitization is nonassociative learning where the animal learns about the properties of a single harmful stimulus. In Aplysia, sensitization of the gill and siphon withdrawal reflex can be induced by a strong stimulus applied to another site, such as the tail or neck. Facillatory interneurons are then activated which synapse on sensory neurons and strengthen synaptic connection between the sensory neurons and target cells. Behavioral memory for sensitization of the gill and siphon withdrawal reflex is graded. A single stimulus to the tail gives rise to short term sensitization whereas repeated stimulation produces long term behavioral sensitization (5,6).

Short and long term memory sensitization is represented by monosynaptic connections between sensory neurons. Serotonin (5 HT), a modulatory neurotransmitter normally released by sensitizing stimuli, can substitute for the shock to the neck or tail. For example, a single application of 5 HT produces short term changes in areas for synaptic effectiveness whereas more applications of 5-HT produce long term changes (7).

Short term cellular changes differ from long term cellular changes in that 1) short term changes involves only modification of pre existing proteins and alterations of pre existing connections. The short term process does not involve ongoing macromolecule synthesis. This is known since short term change is not blocked by transcription/ translation inhibitors. However these inhibitions do block induction of long term changes. Thus there must be a critical time in learning where proteins and RNA are required to change short term changes into long term changes. 2)Long term process involves a structural change which is not seen in the short term. In long term processes, there is a growth in new synaptic connections by sensory neurons onto follower cells (8).

The long term facilitation in Aplysia is similar to the long term potentiation (LTP) in mammalian hippocampus. Studies have found that the start of LTP is associated with increased mRNA expression encoding transcription factors and other proteins needed for growth. This shows that long term changes in both mammals and Aplysia are linked to gene induction which leads to synaptic growth (3,4).

In conclusion, the structural changes implicated in long term memory are due to new protein and mRNA synthesis. These changes in transcription and translation can be initiated by repeated exposure to modulatory transmitters (5 HT) which mimic the effects of hormones and growth factors. Furthermore, through second messenger systems that regulate gene expression, modulatory transmitters utilized for learning can initiate synaptic growth.

WWW Sources

1) dead link

2) WHAT MEMORY IS FOR

3) THE HEBBIAN PARADIGM REINTEGRATED: LOCAL REVERBERATIONS AS INTERNAL REPRESENTATIONS

4) Long-Term Potentiation: What's Learning Got To Do With It?

5) Structure of the Network Mediating Siphon-Elicited Siphon Withdrawal in Aplysia.

6) LONG-TERM CHANGES IN EXCITABILITY INDUCED BY PROTEIN KINASE C ACTIVATION IN APLYSIA SENSORY NEURONS.

7) RAPID DEVELOPMENT OF SYNAPTIC CONNECTIONS AND PLASTICITY BETWEEN SENSORY NEURONS AND MOTOR NEURONS OF APLYSIA IN CELL CULTURE: IMPLICATIONS FOR LEARNING AND REGULATION OF SYNAPTIC STRENGTH.

8) Long-Term Effects of Axotomy on Excitability and Growth of Isolated Aplysia Sensory Neurons in Cell Culture: Role of cAMP.

 

 

Comments made prior to 2007

I have a new book and Web site on what science reveals about improving memory. The book is entitled "Thank You Brain For What You Remember. What You Forgot Was My Fault." Although the book is lay-audience oriented, I do discuss relevant experiments and their applications to everyday memory. In general, there are some 150 specific ideas for improving one's memory. The book's Web site is at http://thankyoubrain.com

The Web site features an advice column and I am starting a blog on summarizing memory research. There is also information about memory workshops that I give ... Bill Klemm, 27 July 2005