Hippocampus Flashcards
The hippocampus is important for
declarative memory, particularly for episodic memory (what happened, where it happened and when it happened)
External events are represented in the brain as
spatio-temporal patterns of neural activity, which must themselves be the agents of synaptic change
What is the location of storage of known as?
Engram of memory
Where must location storage be?
found among those synapses, which support activity-dependent changes in synaptic efficacy
What did Hebb postulate ( and when?)
This follows from Donald Hebb’s postulate in 1949, who hypothesised that “cells that fire together wire together”. Allport’s (1985) elaboration on cell assembly (Figure 1) outlines the process by which if the inputs to a system cause the same pattern of activity to occur repeatedly, the set of active elements constituting that pattern will become increasingly strongly interassociated. That is, each element will tend to turn on every other element, and the pattern, or engram, is said to be ‘auto-associated’. Thus, when prompted to recall something, the experience or one aspect of the memory can drive the activity in the whole network (Figure 1C-D).
What is Hebbian plasticity and what does it underlie?
activity-dependent, positive-feedback process
underlies the strengthening of effective synapses and facilitates the encoding of LTM in the hippocampus
How was plasticity first shown in animals?
These changes in synaptic efficacy were first described in 1966 in the rabbit hippocampus by Terje Lømo, who described a persistent increase in synaptic strength following the pairing of pre- and post-synaptic activity, subsequently characterising the effects of ‘long-lasting potentiation’ (Bliss and Lømo, 1973).
What is LTP?
characterised by associativity, whereby weak inputs can be potentiated if active at the same time as a strong tetanus to a separate, but convergent input.
How does LTP display input specificity?
inputs not active at the time of the tetanus do not share in the potentiation
What does LTP depend on?
the NMDA receptor - for the NMDA channel to open, and thus to trigger LTP the membrane must be sufficiently depolarised to expel the Mg2+ block from NMDA channels, at the same time that L-glutamate has, by binding to the NMDARs, promoted their opening.
What are place cells?
Place cells are a population of cells that collectively signal where in the environment an animal is
What is episodic memory anchored to?
space and time
How do place cells demonstrate LTP?
if the animal repeatedly visits a certain location, there will be a dominant place cell.
What are the properties of place cell firing?
- Independent of animal’s orientation
2. Robust to the removal of sensory cues
How does Sharp’s model indicate that place cell’s learn by autoassociation?
For every location in the environment the rat can be in; it expresses a pattern. When we apply competitive learning: random initial activity will lead to a winner takes all pattern.
Output neurones will become selective to patterns. Neurones in this layer will become selective to cue distances and cue directions. If the rat turns a little bit but stays in a constant location; the pattern in cue directions changes only slightly. This means that the pattern expressed for the new location is very similar to the previous pattern so it’s likely to be associated to the same cell because we’ve just potentiated the weights for the previous neurones that are selective to that pattern. This means that the network has learned the combination of cue distances and directions.
How do BVC’s allow for path integration?
In this situation, each BVC has a higher firing rate when the rat is heading away from the corresponding boundary than when it is heading towards it; implying that integration of the recent path from the boundary contributes to the BVC response. The input from medial entorhinal grid cells to place cells can be seen as providing this (path integrative) component of the BVC input to place cells. In paths integration, you need to reset the starting position input with sensory cues to be in sync with the world.
- shown by directionality of place cells in radial arm maze - but same winner neuron
Properties of BVC firing
Absolute direction rather than subjective
BVC’s have sharper tuning for shorter distances
What demonstrates that LTP is needed between BVC and place cells?
If you block NMDA, which is required for plasticity, the firing of place cells is not stable anymore. To make them stable is to have synaptic plasticity act. This is because synaptic plasiticity is the putative biological instantiation of weight-updating.
less resistant to cues - secondary peak (more than one winner place cell)
Lever 2002
Are BVC-place cell connections just feed forward?
Since learning and adaptation are required, BVC’s can’t really be explained by a simple feed forward model. The BCM model proposes a sliding threshold for LTP or LTD induction, and states that synaptic plasticity is stabilized by a dynamic adaptation of the time-averaged postsynaptic activity. According to the BCM model, when a pre-synaptic neuron fires, the post-synaptic neurons will tend to undergo LTP if it is in a high-activity state (e.g., is firing at high frequency, and/or has high internal calcium concentrations), or LTD if it is in a lower-activity state (e.g., firing in low frequency, low internal calcium concentrations)
Barry and Burgess 2007
What does content-addressable and recall depend on (and how was this found)?
Entorhinal CA1 - content addressable
CA1-CA3 - recall
removed all input from areas CA3 to CA1, thus isolating the CA1 area. Pyramidal cells in the isolated CA1 area developed sharp and stable place fields. Rats with an isolated CA1 area showed normal acquisition of an associative hippocampal-dependent spatial recognition task. Spatial recall was impaired.
Brun et al., 2002
