Posts Tagged ‘medial temporal lobe’

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.’”

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