learning and memory

Biology of Learning and Memory

Learning

  • Definition: Learning is the process whereby we acquire new information about the world.
  • Most of the knowledge we have is learned.
  • Molecular Perspective: Learning involves a molecular problem that results in a change in synaptic strength.

Memory

  • Definition: Memory is the process whereby we retain information over time.
  • Our identity is fundamentally shaped by what we learn and remember.
  • Molecular Problem: Examines the molecular mechanisms of how memory storage occurs at each site in the brain.
  • Systems Problem: Identifies where various memories are stored in the brain, involving two major systems.

Forms of Long-Term Memory

  • There are two main forms:
    • Explicit (Declarative) Memory
    • Requires conscious attention.
    • Involves knowledge of people, objects, and places.
    • Includes facts and events.
    • Associated with the hippocampus and neocortex.
    • Implicit (Procedural) Memory
    • Does not require conscious attention.
    • Involves skills and habits such as motor skills.
    • Associated with the amygdala and cerebellum.

3-Stage Theory of Memory

  • Stages:
    1. Sensory Memory
    • Duration: Less than 1 second.
    • Definition: Involves the lingering stimulation of sense organs.
    1. Short-term Memory
    • Duration: Approximately 15-25 seconds.
    • Working Memory: A component of short-term memory.
    • Chunking: Technique that groups information into manageable units, typically up to 7 ± 2 units.
    • Rehearsal: Process to keep information active in awareness.
    1. Long-term Memory
    • Duration: Potentially a lifetime.
    • Distinction between:
      • Declarative vs. Procedural Memory
      • Semantic vs. Episodic Memory

Consolidation

  • Emotionally significant memories form quickly, influenced by hormones such as epinephrine and cortisol that enhance consolidation of recent experiences.
  • Working Memory: An alternative to short-term memory.
    • Located in the prefrontal cortex
    • Emphasizes temporary information storage for active attention and manipulation.

Types of Long-Term Memory

  • Types:
    • Declarative Memory (Facts)
    • Semantic Memory: Knowledge of facts like word meanings (e.g., President’s name).
    • Episodic Memory: Personal experiences (e.g., last birthday party).
    • Procedural Memory (Skills)
    • Example actions include riding a bike or playing the guitar.

The Hippocampus and Amnesia

  • Amnesia: Memory loss that can provide insights into types of memory and their underlying mechanisms.
  • Different areas of the hippocampus activate during memory formation and recall; damage can lead to amnesia.

Famous Case Study: H.M.

  • Subject H.M. underwent removal of the hippocampus to treat epilepsy.
  • Resulted in severe difficulty forming new long-term memories, while short-term/working memory remained intact, indicating the hippocampus's crucial role in forming new long-term memories.

Brain Anatomy Related to Memory

  • Main anatomical structures discussed include:
    • Frontal Lobe
    • Olfactory Bulb
    • Temporal Lobe
    • Optic Chiasm
    • Mammillary Body

Components of the Hippocampus

  • Key structural areas within the hippocampus include:
    • Pyramidal cell layer
    • Dentate gyrus
    • CA1, CA2, CA3, and CA4 subfields

Types of Amnesia

  • Anterograde Amnesia: Inability to form new memories after brain damage.
  • Retrograde Amnesia: Loss of memories created before the brain injury.
  • H.M. exhibited both types following his surgical procedure.

Characteristics of H.M.

  • Exhibited mild retrograde amnesia but severe anterograde amnesia.
  • Short-term memory remained intact, with notable issues in verbal and visual memory.
  • Retained procedural memory and classical conditioning capabilities.

Theories on Hippocampus Function

  • Research indicates the hippocampus is critical for:
    • Declarative Memory Functioning: Particularly episodic memories.
    • Spatial navigation tasks such as radial mazes and Morris water maze task demonstrate functioning impairment when hippocampal structures are damaged.

The Morris Water Maze and Radial Maze

  • Illustrative tasks used for testing memory and navigation abilities in animals.

Alzheimer’s Disease

  • Characterized by progressive memory loss, particularly in older populations (50% of those over 85; 5% of those aged 65-74).
  • Early Onset: May have a genetic influence; mostly late-onset cases (99%) have no family history.
  • Current treatments are largely ineffective for reversing the condition.

Pathology of Alzheimer’s Disease

  • Associated with the accumulation of amyloid beta and tau proteins, leading to:
    • Plaques: Result from damaged axons and dendrites.
    • Tangles: Develop from neuronal degeneration.
  • Variation in chemical patterns exists between individuals affected by Alzheimer's.

Hebb’s Rule

  • Describes synaptic behavior:
    • If presynaptic input prompts target cells to fire, the synapse strengthens ("cells that fire together wire together")
    • Weak inputs lead to synaptic weakening.
  • Learning and forgetting mechanisms both occur at glutamatergic synapses, where glutamate plays a crucial role.

Glutamate Receptors

  • Types:
    • AMPA Receptors: Allow Na+ entry when activated by glutamate.
    • NMDA Receptors: Function as both transmitter-gated and voltage-gated channels, requiring glutamate binding and depolarization to become active.
  • During LTP, NMDA receptors permit Ca2+ entry, triggering intracellular changes that enhance synaptic response.

Long-Term Potentiation (LTP)

  • Definition: Enhanced synaptic efficiency resulting from high-frequency stimulation; improves the neuron's responsiveness.
  • Properties of LTP:
    1. Input Specificity: Only highly active synapses strengthen.
    2. Cooperativity: Multiple stimulated inputs can drive the cell to threshold, facilitating learning.
    3. Associativity: Classical conditioning basis; pairing weak inputs with strong ones strengthens the synapse.
  • Influences by neurotransmitters such as acetylcholine (ACh), norepinephrine (NE), serotonin (5-HT), and dopamine (DA).

Long-Term Depression (LTD)

  • Definition: A prolonged synaptic response decrease when axons exhibit reduced activity compared to others.
  • Compensatory phenomenon where one synapse's strengthening leads to another’s weakening.

Synaptic Plasticity in the Hippocampus

  • Key mechanisms include LTP, LTD, and glutamate receptor trafficking, which help maintain stable synaptic transmission and adapt plasticity as necessary (e.g., AMPA receptor replacement).

Calcium's Role in Memory Processes

  • High Calcium Levels: Align with LTP processes—associated with memory and learning.
  • Low Calcium Levels: Correlate with LTD processes—associated with forgetting.