What is the role of hippocampal size in spatial memory? How has this been studied? In your answer, be sure to include what you learned this week about taxi drivers, individuals such as H.M., and other animals. Feel free to include what you learned in Chapter 6 (page 163-165), about feeding strategies.
1 The Biology of Learning and Memory
It can be very challenging to define words like learning, emotion, feelings, and sleep. These words are so much in general usage that we assume we know what they mean. However, “working definitions” can be hard to nail down. Learning may be considered a change in the brain and in behavior that is adaptive and is a result of experience. Adaptive indicates something that helps the individual respond to experience in a way that enhances survival. Memory is intimately tied to learning.
It is easy to think about the mechanism of memory in evolutionary terms. The ability to remember something and to retrieve that information when needed, is critical to physical survival. Remembering that a certain path led, last time, to a lion’s territory and changing your route can be very protective. Remembering where you buried a store of food can certainly offer an advantage in the struggle for existence.
Memory can be thought of as the filing away and accessing of past experiences. Memory is necessary for learning. By remembering an experience, an individual can use the past to change or adapt to a similar experience. Learning is the actual gaining of new knowledge, while memory is the process of tucking it away for later use. “Remembering” is later accessing that stored material.
A glimmer of how memory might function has come from studies carried out for almost three decades on a patient whose life has never been the same since he underwent surgery to relieve his epilepsy. In 1957, two scientists, William Scoville and Brenda Miller, at the Montreal Neurological Institute, first wrote extensively about one of their patients, a 27 year old mechanic known only as H.M.
An operation was performed to remove much of his hippocampus and amygdala bilaterally along with some surrounding areas in the temporal lobes. This was done as an attempt to relieve his unrelenting seizures. The results were so life changing that the procedure has never been performed again.
H.M. lost the ability to form new long-term memories but he kept previously acquired ones. He had no evident decline in other intellectual abilities. For example, he knew his own name, his language skills were unimpaired, and his IQ stayed on the high end of normal. He lost, however, the ability to remember recent events. For example, he could not recognize someone he just met or recall what he just had for lunch. The recent would vanish as if it never happened.
What he completely lost was the ability to transfer what he learned from short-term to long-term memory. He lacked the ability to “consolidate.” H.M.’s condition has led us to understand that memory is multifaceted and complicated. Work with H.M. suggests that memory involves distinct steps:
- the acquiring of information: information is coded and entered into specific regions of the brain, such as the hippocampus.
- the storing of information: information is retained in those short-term memory banks (presumably due to Hebbian changes in the synapses). Short-term memory is labile and fragile and obviously short lived. For future use, it must pass into stable, long-term memory, by a process known as consolidation.
Work with H.M. also suggests that different types of information can be stored as different types of memories. A useful distinction is knowing how(procedural or non-declarative) vs. knowing that (declarative). Knowing howobviously includes knowing how to ride a bike, sew, swim etc. Knowing that includes information such as dates and specific facts that all require conscious accessing and recall. H.M.’s defect did not seem to prevent him from learning motor skills of the knowing how or procedural variety but it did prevent him from easily retaining new knowing that information.
It seems that as a memory is formed, a reference file is created. When needed, this can be activated, and in turn, activate, all the appropriate storage areas to connect the varied but relevant information. For example, the memory of the first day of your vacation in Greece, might include recalling the azure sky, the warm sea breezes, shimmering sailboats, the piercing smell of salt water, as well as who was with you, and how you felt about them. This theory of memory suggests that the brain makes connections, reaching back and retrieving separate components in order to recreate the whole. Here is a link to a useful site about memory.
There has been discussion about model for memory’s role in health and disease. It involves a review of what is known about the neurobiology of memory and proceeds to explore the connection between memory impairment and disorders such as schizophrenia, depression, and obsessive compulsive disorder.
Don’t use the disparaging expression “bird brain” around neuroscientists! Birds are fascinating and have the ability to learn songs, remember where they stored food over long periods, and utilize navigational skills in migration. Species of birds that store food for later use have larger hippocampi than birds of those species that don’t cache food. You should also consider the stupendous role of the cerebellum in flight!
And what do taxi drivers in London have to do with birds? They too have changes in their hippocampi that are related to learning to find their way through the maze of city streets. Other research has demonstrated the adult hippocampus can produce new neurons (neurogenesis) in response to an enriched environment.
Researchers are investigating how we actually hold on to information that we need to remember briefly (such as a phone number that must be retained only long enough to dial). New techniques of superimposing MRI and PET scans reveal that neurons in the prefrontal lobes behind the forehead are responsible for temporarily storing concrete information for short-term use. In fact, many researchers feel that “short-term memory” is an outdated term and that “working memory” more clearly expresses what is actually occurring in the brain.
The prefrontal cortex can also pull information from long-term storage depots scattered throughout the brain, keep all the data together, and juggle them long enough to be used. Once the task is accomplished, this area of the brain turns to a new task, drawing its attention away from the old. We will see next week that the prefrontal cortex is intimately tied to what we think of as human intelligence and is deeply involved in language and cognition.
One of the researchers working in this field was Dr. Patricia Goldman-Rakic who died in October of 2003. She was also renown for groundbreaking work in studies on memory and on the role of the prefrontal cortex. A look at her research focus and her accomplishments gives an idea of how active this area of study is and how much she will be missed.
Schizophrenics frequently have difficulty keeping on the path of one thought or following a “train of thought” to the finish. Their attention sometimes drifts off easily. Some research suggests that schizophrenic individuals may have some deficits in pulling together contextual aspects of memory.
3 Brain Studies: Dr. Patricia Goldman-Rakic
Dr. Patricia Goldman-Rakic was one of the most distinguished neuroscientists of her time. Her multidisciplinary research advanced understanding of the relationship between brain and behavior.
Please take a look at the following article about Dr. Goldman-Rakic:
It seems truly awesome that the underlying neurochemical and neuroanatomical bases for memory are conserved and therefore the same from echinoderms to Einstein and from Aplysia to Aristotle. Neuroscience confirms that there is unity in life. Animal models such as Aplysia have contributed extensively to our understanding of the fundamental concepts in learning and memory.
Neurobiologists ask what actual physical changes are occurring in our neurons when we learn. Does learning something even as simple as a phone number or how to tie your shoes leave an actual physical trace or effect on our neurons? Karl Lashley referred to this physical evidence of learning as an “engram.”
We also want to know how these changed cells can then communicate and cooperate with each other in such a fashion that new adaptive behaviors can evolve. Any model of learning, has to include a mechanism in which something once learned comes easier the next time. In 1949, Donald O. Hebb suggested that if two neurons are active at the same time they will tend to associate more with each other and affect activity in one another, with increasing effectiveness. This article discusses the groundbreaking work of Dr. Hebb.
A Hebbian synapse, therefore, is a synapse that becomes more effective because activity in the presynaptic and postsynaptic neurons has occurred at the same time. To put this another way, a presynaptic neuron that has already connected with a particular postsynaptic neuron, subsequently forms more effective and more efficient synapses. This is due to a change in one or both cells.
5 Long Term Potentiation
When we try to study learning at the cellular level, we need a model that allows for the second (subsequent) stimulation of a neuron to be more powerful as a consequence of the first. When we look for a model of learning in cellular terms we need a demonstration of the second response being better because of what the cell has already experienced. This comes close to the definition of a Hebbian synapse and one model is Long Term Potentiation (LTP). This occurs when a neuron is stimulated rapidly and repeatedly and as a result becomes more responsive to new stimulation. LTP has been studied most extensively in the hippocampus.
At many hippocampal synapses, LTP depends on the activation of one type of glutamate receptor (NMDA receptor). As mentioned in Week 5: Two glutamate receptors known as AMPA and NMDAhave a strong effect on each other and may hold a clue to learning and memory. (Check out this 3 minute animation. It can be useful to hear the explanation so be sure and use the narrated version if you can.)
Studies are trying to tease out the role of NMDA receptors and their relationship to excitotoxicity in stroke patients. The only way to activate the NMDA receptors is to first, repeatedly, activate the non-NMDA glutamate receptors (AMPA). This depolarizes the neuron. The depolarization removes the magnesium ions that block the way and allows glutamate to open the NMDA channels through which sodium and calcium can enter. This entry of ions eventually causes the expression of genes in the cell which will encourage responsiveness in non-NMDA receptors in the future. (Sound familiar? Refer back to what you learned about these receptors.)
When calcium is able to enter the cell, it can cause the expression of previously inactive genes. It is the activity of these genes that facilitates the future responsiveness of the active, non-NMDA receptors in the area. Protein synthesis is required for LTP and for learning. In research studies, protein synthesis inhibitors can block memory formation when given to a research subject immediately prior to or immediately following a learning trial. This may play a role in decreasing the incidence and severity of posttraumatic stress disorder and is under investigation.
The key words in discussing LTP are facilitate and potentiate and indicate that the cells involved are more responsive after the first time they are stimulated. This fits our model for learning. Fascinating studies on the effects of cocaine administration on long-term potentiation offer a view of the role memory might play in addiction.
6 Remembering H.M.
In 1953, a young Henry Gustav Molaison, better known as H.M., had brain surgery that left him with almost no long-term memory. He became a key research subject in neuroscience. Please listen to the following National Public Radio (NPR) podcast about H.M.’s brain story: http://www.npr.org/templates/story/story.php?storyId=7584970
Also, take a look at the PBS NOVA Science Now “The Man Who Couldn’t Remember:” http://www.pbs.org/wgbh/nova/body/corkin-hm-memory.html