NEUR30003 Lecture Notes - Lecture 22: Dead Reckoning, Edvard Moser, Place Cell
18 Jun 2018
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Lecture 22
1. Learning and retaining information is, typically, limited by how much information is
involved, how long you need to retain it, and how often you rehearse the information. Over
100 years ago the psychologist Herman Ebbinghaus quantified this well-known phenomenon
as an exponential decay function. Attempts to find the location of memory traces, such as
the cortical lesions studies in rodents by Karl Lashley in the 1950s, revealed there wasn’t a
discrete cortical location for memory; very large lesions were needed to eliminate memories
of tasks learned by rats (such as maze solutions).
2. Spatial memory is relatively easy to assess in experimental animals. Psychologists and
neuroscientists have identified two means by which animals appear to learn about their
spatial environment and their location and orientation within it: 1) they can navigate by “dead
reckoning” – by keeping track of the direction and duration of travel in straight lines and
summing these vectors to calculate current position in relation to the starting position, 2)
detailed exploration of environments allows spatial information to be integrated into cognitive
maps.
3. In rodents, spatial navigation and spatial memory was demonstrated (e.g. by lesion
studies) to be hippocampal dependent. In humans however, it seemed that he hippocampus
was mainly concerned with consolidation of episodic memory (patient H.M. lived in a small
window - about 2 minutes - of the present, his only longer term memory was what he wrote
into notebooks).
4. Because it is a good slice preparation (the connectivity between neurons is preserved
after cutting), the rodent hippocampus has been used to study synaptic transmission. It was
in hippocampal slices that neuroscientists Bliss and Lomo discovered LTP. It makes sense
that hippocampal synapses are capable of profound and long-term changes in strength, as
this would seem to be a good candidate for how memories are formed.
5. Measuring hippocampal neurotransmission in awake behaving rodents lead to the
discovery, by John O’Keefe, of place cells: neurons that increased their firing rates when the
animal was in a specific regions of its environment. May-Britt and Edvard Moser showed that
the place cells get input, and get reassigned their place fields, from a collection of grid cells
in the entorhinal cortex – Unlike place cells (that fire when the rat is in a particular location)
grid cells fire when a rat is in a multiple of a particular distance. Drugs that block LTP block
the reassignment of place cells. In addition to place cells, a variety of other navigation-
related neural types have been described – speed cells, head orientation cells, and boarder
cells.
6. Importantly, place cells also receive input from sensory systems, and thus encode not
only location but also events and objects associated with that location - a place field is also
what happens in a place and is thus also a type of episodic memory. Place cells not only
encode where the rat is, but where it had been recently; the sequential pattern of place cell
activation was replayed when animals slept. It is suggested that this is a process of
consolidating the spatial memory – once consolidated (and probably stored outside the
hippocampus) a spatial memory can be recalled when returning to the environment, or even
perhaps recalled into the imagination to plan journeys.
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7. The activities of place cells, during exploration and rehearsal, occur in the context of
rhythmic, coherent hippocampal activity of around 8Hz – the theta rhythm. This rhythm, like
others present in other brain systems, has a number of postulated functions is but its
significance is not currently understood – we know a good deal about how individual neurons
work, but very little about what dictates behaviors of large networks of neurons and their
interactions with other neural populations.
8. The broader significance of the discovery of how spatial memory is organized at a
neuronal level in the hippocampus is that it might also be a mechanism for how other related
types of memory form – it is possible that, just like the dead reckoning determines spatial
relationships that ultimately create a cognitive map, the same process could occur for
episodic memory (the location of events and objects and people); and even semantic
memory might be accreted by repeated similar instances of objects and concepts so that
they get “mapped” into conceptual and relational categories.
-
- Got people to remember list of nonsense syllables → then tested them immediately
→ tested later and later
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find more resources at oneclass.com
-
- Maze running in rats - remove parts of brain to localise memory, unsuccessful but
maybe should have checked basal ganglia or cerebellum [procedural memory, motor
skill] instead of cerebral cortex
- Didn’t make a difference which part of brain lesion was
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- Dead reckoning - if heading in a certain direction for a certain time → turn 90 degrees
and head in that direction for a certain time (know in head where one is from where
one started because went so far a certain way and so far another way)
- Navigated way home through dead reckoning
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Lecture 22: learning and retaining information is, typically, limited by how much information is involved, how long you need to retain it, and how often you rehearse the information. 100 years ago the psychologist herman ebbinghaus quantified this well-known phenomenon as an exponential decay function. It was in hippocampal slices that neuroscientists bliss and lomo discovered ltp. Drugs that block ltp block the reassignment of place cells. Place cells not only encode where the rat is, but where it had been recently; the sequential pattern of place cell activation was replayed when animals slept. Got people to remember list of nonsense syllables then tested them immediately. Maze running in rats - remove parts of brain to localise memory, unsuccessful but maybe should have checked basal ganglia or cerebellum [procedural memory, motor skill] instead of cerebral cortex. Didn"t make a difference which part of brain lesion was.
