Neurobiology of Memory: Encoding, Storage, Retrieval

• The lecture discusses the neurobiology of memory, focusing on memory encoding, storage, and retrieval

Memory Processes

• Encoding transforms sensory data into a mental representation
• Storage keeps encoded information in memory
• Retrieval pulls out or uses information stored in memory

Henry Gustav Molaison (1926 – 2008)

• Had a large portion of his medial temporal lobes surgically removed, including the hippocampus as a treatment for intractable epilepsy
• Later suffered anterograde amnesia, unable to commit new events to his explicit memory

Types of Memory

• Long-term memory
• Working memory

Neuroanatomy

• Cortex is Latin for "shell"
• Lateral means side, from the Latin lateralis ("towards the side")
• Medial means “towards the centre", from the Latin medius ("middle")
• Gyrus is a ridge, from the Latin gyrus ("circle”)
• Sulcus is a groove, from the Latin for “furrow”

Medial Temporal Lobe

• Plays an important role in long-term memory
• Parahippocampal gyrus includes the perirhinal ("around” “nose”) and entorhinal ("inner” “nose") cortex
• Hippocampus in Greek means sea horse
• The hippocampus receives input from the entorhinal cortex, and then sends information through the entorhinal cortex & the fornix (Latin for "arch") Evidence for the role of the medial temporal lobe in long-term memory from neurology
• Alzheimer's disease's plaques and tangles congregate in the medial temporal lobe & atrophy of the medial temporal lobe is closely related to episodic memory deficits in the disorder
• Healthy older adults show increased neural response in the right hippocampus compared to patients with Alzheimer's disease during episodic memory tasks, according to a meta-analysis of 14 fMRI studies

Animal Studies

• The delayed nonmatch-to-sample task is used in primate research
• During this task, primates are shown an object, and after a delay they are shown the same object and a new object, and they must select the new object
• Primates with medial temporal lesions perform more poorly on this task
• The Morris water maze (Morris, 1981) is used to measure spatial navigation learning in rats
• The time it takes the rat to reach the platform becomes shorter over time because they have learned where the platform is in relation to visual cues
• Rats with hippocampal lesions swim randomly about.

fMRI Studies

• The subsequent-memory paradigm; to study memory encoding in the hippocampus
• Depending on the colour of the presented word, participants must make a judgement about either size (green) or animacy (red)
• Increased neural response in the hippocampus and parahippocampal cortex were associated with correctly recollected words
• Increased perirhinal cortex activity associated with increased recognition confidence (familiarity), suggesting hippocampus for recognition based on recollection of info, perirhinal cortex for recognition based on familiarity

Temporal Lobe Studies

• Encoding processes that identify an item as familiar and encoding processes that identify an item as having been seen before (recollection) depend on different parts of the medial temporal lobes, according to Aggleton and Brown (1999)
• There is neural activity in the cortical sensory regions during recall that provided input to the hippocampus during encoding, according to Wheeler et al., 2000

Frontal Cortex

• Plays a role in encoding and retrieval of information
• (Cabeza and colleagues theory) Encoding involves the left frontal cortex, while retrieval involves the right frontal cortex
• (Kelley and colleagues theory) the left frontal cortex is involved in encoding/retrieval of linguistic information; the right frontal cortex is involved in encoding/retrieval of

Memory Consolidation

• The process that stabilises a memory over time after it is first acquired consisting of a rapid consolidation process (dependent on the hippocampus), and a slower permanent consolidation process
• Both ECT and head injury are more likely to affect recently learned items, providing evidence that these processes affect rapid consolidation

Limbic Scars

• Childhood maltreatment is associated with reduced hippocampal volume (n = 145), according to Dannlowski et al., 2012, Biological Psychiatry
• The hippocampus is highly susceptible to stress, including high expression of glucocorticoid receptors

Cellular Basis of Learning and Memory

• In 1949, Donald Hebb suggested that learning had a biological basis which could be summarised with the phrase: “Cells that fire together wire together.”
• Santiago Ramon y Cajal in 1894 proposed that memory is stored in the growth of new connections between neurons

Cellular Basis of Memory

• Hebb's law (1949) states “If a synapse is active when a postsynaptic neuron is active, the synapse will be strengthened.”
• Bliss and Lømo (1973) stimulated the axons in the rabbit hippocampus resulting in an increase in the magnitude of the action potentials in the downstream neurons
• Bliss and Lømo stimulated the axons of the performant pathway with a burst of pulses causing a several-hour increase in the amplitude of excitatory postsynaptic potentials (EPSPs) in the granule cells of the dentate gyrus

Long-Term Potentiation (LTP)

• A synaptic strengthening mechanism that includes NMDA (N-methyl-D-aspartate) receptors; blocked by magnesium, receptors only open when the neurotransmitter glutamate binds to the receptor and when the membrane is de-polarised
• The open ion channel allows calcium ions to enter the cell
• Calcium acts as intracellular messenger and changes enzyme activities to influence synaptic strength (e.g. increases number of