This paper reflects the research and thoughts of a student at the time the paper was written for a course at Bryn Mawr College. Like other materials on Serendip, it is not intended to be "authoritative" but rather to help others further develop their own explorations. Web links were active as of the time the paper was posted but are not updated.
2006 Second Web Paper
In my last paper, I examined memory in terms of what its loss, retention and recovery revealed about how it worked and what it was (1). This macro-level, behavior oriented approach revealed several characteristics of memory: it is dependent on molecules within the brain and on brain cells, it includes long and short term memory, and it is affected by the external environment. From another angle, memory, especially human memory, is essential for the development and evolution of human culture, arguably the trait that most clearly sets humans apart from other animals (2). The ability to form and use memories of different kinds played an increasing role in the evolution of humans. However, culture and the behavioral results of memories arise out of a biological framework, which must create or allow for each of these characteristics and capabilities.
The goal for this paper is to reach an understanding of the biology of memory, the basic mechanism behind these more macro-level patterns and interactions. Specifically, how are memories stored, where are they stored and what exactly are memories? The search for answers to these questions leads to secondary questions: Is there a difference on the cellular level between long term and short term memory? Are there qualitatively different kinds of memories? Where, in the brain, does remembrance occur?
To begin to look at a set of processes as complex as those involved in memory, it is useful to start with more straight-forward memories in less complex nervous systems (3). Kandel studied sea snails and other animals with simple nervous systems in an attempt to explain what memories are: synapses or cells? In the 1950's the debate centered around whether memories resulted from the growth of cells or from a change in the properties in a synapse (the area between a sending and a receiving cell) that would make synaptic signals between the two cells more or less frequent. The nerve cells of organisms taught a certain response were observed at all points in the learning, or creation of the memory. Repeated firing of certain synapses leads to a build up of serotonin which fixes short term memory in the cell by altering synaptic potentials (making signals more likely to be sent between nerve cells). Repeated and temporally spaced serotonin accumulation eventually causes the activation of certain DNA segments through a chain of molecular interactions. Proteins are produced that converge on the serotonin-high synapses of the nerve cell and begin the interactions necessary to begin growth of new synapses between the two cells. (3)
This cellular level understanding allows for experience to cause changes in connections between cells -strengthening or weakening them- and allows for both long-term and short term memory, which are qualitatively different. But, the model is imited to memories that pattern behavior, called implicit or procedural memories. It needs to be expanded to explain what are called declarative or explicit memories -the memories of events that occurred years ago that people usually refer to when they discuss memory (3). Memories that can be narrated are different from learned behaviors. Strengthening connections between input and output neurons that already exist results in an increase in a certain behavior, but remembering smells from ten years ago requires a storage site beyond input-output chains and must in some way link to consciousness.
The current theory for declarative memories does build off of the simpler implicit memory model to build more complex memory abilities (3). It is thought that cells in the hippocampus (a part of the telencephalon, the most recent part of the brain to evolve, (4)) process sensory input signals -images, sounds, smells, emotions- storing them in other parts of the brain by connecting the related inputs to certain cells in the hippocampus with the same cellular-level process used for procedural memories (5). Over time, the connections between storage cells may increase to the extent that the hippocampus cells are not required by the memory. Procedural memories are learned in many parts of the nervous system and in a wide range of animals. Declarative memories seem to be processed through the hippocampus, but connected and stored throughout the brains of a select group of animals (5). Kandel (3) described memory as "the pattern of functional interconnections of . . . cells" (p.567) and this description seems to apply to both kinds of memory although more research remains to be done on declarative memories.
Declarative memories, while they appear later in life and in evolution, are not less important than procedural memories. In fact, for about the first year of life, humans very actively create procedural memories, and then later begin to form declarative memories (6), (7). Children and adults continue to form procedural memories throughout their lives. In many ways these procedural memories dictate behavior even more especially because we are less conscious of these memories and have a more difficult time describing them with language (8). Other studies have found that declarative memories of events preceding positive feelings are more likely to form than events followed by negative feelings. This suggests an evolutionary connection between procedural and declarative memories. Procedures that had positive results should be remembered and performed again for improved survival. While this is less important, for declarative memories, if it is built on procedural mechanisms, the trait may remain. (9)
A biological basis for learned behavior and memory does exist at the cellular level. The biological observations and theories (where observations remain to be made) provide for memory formation, learned behavior, and the exchange of culture/behavior, which in turn changed the forces moving human evolution. Evolution occured in some sense through behavioral changes passed down culturally in place of physical changes passed down genetically. Also, this biological model allows for biology and experience to interact directly. Experience quickly leads to changes in the brain, which allow for a change in response to the world beyond the individual. Thus, the external and internal processes are interdependent. The biological model explains how molecular and cellular level problems cause memory loss: chains of molecular reactions are necessary for memory production and cell connections serve to store memories. At the same time, the biology allows for cultural, emotional, and attention-related influences on memory formation, though more research is required for a full understanding of these relationships. Finally this model clearly and qualitatively differentiates between short term and long term memory.
This essay began with a series of questions. The answers detail a biological process for procedural memory which hints at the workings of declarative memory and pathways for outside influences on memory and raise more questions: what is the biological difference between declarative and procedural memories? How much of the brain framework is laid out through genetically controlled development and how much is created through memory and learning? Can new cells imitate the connections of the cells they replace, or must the whole process of memory formation reoccur for memory recovery? Through what biologically mediated pathways do attention, emotion and culture affect memory formation?
1)Schifeling, Erin. "Memory Loss and Recovery."
2) Donald, Merlin. The Origins of the Modern Mind: Three Stages in the Evolution of Culture and Cognition. Cambridge: Harvard University Press, 1991.
3)Kandel, Eric R. "The Molecular Biology of Memory Storage: A Dialog Between Genes and Synapses." Bioscience Reports. V. 21, No. 5. Plenum Publishing Corporation, 2002.
4)"Adventures in Neuroanatomy: Parts of the Nervous System."
5)"The Hippocampus: Memories are Made of This."
6)Edelson, Mat. "Researchers Map the Biology of Memory Formation." John Hopkins University, 2001.
7) Ingram, Jay. "Making Memories: Why You Can't Remember Your First Birthday." Muse, April 2006.
8)Kirschner, Gordon. "Implicit Memory and Psychotherapy."
9)Pendick, Daniel. "Have a Nice Memory." Memory Loss and the Brain, 2005.
10)Rohatgi, Ruchi. "Learning and Memory."
11)Webster, Jennifer. "Memories are Made of This."
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