Amnesia and the Neuropsychology of Memory

Explicit Memory

  • Definition: Conscious recollection of facts or events.

  • Also known as Declarative knowledge.

    • Involves recall and recognition.

  • Examples:

    • Remembering a word list.

    • Remembering your birthday.

  • Key Brain Region: Medial temporal lobe, especially the hippocampus.

Implicit Memory

  • Definition: Unconscious change in memory.

  • Includes procedural knowledge (skills).

    • Example: Riding a bike.

  • Priming (test) is also an implicit memory process.

  • Key Brain Regions: Cortex, basal ganglia, cerebellum, and perceptual regions.

Distinctions between Implicit and Explicit Memory

  • These distinctions have often been illustrated by examining patients with amnesia.

Recall vs Recognition

  1. Recall:

    • Definition: Actively retrieve information from memory.

    • Requires self-initiated retrieval, places a heavier load on working memory (WM) and executive control.

  2. Recognition:

    • Definition: Identify previously seen information among options (e.g., in multiple-choice tests).

    • Relies on familiarity and must recognize previous exposure.

    • Note: Both recall and recognition are explicit memory tests as they require conscious recollection.

Amnesia

  1. Types of Amnesia:

    • Psychogenic Amnesia:

      • Relatively rare in the real world.

      • Caused by psychological, not physical trauma.

      • Debate exists on its authenticity in real-world contexts; not the focus of this discussion.

    • Organic Amnesia:

      • Results from brain damage, injury, accident, or surgery.

  2. Types Based on Memory:

    • Anterograde Amnesia:

      • Most common form.

      • Characterized by the inability to form new explicit memories after the trauma.

    • Retrograde Amnesia:

      • Involves inability to retrieve explicit memories formed before the trauma.

      • Often categorized as temporarily graded: memory for older information remains intact while more recent information is vulnerable.

Brain Damage in Amnesia

  • Common areas affected include:

    • Hippocampus

    • Basal ganglia

    • Amygdala

Patient H.M. (Henry Molaison)

  • Medical History:

    • Experienced epilepsy beginning in childhood, localized in the temporal lobe.

    • Surgical procedure aimed at alleviating epilepsy symptoms involved removal of parts of the temporal lobe.

  • Result: Patient exhibited anterograde amnesia.

Empirical Studies

Mirror Reading Study

  • Participants: Anterograde amnesic patients.

  • Results:

    • New words: Both normal individuals and amnesics improved similarly (showcasing implicit learning).

    • Old words: Normal participants outperformed amnesic patients (both implicit and explicit memory evaluated).

Tower of Hanoi Task

  • Objective: Move all disks from the first to the last peg with three rules:

    1. Only move one disk at a time.

    2. Only the top disk can be moved on a given peg.

    3. A larger disk cannot be placed on top of a smaller disk.

Priming

  • Definition: Improved processing of a stimulus due to prior exposure, occurring without conscious recollection.

  • Word Fragment Completion Task:

    • Amnesic patients demonstrated a higher completion rate of old word fragments compared to new ones, indicating preserved implicit memory.

    • However, when evaluated on explicit recognition (e.g., "Did you see this word before?"), their performance was poor.

PET Study on Explicit and Implicit Memory

  • Explicit Task: Recall the word from the list that begins with a specific stem.

    • Result: Activated hippocampus and frontal lobe activity.

  • Implicit Task: Say the first word that comes to mind.

    • Result: Activated posterior visual areas, without hippocampal involvement.

  • Explanation of Activation: Prior exposure facilitates task completion, resulting in decreased neural activity, indicating lesser cognitive effort required.

Behavioral Double Dissociation

  • Modality of Presentation:

    • Implicit test (priming - fragment completion): Examining speed of completion for fragments using words from the list.

    • Explicit test (recall the words): Evaluating the retrieval of words.

  • Results: Depth of processing significantly affects explicit memory but does not affect implicit memory.

Depth of Processing (Levels of Processing)

  • Definition: Words can be encoded at various depths or levels, which affects memory retention.

  • Categories of Depth:

    1. Shallow: Physical encoding (appearance of letters).

    2. Intermediate: Acoustic encoding (sound of the word).

    3. Deep: Semantic encoding (meaning of the word).

  • Conclusion: Deeper encoding correlates with easier recall and, therefore, better explicit memory retention.

Overview of Implicit and Explicit Memory Types

Long Term Memory

  • Explicit Memory:

    • Sub-Classifications:

    • Semantic (facts)

    • Episodic (personal episodes)

  • Implicit Memory:

    • Sub-Classifications:

    • Skills & Habits

    • Priming

    • Classical Conditioning

Brain Regions Associated with Memory Types

  • Explicit Memory: Hippocampal Region

  • Implicit Memory: Striatum (for skills), Cortex (for priming), Amygdala (for emotional aspect), Cerebellum (for coordination and procedural tasks).