Chapter 52 Learning and Memory

Learning and Memory

Principles of Neural Science CH 52

Short Term and Long Term Memory Involve Different Neural Systems

• Learning: Defined as the process where experience results in a relatively permanent change in the behavior of an organism.

• Memory: The retention of what has been learned, which can be retrieved at a later time.

• Short Term Memory (STM): Duration ranges from seconds to minutes.

• Long Term Memory (LTM): Duration spreads from hours to a lifetime.

• Selectivity of Lesions and Drugs: Lesions or pharmacological agents can selectively impact STM or LTM without affecting the other, implying that distinct but interacting circuits and molecular mechanisms exist for STM and LTM.

Working Memory and Neural Systems

• Working Memory (also known as STM):

  • Relies on neural circuits involving the prefrontal cortex and parietal networks which are crucial for actively maintaining and manipulating information.

• Long Term Memory:

  • Relies on a combination of cortical and subcortical networks.

  • The hippocampus plays an essential role in early encoding and consolidation of LTM.

Role of Short Term Memory

Goal-Directed Functionality

• STM maintains transient representations of information relevant to immediate goals.

• It supports processes such as decision-making, reasoning, and planning.

Neural Basis of STM

• Accomplished through the sustained firing of neurons in the prefrontal cortex (PFC) and posterior parietal cortex during memory delay tasks.

• This persistent neural activity functions as a correlate for “holding something in mind.”

Factors Affecting STM

• Influenced by attentional load, interference, and distractions.

Memory Transfer to Long Term Memory

• Information stored in STM is selectively transferred to LTM.

• Consolidation: The process by which information moves from STM to LTM.

Influences on Consolidation

• Attention: Information must be attended to for effective encoding.

• Rehearsal: Repetition facilitates the likelihood of long-term storage.

• Salience/Emotion: Emotionally charged events are more effectively consolidated due to the amygdala's influence on the hippocampus.

• Memory transfer is also time-dependent and especially vulnerable to disruption shortly after acquisition, as demonstrated in cases of retrograde amnesia post head trauma, where recent memories may be lost but older ones remain intact.

Forms of Long Term Memory

• Two major types of long-term memory are distinct in their mechanisms:

  • Implicit Memory (Nondeclarative):

  • Procedural Memory: Skills and habits.

  • Associative Learning: Encompasses classical and operant conditioning.

  • Nonassociative Learning: Involves habituation and sensitization.

  • Explicit Memory (Declarative):

  • Semantic Memory: Facts.

  • Episodic Memory: Events.

Medial Temporal Lobe and Episodic Memory

• The medial temporal lobe (MTL) is pivotal in functioning related to episodic long-term memory and comprises the following important structures:

  • Hippocampus

  • Entorhinal Cortex

  • Perirhinal Cortex

  • Parahippocampal Cortex

Impact of MTL Damage

• Damage to the MTL can lead to significant anterograde amnesia, especially for episodic memories.

• However, this damage does not typically affect:

  • Working memory.

  • Implicit memory (skills, priming).

• The MTL is vital for both the encoding and early consolidation of episodic memories, but it is not necessarily the long-term storage location.

• Other structures such as the diencephalon and prefrontal cortex also contribute to the networks involved in episodic memory.

Processes Involved in Episodic Memory

• Episodic Memory Processing: Engages four key functions:

  • Encoding: The transformation of sensory experiences into a neural trace, predominantly supported by the MTL and associated cortex.

  • Storage: The gradual embedding of memory across distributed cortical networks.

  • Retrieval: Involves the reactivation of stored traces with the MTL potentially acting as an index to restore cortical activity.

  • Consolidation: The time-dependent stabilization of memory traces, which can be vulnerable to disruption, particularly at initial stages.

• Role of Sleep in Consolidation:

  • Sleep, especially slow-wave sleep, is crucial for consolidation as it enables the hippocampal replay of neural activity patterns.

Interactions in Episodic Memory

• Episodic memory derives from interactions between the medial temporal lobe and association cortices.

  • The MTL collaborates with unimodal and polymodal association areas that encode various features of an event (e.g., visual, auditory, spatial characteristics).

  • The hippocampus integrates these diverse elements into a coherent relational representation, constituting a key feature of episodic memory, which encompasses the “where, when, what” of experiences.

  • Over time, as connections among cortical areas strengthen, the memory becomes increasingly independent of the hippocampus, known as systems consolidation.

Contribution of Episodic Memory to Future Planning

• Episodic memory's Role: It transcends merely recalling past events; it enables mental time travel towards future scenarios.

• When imagining hypothetical situations or planning future actions, the hippocampal networks employed for retrieving past episodes are likewise engaged.

• Essential for goal-directed behavior, decision-making, and adaptive planning, patients with hippocampal damage often struggle to formulate coherent future events.

Role of the Hippocampus in Relational Associations

• The hippocampus underpins episodic memory by forging relational associations.

• Specific circuits in the hippocampus, particularly CA3, excel at linking multiple inputs, resulting in relational or associative memories.

• This mechanism facilitates flexible retrieval, where a partial cue can reinstate the complete memory (pattern completion).

• The hippocampus also supports pattern separation—the capability to distinguish between similar yet non-identical experiences.

• This function is crucial for preventing memory interference and is vital for spatial navigation, involving structures known as place cells.

Implicit Memory and Its Behavioral Impact

Overview

• Implicit (Nondeclarative) Memory: Encompasses various cognitive behaviors in both humans and animals, including:

  • Procedural Memory: Skills and habits.

  • Priming Effects: Enhanced processing of stimuli due to prior exposure.

  • Classical Conditioning: Such as Pavlovian fear conditioning.

  • Operant Conditioning: Involves instrumental learning.

  • Nonassociative Learning: Includes habituation and sensitization.

• These processes function without the need for conscious recollection and utilize distinct neural systems that differ from those utilized in episodic memory.

Different Neural Circuits in Implicit Memory

• The various forms of implicit memory recruit different neural circuits:

  • Skills & Habits: Engaged using the basal ganglia (striatum), motor cortex, and cerebellum.

  • Priming: Conducted through neocortical regions pertinent to the stimulus being primed (e.g., visual cortex for visual priming).

  • Classical Conditioning:

  • Fear conditioning relies on the amygdala.

  • Eyeblink conditioning involves circuits in the cerebellum and brainstem.

  • Nonassociative Learning: Engages simple reflex pathways that encompass synaptic plasticity.

Non-Associative vs. Associative Learning

Non-Associative Learning

• Habitual Response: A diminished response to repeated benign stimuli (e.g., white noise).

• Sensitization: Heightened response prompted by repeated strong stimuli.

Associative Learning

• Classical Conditioning: Establishes a bond between a conditioned stimulus (CS) and an unconditioned stimulus (US).

• Operant Conditioning: Links specific behaviors with their consequences.

• Both pathways depend on alterations in synaptic strength; however, associative learning is conditioned by the existence of a contingency between events, leading to more intricate forms of plasticity.

Operant Conditioning and Behavioral Associations

• Operant Conditioning Defined: Focuses on associating a specific behavior with a reinforcing event, classifying it as goal-oriented and responsive to different reinforcement schedules.

Neural Substrates of Operant Conditioning

• Key structures include the basal ganglia and the dopamine-producing midbrain circuits (specifically VTA and nucleus accumbens), both integral to mediating reinforcement learning signals.

• Underlying this mechanism, dopamine encodes prediction errors—representing the discrepancy between expected rewards and actual outcomes—which propels further learning.

Biological Constraints on Associative Learning

• Certain forms of associations are naturally learned more easily than others due to biological constraints that may stem from genetic or physiological factors.

  • Example of Biological Influence:

  • Taste Aversion: A biological readiness to associate sickness with a specific food.

  • Inherited Fears and Phobias: Readiness to averse stimuli such as heights or spiders.

Learning Exhibits Natural Constraints

• Examples of these biologically driven learning constraints include:

  • Pigeons have natural instincts such as pecking which can be conditioned to receive food.

  • In contrast, pigeons can learn to flap their wings to evade a shock but cannot be taught to flap wings to receive food.

Insights from Memory Errors

• Nature of Memory: It is a constructive process, rather than a precise recording of events, with incidences of false memories and confabulations demonstrating the reconstructive nature of memories influenced by contextual factors.

• Forgetting Mechanism: Forgetting is an adaptive feature, allowing irrelevant information to be discarded from memory.

• Reconsolidation: Reactivated memories can undergo updates or disruptions; memory traces remain plastic even after initial consolidation.

• Investigating memory errors can illuminate the principles governing encoding, retrieval, and the influence of schemas and expectations in memory formation.