Lecture 17 Memory I

Memory Systems

Goals

  • Review memory deficits associated with dorsolateral prefrontal cortex (dlPFC) lesions.
  • Explore the taxonomy and neuroscience of memory systems.
  • Examine case studies of patients with memory disorders.
  • Discuss the role of the medial temporal lobe (MTL) and the hippocampus in memory.
  • Highlight the evolutionary and adaptive significance of episodic memory.

Memory Systems Overview

Memory encompasses a range of processes and types, including: – Working Memory: – Short-term retention for immediate use (e.g., holding a phone number long enough to dial it). – dlPFC involvement in attention to internal representations and executive control. – Episodic Memory: – Memory for events, including the what, when, and where of experiences. – Semantic Memory: – Knowledge of facts and concepts, such as understanding the meaning of words. – Procedural Memory: – Implicit learning of skills, such as playing a musical instrument. – Priming and Conditioning: – Automatic responses and learned associations (e.g., blinking to a tone paired with an air puff).

Frontal Lobes and Working Memory

Key Findings

  • dlPFC lesions lead to deficits in:
    • Temporal Order: Difficulty remembering the sequence of events.
    • Source Memory: Difficulty identifying the origin of learned information.
  • Encoding and Retrieval:
    • dlPFC plays a role in both encoding new memories and retrieving old ones.
    • Studies using Subsequent Memory Tasks show dlPFC activation is higher for items later remembered than those forgotten.

Material-Specific Encoding

  • Activation is content-specific:
    • Left dlPFC: Enhanced activation for verbal materials.
    • Right dlPFC: Enhanced activation for visual materials (e.g., faces).
    • Bilateral dlPFC: Involved in encoding objects.

Case Studies of Memory Disorders

Patient H.M. (Henry Molaison)

  • Surgery: Bilateral hippocampal resection to treat epilepsy.
  • Memory Impairments:
    • Profound anterograde amnesia: Inability to form new episodic memories.
    • Moderate retrograde amnesia: Loss of memories from 1-2 years before surgery, with a graded loss extending back 11 years.
  • Preserved Abilities:
    • Normal short-term memory (digit span).
    • Retained procedural skills (e.g., mirror tracing).

Patient K.F.

  • Lesion: Left parieto-occipital damage from a motorcycle accident.
  • Dissociation:
    • Poor short-term memory but intact long-term memory.
    • Demonstrates a single dissociation between STM and LTM.

Clive Wearing

  • Cause: Herpes encephalitis resulting in severe damage to the medial temporal lobes.
  • Deficits:
    • Severe anterograde amnesia: Memories last 7-30 seconds.
    • Incomplete retrograde amnesia: Retains knowledge of his wife and children but cannot form new episodic memories.
    • Preserved procedural memory (e.g., playing piano).
  • Behavioral Phenomena:
    • Believes every waking moment is his first, reflected in repetitive diary entries.

Korsakoff Syndrome

  • Cause: Thiamine deficiency due to chronic alcoholism.
  • Deficits:
    • Severe anterograde and retrograde amnesia.
    • Confabulation: Filling memory gaps with fabricated or distorted recollections.
  • Anatomy:
    • Damage to the mammillary bodies, which are part of the hippocampal output system.

The Hippocampus and Episodic Memory

Anatomy and Function

  • Inputs from:
    • What pathway: Lateral entorhinal cortex.
    • Where pathway: Medial entorhinal cortex.
  • Outputs to the cortex via the fornix and mammillary bodies.
  • Role in integrating what, where, and when into cohesive episodic memories.

Cognitive Maps

  • The hippocampus helps create cognitive maps for navigation and memory:
    • Place Cells: Fire when in specific locations.
    • Grid Cells: Provide a coordinate system for spatial navigation.
    • Head Direction Cells: Encode orientation in space.

Evolutionary Perspective

Episodic Memory in Non-Humans

  • Studies on scrub jays demonstrate episodic-like memory:
    • Remember what, where, and when food was cached.
    • Episodic memory may enhance survival by supporting adaptive behavior.

Adaptive Significance

  • Episodic memory supports:
    • Future planning: Predicting outcomes based on past experiences.
    • Behavioral flexibility: Adapting to novel situations using memory-based predictions.

Taxonomy of Memory

Declarative Memory

  • Episodic:
    • Memory for personal experiences (e.g., recalling a birthday party).
  • Semantic:
    • Context-free factual knowledge (e.g., knowing what a seahorse is).

Non-Declarative Memory

  • Procedural: Skills and habits.
  • Priming: Facilitation of responses due to prior exposure.
  • Conditioning: Learned associations.

Key Terms

  • Anterograde Amnesia: Inability to form new memories.
  • Retrograde Amnesia: Loss of existing memories.
  • Cognitive Maps: Mental representations of spatial and relational environments.
  • Place and Grid Cells: Neurons in the hippocampus encoding spatial information.
  • Korsakoff Syndrome: Memory disorder associated with thiamine deficiency.

This lecture integrated clinical case studies, neural mechanisms, and evolutionary insights to deepen understanding of memory systems. Future lectures will explore the Complementary Learning Systems theory and the role of neurogenesis in memory.

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