lecture 17 Flashcards
explain the case of Henry Gustav Molaison (HM) – first described by Brenda Milner in 1957
Doctors cut out his hippocampus bilaterally to cure his epilepsy.
• It worked, but he lost the ability to form new explicit memories (severe anterograde amnesia).
• He also suffered from a graded retrograde amnesia (events that occurred within 1 or 2 years were lost as well as some that happened even longer ago than that.
• He still had a brief working memory and a high IQ, but he could not learn new consciously accessible declarative memories.
In 1971, O’Keefe and Dostrovsky showed that pyramidal cells in the hippocampus showed a high rate of firing when rats were doing what
located in a particular place
what are place cells
Different neurons had different receptive fields; they fired when the rat was in a specifc place. These were called place cells.
what are Border cells
when animal is near one or more boundaries of environment, such as walls of a box
Since discovery of place cells, researchers found that hippocampal regions also contain what kind of cells
border cells, head direction cells, grid cells, and time cells
what are Grid cells
Grid cells show evenly spaced, crystal-like
coverage of entire environment in which animal is located.
what is a stable feature of grid cells
Grid Cells: the size of each grid-based hotspot and the distance between these spots is a stable feature of grid cells
Grid Cells: the size of each grid-based hotspot and the distance between these spots is a stable feature of grid cells.
However, the neural activity of neurons located deeper and deeper in the hippocampus (along the dorsoventral extent of some hippocampal areas) corresponds what
larger and larger location-based grids
what Information is Encoded in the Hippocampus
Position -> place cells
• Distance -> grid and border cells
• Direction -> head direction cells
Border cells and head direction cells, by definition, respond similarly in different contexts.
How do place and grid cells vary room to room?
How are two similarly sized rooms differentially represented?
In different environments, place cells and grid cells slightly remap.
Grid Cells: Two properties of grid cells hold constant from room to room: what are they and what changes
the size of each spot on the gird and the distance between spots. However, in different environments, the correspondence between grid cell neural activity and the animal’s location in the room is slightly rotated
Hippocampal activity provides information about more than space. They seem to encode what as well
contextual information as well.
Rats were trained to turn left and then right on alternating
trials to receive a piece of food. When an animal was approaching the decision point, different hippocampal cells were active depending on what
whether the animal had last turned left or right
Grid cells are arranged as semi-independent modules, which probably provides immense combinatorial power. We think this allows for what
new spaces to be mapped without interfering with the memories of well-established spaces.
Even slight changes in the location of landmarks, time of day, or even motivational state can cause grid cells to do what
remap
Hippocampal neuronal networks seem to encode aspects of what
both time and space. (Most neurons are strongly influenced by both factors. Some are significantly influenced by one or the other.)
Hippocampal neuronal networks seem to encode aspects of both time and space. (Most neurons are strongly influenced by both factors. Some are significantly influenced by one or the other.)
• What does this all have to do with explicit memory formation?
People don’t really know but think the key to understanding explicit memory is understanding the how grid cells organize and process information
During any given moment, a unique pattern of neural activity (spread across the cerebral cortex) reflects what
the constellation of sensory input, thought processes, and emotion you are currently experiencing
During any given moment, a unique pattern of neural activity (spread across the cerebral cortex) reflects the constellation of sensory input, thought processes, and emotion you are currently experiencing.
And for every distinct context (time and place), there is thought be a unique organization of grid cell neural activity in the hippocampus (and related structures).
Perhaps the cortical activity that reflects any given moment can be represented in some manner in the hippocampus. It is generally thought that memories are not
actually stored in the hippocampus, but that the hippocampus can do what
form a hub, node, or index that is capable of both representing and reactivating the sensory systems that initially encoded any given event/experience.
what is Memory Encoding
Over time (years in humans), memory gradually becomes less and less dependent on the hippocampus, which simply means the memory will still be there if you lose your hippocampus.
What is happening in the years when memories are dependent on the hippocampus?
A prominent theory is that hippocampal activity (during recall events and during sleep) is “training” the cortex, causing a reorganization of the synaptic weights in the cortex so that intra-cortical connections can support memory recall on their own.
Cortical memories seem to mostly be semantic (i.e., information that is not bound to any particular time or place).
Hippocampus->Cortex Memory Consolidation, ; explain the idea of cortical memories
The hippocampus acts a hub or node that, with pattern completion, can activate all the neurons in the cortex that were involved in the initial memory.
According to this model, the hippocampus is eventually not needed for any memory to endure, to be recalled. The hippocampus is only needed to initially strengthen and stabilize connections within the cortex.
Hippocampus->Cortex Memory Consolidation, explain semantic transformation
Some people argue that the cortex only contains semantic information (facts).
In this model, all memory starts off as episodic memory, which is always dependent on hippocampal nodes interacting with the cortex. Over time, as facts emerge from repeated episodic experiences, these semantic memories are permanently stored in the cortex in a hippocampal-independent manner.
Hippocampus->Cortex Memory Consolidation, explain schema modification
A related idea is that ‘schemas’ are stored in the cortex. (This is similar to the semantic transfer idea, but the idea of a schema is more general.)
what is a schema
an organization of related knowledge that contains both semantic information and episodic details; a mental structure of preconceived ideas;
a framework representing some aspect of the world;
a pattern of thought that organizes categories of information and the relationships among them.
Schemas interleave all memories via common elements.
