Amnesia case studies

HM: what part of his brain did he have removed + why?

• Medial temporal lobe (hippocampus, amygdala + surrounding medial temporal cortices) to control seizures

HM’s amnesia: 2 aspects

• Anterograde amnesia

• Temporally graded retrograde amnesia

Anterograde amnesia def (1)

Inability to form/store new episodic or semantic memories (after surgery)

Temporally graded retrograde amnesia def (1)

• Events closer to surgery more likely to be forgotten (known as Ribot’s law)

What type of memory was intact in HM?

• Short-term/WM

• Procedural

Double dissociation def (1)

A neuroscientific concept used to prove that two different mental functions are independent of one another and rely on distinct brain regions

Example of double dissociation: named, description (2)

Opposite memory impairments in amnesia vs Parkinson’s patients

• Amnesic Patients (MTL Damage): Deficit in episodic memory (recognition of task features) but intact procedural learning

• Parkinson’s Patients (Basal Ganglia Damage): Deficit in procedural/reward learning but intact episodic recognition of task features

What is the basal ganglia implicated in?

• Reward learning + procedural learning

What does fMRI data suggest about relationship between MTL and caudate (basal ganglia) activation?

• Negative correlation

  • MTL highly active early in task → rely heavily on episodic memory

  • Throughout task, MTL activity decreases + basal ganglia activity increases

What does the fMRI finding suggest?

There may be a third brain region that is actively modulating activity between MTL + caudate to resolve competition

2 levels at which we can consider memory storage

• Synaptic consolidation

• Systems consolidation (what psych is interested in)

Synaptic consolidation mechanism (1→1, 1)

• Initial encoding relies on synaptic plasticity / flexibility of synapse

  • Through receptor changes: increased conductance or number of receptors to make it

• Stabilisation of new receptors

Synaptic consolidation timeframe

• Hours

Systems consolidation timeframe

• Days, weeks, months

Systems consolidation theories (3)

• Standard consolidation theory

• Multiple trace/trace transformation theory

• Cognitive map theory

Simplified explanation of where info is stored based on how recent

• More recent info → hippocampus

• Less recent info → cortex

Systems consolidation explained (1)

• Storage of info shifted from hippocampus to cortical regions

Standard consolidation theory explained (3)

• Different aspects of an event are stored in separate cortical regions

• Hippocampus binds together info from different cortical areas

• Over time, cortical regions form direct links between themselves and no longer require hippocampus as a ‘binder’

How standard consolidation theory can account for temporally graded retrograde amnesia (1)

• Less recent memories stored in cortex rather hippocampus → can survive hippocampal damage often experienced by amnesia patients

Catastrophic interference def

A fundamental problem in neural networks where newly learned information rapidly overwrites or displaces old information

Solution to catastrophic interference (name)

Complementary learning systems

Complementary learning systems explained: neocortex + hippocampus

• The Neocortex:

  • Slow to learn, relying on gradual and repetitive training. By learning slowly, it avoids the massive weight shifts that lead to catastrophic interference, allowing it to build stable, long-term representations

• The Hippocampus:

  • This system is capable of rapid learning. It stores new information quickly and then acts as a "teacher," gradually "training" the neocortex over time so the information can be integrated without destroying existing memories

Reversed temporal gradient in semantic dementia vs amnesia patients (1)

• Semantic dementia patients have better memory for more recent events rather than older events (because cortex (LTM) is atrophying, rather than hippocampus (which is relatively intact))

Multiple trace theory description (2)

• Hippocampus stores initial, highly contextualised episodic memory

• Over time, info is created into ‘schematic’ (general knowledge/gist) memory + stored in cortex

Evidence for multiple trace theory?

In fMRI, hippocampus is activated by remote as well as recent memories

• Suggests LT episodic memory might be stored there

Multiple trace theory benefit (1)

• Schematic memories important because they help us generalise + make sense of the world

Cognitive map theory purpose

• Doesn’t explain intricacies of memory systems, but rather why we might have episodic memory

Core role of hippocampus in cognitive map theory

To facilitate navigation

Cog map theory description (2)

• Posits that the hippocampus stores "spatial maps" of the environment

• which are “allocentric” (represent space independently of the individual's specific viewpoint or direction, opposite of egocentric)

Evidence for cog map theory (2)

• Place Cells: Foundationally, the theory is supported by the discovery of "place cells" in the rat hippocampus. These are specific neurons that fire only when an animal is in a particular region of space

• Navigation Deficits: Behavioral evidence comes from tasks like the Morris water maze. Rats with hippocampal lesions are unable to find a hidden platform because they cannot remember the spatial layout of the pool, whereas those with intact hippocampi navigate directly to it

Function of cog. maps (3)

• Represent physical space

• but also temporal maps

• and relationships