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Understanding Memory Issue and Stroke

By David Wasielewski

 

 

Many stroke survivors need to deal with changes in their memory following a stroke. These issues can vary widely and may be difficult for the patient and caregiver to understand. Some individuals will experience a specific inability to remember faces. Some may have problems with numbers and calculations. In my personal experience there is often a decrease in the speed at which I can retrieve a certain memory.

 

In severe cases survivors can experience a complete inability to form any new memories (short term memory deficit). But how do strokes affect the ability to store and access one’s memories? Neurologists are only just beginning to understand the complexities of what are two very different brain functions, 1) The formation of new memories and 2) The storage of those memories once they are created.

 

As with much of our knowledge of the brain, clues to how the brain works are often discovered by observing the effects of accidents and traumas experienced by others. For obvious reasons scientists cannot simply perform brain related experiments on human subjects. They can however, learn much from close observation of those who experience accidental brain injuries.

 

One of the first discoveries about how the brain stores information came from observing a famous patient known as H. M. This patient was treated for severe epilepsy. Epilepsy often involves the uncontrolled firing of nerve centers on opposite sides of the central brain area. To treat H. M.’s severe epilepsy sections of his central brain tissue including the hippocampus and surrounding tissue were removed.( see Figure 2 ).

 

While the procedure successfully reduced his epilepsy symptoms it also, unfortunately, left him with the complete inability to form any new memories. He lived for quite a few years after the surgery without forming any new memories. This was especially interesting because while no new memories formed H. M. could easily remember events that took place before his surgery.

 

He had total recall of events from his early childhood but could not remember what he had eaten for dinner that day. He also retained ability to remember physical procedures that he was taught after the surgery. Neurologists of the time were led to several conclusions from their observation of H. M.

 

The first observation was that the brain mechanism for creating and storing declarative memory is different from the mechanism for creation and storage of procedural memory. Declarative memory is the mechanism we use to study for an exam or remember the name of the person we just met.

 

Procedural memory allows us to learn and remember physical actions such as how to ride a bicycle or hit a baseball. It helps us improve performance in sports. Declarative memories involve processing of information through a brain circuit that includes the hippocampus and closely surrounding tissue. Procedural memories did not seem to involve this mechanism.

 

The second observation was that while memories are created within the hippocampus these same memories are stored in other areas of the brain. H.M. could not form new memories using the hippocampus but held on to his older memories stored in other areas of the brain. Using this information as a base scientists began using advanced technologies to carry out more complex brain experiments.

 

Neuroscientists have, for some time, had the ability to measure increased electrical activity in certain areas of the brain. This allows them to theorize where specific memories are stored in the brain. One group of scientists realized the importance of recognizing faces in monkeys. Monkeys are social animals that depend on identification of family and group members.

 

The researchers set out to identify if there was a specific area in the monkey brain where these important images were stored. They eventually were able to measure increased activity in the inferiotemporal cortex when the monkeys were shown a series of familiar facial images (see Figure 1). More recent human research also confirmed that different types of memories are stored in different areas of the brain.

 

Neurosurgery is often carried while the patient is conscious in order for the surgeon to accurately identify that the surgery is affecting the proper area of the brain. Surgeons often use this opportunity to help them map brain functions. The surgeon can, with the patient’s permission, stimulate specific brain areas and ask the patient for their reaction.

 

They found that stimulation of a certain brain area will consistently evoke certain memories of smell, color or vision. The conclusion is that each of those memories is stored in a different, specific area of the brain. Dr. Shawn Murphy, who teaches neurobiology at Harvard, uses the following analogy to explain how the brain memory functions.

 

“The hippocampus acts like the card catalog in a library. It creates a memory and then somehow directs the brain where to file it”. It also seems able to create directions or index that allow the individual to access that information in the future. The concept seems simple but how does the brain accomplish such a complex task?

 

Science continues to research the memory mechanism. While it remains a mystery, there are some advantages to knowing about this memory creation and storage function.

 

Observation of memory difficulties can help doctors to identify the area of the brain that has been affected by a stroke. A loss of short term memory indicates damage to the hippocampus. Loss of facial memory indicates damage to the inferiotemporal cortex. Conversely, if a neurologist identifies the location of stroke in a patient he or she can predict, to some extent, what memory problems a patient or caregiver should be prepared for.

 

This research helps stroke survivors and caregivers to make sense out of their own experiences with changes in their memory. They can remember events from years past but not what happened yesterday. There is damage to the hippocampus. They can remember numbers but not the name of the person they just met. These different memories are stored in different parts of the brain

 

Figure 1

Description: face cell

Brain activity in the inferiotemporal lobe increases as a familiar face is shown to a monkey

 

 

 

Figure 2

 

Description: medial temporal lobe

Location of the hippocampus deep in the central brain tissue of the human brain

 

 

Copyright © August 2011

The Stroke Network, Inc.

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