Posts Tagged ‘episodic memory’

The Hippocampus

May 18, 2017

The hippocampus receives considerable attention in “The Truth About Language” by Michael C. Corvallis.  As the hippocampus plays a critical role in memory, it is not surprising that it is central to language and time travel.  As we each have a hippocampus in each hemisphere of the brain, we have two hippocampi.

The importance of the hippocampus was first realized when an Englishman underwent surgery for epilepsy, and the surgery destroyed major parts of both hippocampi.  After this surgery he could no longer form new episodic memories.  Episodic memory involves memories having to do with the specific episodes of our lives.   Although his semantic memory, his general knowledge, remained intact.  Not only was he unable to recall the past, he was also incapable of imagining the future.

In the final years of my Mom’s life she suffered from dementia.  When I visited her, she was always glad to see me.  However, if an attendant took her to the restroom while I was visiting, when she returned she acted as if I had just arrived.  That is, she had stored no memory of my being there.

The hippocampus is the hub of the brain circuit involved in episodic memory and mental time travel.  Brain imaging shows it to be activated both when people remember past events and when they imagine possible future events.  It is also activated when people are asked to imagine purely fictitious  episodes.   Although other brain regions are involved, reflecting the fact that memory and imagination involve information stored in widely dispersed areas, the hippocampus appears to be the most critical component in that damage to it has the most debilitating effect on the ability to mentally escape the present.

The default-mode network, responsible for our mind wandering, is identifiable in primates and even in rats.  The hippocampus plays a critical role in both rat and human memory.  Recording from the hippocampus of the rat reveals that single neurons code where the animal is located in the spatial environment.  These neurons serve as place cells and together generate what has been termed a cognitive map of the environment that tells the rat where it is.  It plays the same role in humans.  Studies have shown that the hippocampus  is enlarged in licensed taxi drivers in London, who are required to memorize the map of London for their licenses.

Research using rats has indicated a similar competence.  In an experiment rats were trained to alternate left and right turns at a particular location in the maze.  Between trials they were introduced to a running wheel and, while they were running, activity in their hippocampi was recorded.  This activity coded which way the rats planned to turn in the maze on the next trial.  Apparently these rats were planning ahead for their next try at the maze.  The researchers also noted that autonomous activity in the hippocampus involved the computation of distances, and also supported the episodic recall of events and the planning of action sequences and goals.  One researcher wrote that “replay in the rat hippocampus can either lead or follow the behavior once the map of space is established.  This suggests that replay phenomena may support ‘mental time travel’ through the spatial map, both forward and backward in time.

Research on human patients about to undergo surgery had electrodes placed in cells in the medial temporal lobe, in an attempt to locate the source of epileptic seizures.  They were then asked to navigate a virtual town on a computer screen and to deliver items to one of the stores in the town.  Then were asked to recall only the items and not the location to which they were delivered.  However, the act of recall activated the place cells corresponding to that location, effectively mirroring the replay of place cells in the rat brain.

In another study, people were shown sequences of four videos of different events.  At one level. narratives were linked to each video, encouraging attention to individual details. At the next level, narratives linked a par of videos, and at the final level a narrative linked all four videos.  As the people processed these narratives, activation in the hippocampus progressed from the rearward end to the forward end as the scale of the narrative shifted from small and detailed to larger and more global.    Dr. Corvallis notes that this probably happens when we read novels.  Page by page, we focus on the details, but as the story progresses we build a more global understanding of what the story is about.  Dr. Corvallis writes, be thankful to your hippocampi that you can make sense of a novel at all.

Dr. Corvallis suggests that although  the generativity spatial mapping is nonlinguistic, it may well underlie the generativity of language itself.  “In the rat these elements may be restricted to simple aspects like sounds or smells, and we may perhaps allow ourselves the luxury of believing our own experiences to be incomparably richer.  Yet the generative component itself probably has a long evolutionary history.  As Darwin famously put it:  ‘The difference in mind between man the the higher animals, great as it is, certainly is one of degree, and not of kind.’”

© Douglas Griffith and healthymemory.wordpress.com, 2017. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Douglas Griffith and healthymemory.wordpress.com with appropriate and specific direction to the original content.

Time Travel: The Ultimate Purpose of Memory?

October 28, 2012

Most of the time we think of memory as being a place of historical storage where old information and experiences are kept. But another way of thinking about it is as a vehicle for time travel (see the Healthymemory Blog Post, “Human Memory: A Machine for Time Travel”). You are able to travel to times long before you were born using what you have learned and your imagination. You can also project yourself into the future with science fiction or your own imagination. Actually we do quite a bit of projection in our daily lives, imagining what it will be like and making appropriate plans. Brain images of people when they are remembering the past and imagining the future show a great degree of overlap in the areas of the brain that are responding.

The distinguished memory researcher Endel Tulving found an unfortunate individual with amnesia who could remember facts but not episodic memories relating to past events in his life. When this person was asked about plans, be it for later in the day, the next day, or in summer, his mind went blank. Brain scans support this idea. When we think of a possible future, we tear through our memories in autobiographical memory and stitch together fragments into a montage that represents a new scenario. Our memories become frayed and reorganized in the process.1

So it appears that the ability to project ourselves forward in time, using what we have learned and experienced to guide the projection, might be the ultimate purpose of memory. Gestalt psychologists believe that in both the processing of information and its memory that laws were operating to create order and make information more meaningful. Emergence was an important concept in which new ideas emerged from the information at hand. These processes help us deal with the future.

Although our brains are working from the time we are born (and there is data indicating that they are working before we are born) to understand and make sense of the world in order to cope with it. In the early stages of life we are preoccupied with mastering language and moving about our environments. Consequently we rarely remember specific events before the ages of 2 or 3, when our autobiographical memories begin to develop And they develop slowly as it is difficult to remember much before our sixth birthday. We are also developing a sense of identity. When we are able to recognize ourselves in a mirror, we have achieved a critical stage of development. A child’s ability to imagine the future seems to develop in tandem with autobiographical memory. Obviously our culture and our families have a profound influence on these memories and our preparation for coping with the world. Our autobiographical memories continue to mature when we leave our parents. A ten year old can rarely relay a coherent life story, but a twenty year old can ramble on for hours. There is a “reminiscence bump,” where we are able to recall much more information that occurs in late adolescence.2 Consequently we are prepared or semi-prepared to assume responsibilities just in the nick of time.

1Robson, D. (2012). Memory: The Ultimate Guide. New Scientist, 6 October, p.33.

2Weir, K. (2012). A Likely Story. New Scientist, 6 October, 36-37.

© Douglas Griffith and healthymemory.wordpress.com, 2012. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Douglas Griffith and healthymemory.wordpress.com with appropriate and specific direction to the original content.

SuperAgers with a Super Memory

October 3, 2012

In a recent experiment1 SuperAgers were defined as individuals over 80 with episodic memory performance at least as good as normative values for 50- to 65-year olds. The performance of these SuperAgers was compared to two cognitively normal cohorts: age-matched elderly and 50- to 65-year olds. The brains of all three groups were compared using cortical morphometry.

With respect to memory performance, the SuperAgers performed better than both control groups (but the difference between the SuperAgers and the middle-age controls was not statistically significant, p>0.05). The sample consisted of 12 SuperAgers, 10 elderly controls, and 14 middle-age controls. The elderly control group performed significantly worse than the other two groups.

With respect to whole-brain cortical thickness elderly controls exhibited significant atrophy in the older cohort compared against the middle-aged controls in multiple regions across the frontal, parietal, and occipital lobes, including medial temporal regions important for memory. However, the whole brain cortical thickness analysis comparing the SuperAgers with the middle-aged controls did not reveal significant atrophy in the SuperAgers.

With respect to the thickness of the Anterior Cingulate Cortex, the thickness of the SuperAgers was higher than both the Elderly Controls and the Middle-Aged Controls. Somewhat surprisingly, only the difference between the SuperAgers and the Middle-Aged controls was statistically significant (p<0.05). However, the likelihood of achieving statistical significance increases as sample size increases. Research has indicated that the cingulate constitutes a critical site of transmodel integration related to episodic memory, spatial attention, cognitive control, and motivational modulation. It is unclear whether the SuperAgers were born with a particularly thick cortex or whether they resisted cortical change over time.

The relationship between brain and memory is an interesting one. The notion that more brain equates to more memory is fairly common, but this finding needs to be placed in context. Alzheimer’s cannot be diagnosed conclusively until an autopsy has been done. The key signatures for the diagnosis are amyloid plaques and neurofibrillary tangles. But these same signatures have been found in autopsies of people WHO HAD SHOWN NO SYMPTOMS OF ALZHEIMER’S WHEN THEY WERE ALIVE! So it would appear that these amyloid plaques and neurofibrillary tangles are a necessary, but not a sufficient condition for Alzheimer’s.

I remember reading an article when I was in graduate school about someone who had hydroencephalocele, which is more commonly called “water in the brain.” As a result of this condition, this individual had only about 10% of the normal volume of cortex. Yet this person led a normal life and earned a Bachelor of Science Degree in mathematics!

The plasticity of the brain is truly remarkable. Healthymemory believes that this plasticity is fostered by cognitive exercise and cognitive challenges. So, stay cognitively active and seek cognitive growth!

1Harrison, T.M., Weintraub, S., Mesulam, M.-M, & Rogalski, E. (2012). Superior Memory and Higher Cortical Volumes in Unusually Successful Aging, Journal of the International Neuropsychological Society, 18, 1-5.

© Douglas Griffith and healthymemory.wordpress.com, 2012. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Douglas Griffith and healthymemory.wordpress.com with appropriate and specific direction to the original content.

More on How Memory Works

November 4, 2009

  Understanding memory failures are key to understanding how memory works. Why can we not always recall the information stored in our brains? Well, one reason might be the enormous size of the brain in terms of nerve cells and synaptic connections. Memory theorists have made a distinction between information that is available in LTM and information that is accessible in LTM. There is much more information available in LTM than can be accessed at any one time. To retrieve information, the right retrieval cue must be used. This is known as the Principle of Encoding Specificity. The cue that was used to store the information is needed at the time the information is retrieved. If this cue cannot be found, or if the person is thinking in a different context, the information will fail to be retrieved. During these failed retrieval attempts we can often think of other items. We can also feel that we can almost recall the item. This is called the tip of the tongue (TOT) phenomenon. This also reveals yet another type of memory, metamemory. Metamemory is knowledge you have about your own memory. If asked a question about which we know nothing, we will not even bother to try to retrieve it. If we think we might know, we shall try to retrieve it. What is especially annoying is when we know we know something, but just can’t remember it. Then, at some later time, when we are not even trying to remember, the answer will come to us. Why this happens and techniques you can use to prevent this from happening will be discussed later in this blog.

LTM can be subdivided into other types of memory. Episodic memory refers to events we have personally experienced, that is, episodes. Amnesia, when people forget who they are and where they came from is commonly referred to as a loss of memory Actually, it is usually a loss of a specific type of memory, autobiographical memory, which is a component of episodic memory. This is the memory of someone’s own specific history. When someone loses all memory, they lose the ability to function. The final stages of Alzheimer’s disease provide a graphic example of what it means to lose all memory. What something was, when and how it occurred are all examples of episodic memory. Remembering that the Declaration of Independence was signed on July 4, 1776 in Philadelphia is an example of episodic memory. However, the wider significance of that event would be stored in semantic memory. Semantic memory is the storehouse of general knowledge. To solve a problem or to answer an essay question on an exam requires semantic memory.

 

 

© Douglas Griffith and healthymemory.wordpress.com, 2009. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Douglas Griffith and healthymemory.wordpress.com with appropriate and specific direction to the original content.