P2- Memory

Neural Basis of Memory

• Memory is the brain's ability to encode, store, and retrieve information.
• Encoding transforms information into neural representations.
• Storage modifies neural connections (synaptic plasticity), with long-term storage involving physical and chemical alterations in neurons.
• Retrieval involves accessing stored information using cues; memories can be rebuilt from their encoded state.

Different Kinds of Memory

• Declarative Memory (Explicit): Consciously recalled facts and events.

  • Episodic Memory: Personal experiences, events with context (time, place). Includes perceptual details (e.g., remembering what you ate yesterday).
  • Semantic Memory: General knowledge and facts. No personal context. (e.g., capital cities, facts).

• Non-Declarative Memory (Procedural/Implicit): Skills and habits, operates unconsciously (e.g., riding a bike).

Temporal Distinction of Memory

• Sensory Memory: Short-term storage of sensory data (seconds).
• Short-term Memory: Limited capacity, short duration (fades after ~30 seconds). Requires attention to move to long-term memory.
• Working Memory: Manipulate information in short-term memory; tightly linked to short-term memory, involves more active processing that requires more brain areas and higher levels of neural representation. The prefrontal cortex (PFC) plays a key role in working memory.
• Long-term Memory: Large capacity, prolonged storage of information.

Assessing Memory

• Short-term memory test: Repeat sequences of digits in order. Average recall is 6–7 digits.
• Working memory test: Repeat sequences of digits in reverse order; typically more difficult, as it requires manipulation. Prevents forgetting. The prefrontal cortex (PFC) is critical.
• Test performance must be adjusted for age. A significant deviation from age-matched norms indicates a deficit.

Memory Location

• Memories are distributed across the brain (prefrontal, temporal, and parietal areas).
• Hippocampus plays a crucial role in memory formation and storage but is not the storage location.
• Systems consolidation: Hippocampus guides reorganization of neocortical information, making memories independent from the hippocampus over time.
• Retrograde amnesia (memory loss for past events) studies support this. Recent memories rely on both hippocampus and neocortex. Remote memories have become independent of the hippocampus.

Role of Sleep

• Slow-wave sleep likely plays a role in memory consolidation.
• Neural replay during sleep may support consolidation.

Medial Temporal Lobe (MTL) Role

• The MTL, including the hippocampus, is crucial for episodic learning.
• The hippocampus acts as a pointer to the cortex.
• Different cortical areas (ventral/dorsal streams, FFA, PPA) process different memory attributes (e.g., shape, color, motion, faces, places).
• The hippocampus and MTL create connections between these areas to generate a comprehensive representation, encoding both structural and semantic information.
• After a few weeks, the representation becomes independent of the hippocampus, requiring less extensive neural activity (activating only one area of the network).

Six Key Steps in Memory Formation and Retrieval

• 1. Encoding-Initial learning involves encoding by cortex; information then goes to MTL/hippocampus.
• 2. MTL-cortex resonance: MTL and cortex "resonate" to initiate memory trace establishment.
• 3. Independence from external support: Memory trace is independent of the initial stimulus.
• 4. Consolidation: Memory traces become permanent and distinct via synaptic changes in MTL/cortex.
• 5. Reminder cue: Subsequent presentation of one memory component acts as a retrievabl cue e.g. recalling a memory component from the original event.
• 6. Memory retrieval: Resonating activity between the MTL/cortex retrieves the memory.

A Representational View of Memory

• The representational view of memory challenges traditional short-term/long-term models.
• It emphasizes the content and function of memories rather than the storage locations.

Memory Schemas

• Memory schemas are crucial for effective encoding and retrieval.
• Constructed from co-activated long-term representations in posterior cortex (retrosplenial cortex, medial temporal gyrus, superior temporal gyrus, anterior temporal lobe, temporo-parietal junction).
• Ventromedial prefrontal cortex (vmPFC) acts as a neural connector, binding these regions in time.

Prefrontal Cortex's Role

• Actively maintains information in working memory through sustained neural activity (humans and macaques).
• PFC stops active maintenance when an action phase initiates.
• Prefrontal cortex lesions lead to dysexecutive syndromes, impacting the ability to maintain information.

Prefrontal Cortex vs. Medial Temporal Lobe

• Left temporoparietal damage disrupts working memory storage, particularly involving auditory-verbal information.
• Prefrontal cortex damage might not affect passive maintenance but can hinder manipulation of information.

HM Case Study

• Bilateral hippocampal resection caused severe anterograde amnesia (inability to form new conscious memories) and retrograde amnesia (partial loss of memories before surgery).
• HM's experience highlights a potential unconscious memory pathway in the limbic system, which is not always reliant on the hippocampus.