Learning and memory

Question 1

What did 19th century french psychologist Ribot note?

Answer 1

Memory loss after brain injury often varied with age of the memory. Recent episodic memories more susceptible to loss than remote ones.
Suggests memories re organised over time.

Question 2

What is hippocampus function?

Answer 2

Rapid acquisition

Question 3

What is the neo cortex function?

Answer 3

Slower consolidation of memories

Question 4

Describe the case of HM.

Answer 4

Had severe epilepsy.
Doctors removed part of his medial temporal lobe.
Old memories intact 
No retention of new memories
Could still learn skills.

Question 5

What is anterograde amnesia?

Answer 5

When new memories can not be retained.

Question 6

Describe the hippocampus structure /position

Answer 6

Found in the medial temporal lobe
Cell bodies are very neatly arranged
Includes CA1, CA2, and CA3 structures, as well as the dentate gyrus

Question 7

Describe the trisynaptic pathway of the hippocampus.

Answer 7

1. Input comes through via the Entorhinal cortex via the perforant pathway to the dentate gyrus granule cells.
2. Granule cells project to the CA3 pyrimidal cells via mossy fibres.
3. CA3 inputs project to CA1 cells via schaffer’s collateral pathway.

Question 8

Describe the synaptic connectivity overview of the circuits of the hippocampus.

Answer 8

The dentate gyrus has a laminal structure, where each layer projects to a different part of the hippocampus. 
There is a dense association network interconnecting CA3 cells. 
Some layers (2 and 3) in the detate gyrus project directly to the CA1 region 
Pyrimidal cells placed in a neat monolayer in CA sections (especially CA3).

Question 9

Describe the the circuits of the hippocampus in detail.

Answer 9

Sensory information from cortical areas are excited when a memory is acquired.
Information from cortical areas excite neurones in the parahipocampal and perirhinal corices.
Info from these cortices project through the entorhinal cortex to the dentate gyrus (mostly), but also there are weak connections to CA3, CA1 and the subiculum directly.
Dentate gyrus project to the CA3 region via mossy fibres, and CA3 regions project to the CA1 region via schaffer’s collateral pathway.
The main output of the hippocampus is via the subiculum back to the entorhinal cortex
Entorhinal cortex is main output as well as input region
There is also output from the CA1 region to the fornix, which then project directly BACK to sensory cortical areas.

Question 10

Describe the cells of the CA3 region

Answer 10

• Dendrites of the neurones project back to itself (recurrent circuit). Allow formation of auto associative networks.
• This is external inputs from the granule cells (when sensory cortical areas send output through mossy fibres, which synapse onto nodes of CA3 neurones).
• This causes a pattern of activity at CA3 synapses (only some synapses are activated).
• Every time there is the same stimulus, these synapses will be excited again and again,
• It is a positive feed back mechanism. Much elevated excitability at these synapses
• CA3 has positive feedback connections. Receives inputs from entorhinal cortex (EC) and projects its output to CA1.

Question 11

What did Timothy Bliss discover?

Answer 11

That hippocampal pathways are very sensitive to their previous activity history.

Question 12

Proof for importance of hippocampus (experiment)

Answer 12

Some birds cache seeds in times of plenty and then retrieve them when food is scarce. Species of passerine birds which cache food have a bigger hippocampus than non-caching species of the same family.
London taxi drivers

Question 13

Role of the hippocampus in spatial learning (experiment)

Answer 13

Morris water maze. Platfrom can not be seen. Rats swim around and eventually find the platform. Put queues around the pool to help rat to navigate. Rats eventually learn location of platform. If hippocampus is lesioned, the rat never learns to navigate to the platform.

Question 14

The role of NMDA receptors in spatial learning (experiment)

Answer 14

Rats treated with NMDA blocker in hippocampus. Rats were unable to locate the platform.

Question 15

importance of CA3 region (experiment)

Answer 15

NMDA receptors in CA3 are knocked out, leaving CA1 intact. Normal long-term learning is unaffected, but rapid learning and pattern completion are disrupted.
CA3 NMDARs crucial fro memory retrieval when patter completion is required.
Learning is mediated by synapses with NMDR receptors.

Question 16

What are place cells?

Answer 16

Cells that fire when the animal is at a specific position.
Place specific firing is not attached to a single cell. A given place is coded by a population of simultaneously active cells with partly overlapping fields.
They follow visual cues

Question 17

Experimental example of place cells

Answer 17

Animal moves in cylinder finding food connected to a commutator. A given cell in the hippocampus will fire only when the rat is in a particular region of the cylinder.

Question 18

Why do we need long term memories?

Answer 18

Limited storage in hippocampus

Question 19

What is consolidation?

Answer 19

hypothetical process in which a memory
item is transformed to a long-term form. (Memory is labile for a short time after acquisition and sensitive to e.g. protein synthesis inhibition during this period but not later when the memory considered to be consolidated.

Question 20

What are the two types of consolidation?

Answer 20

Synaptic consolidationpost encoding transformation of information into a long-term form at local
nodes in the neural circuit that encodes the memory. (Stimulus induced activation of intracellular signalling cascades, resulting in posttranslational modifications, modification of gene expressions, synthesis of gene products that alter synaptic efficacy.) Lasts for hours.
(LTM stored in the same circuit, due to changes in synaptic efficacy.)

systems consolidation post encoding reorganization of long term memory (LTM) over distributed brain circuits. Lasts for days to years.
(Post-encoding reorganization of LTM over distributed brain circuits.)

Question 21

What are the 3 mechanisms of systems consolidation?

Answer 21

TTM - Trace Transformation Model (Over time trace migrates to different brain areas.)
MTM - Multiple Trace Model (Encoding in multiple locations.)
SAM – Schema Assimilation Model (Already existing information manipulated.)