Long Term Memory Notes

Introduction to Long Term Memory

This section discusses long-term memory, emphasizing its storage mechanisms and retrieval processes. It builds upon the previous lecture on short-term memory, so it's recommended to review that material first.

Stages of Long Term Memory

Long-term memory can be divided into stages, often described using the filing cabinet analogy:

  • Encoding: This involves processing new information, organizing it, and preparing it for storage. It's like creating a file with all relevant information, organized in a way that makes sense for later retrieval.

  • Storage: This is the process of maintaining the organized information over time, similar to placing the file in a filing cabinet. The brain organizes information based on semantic similarity and relationships to other concepts.

  • Retrieval: This involves accessing the stored information when needed, like retrieving the file from the filing cabinet. It entails finding the right location and extracting the necessary information.

Consolidation: This is an ongoing process that strengthens memories over time, making them more resilient against decay. This process is closely related to encoding and storage by reinforcing the information. Sleep is crucial for consolidation.

How Long Term Memory Works in the Brain

Learning and Neural Activity

Learning involves the brain storing information for later use. This is achieved by storing a representation of neural activity patterns associated with a stimulus. When the brain learns something, it stores the pattern of neural activity that occurred when the stimulus was first presented. This allows the brain to reactivate that pattern later, even without the original stimulus being present.

Hebbian Theory

Hebbian Theory: "neurons that fire together, wire together."

When two neurons near each other fire action potentials simultaneously, the connection (synapse) between them becomes stronger. This increased synaptic strength means that if one neuron fires in the future, it is more likely to trigger the other.

Imagine two neurons with a weak connection (dotted line). If both neurons fire an action potential simultaneously (lightning bolt), the connection between them strengthens (solid line). Now, if one neuron fires, the other is more likely to fire as well.

Long Term Potentiation (LTP)

Long-term potentiation (LTP) is the process that strengthens the connections between neurons after they fire an action potential. LTP is the mechanism that transforms a weak connection (dotted line) into a strong connection (solid line).

Synaptic Plasticity

LTP leverages synaptic plasticity, the ability of synapses to change. This can occur through:

  1. Increased Neurotransmitter Release: Potentiated synapses release more neurotransmitters into the synapse.

  2. Increased Postsynaptic Receptors: More receptors on the postsynaptic neuron allow more neurotransmitters to bind.

More neurotransmitters and receptors lead to increased current flow into the postsynaptic neuron, increasing the likelihood that it will fire if the presynaptic neuron fires.

Memory as Reactivation

Memory involves reactivating a pattern of neural activity that was initially generated by a stimulus, even without the stimulus being present. This requires turning a partial representation into a full representation.

  1. Initial Stimulus: A stimulus (e.g., seeing a cow) generates a pattern of activity in the brain.

  2. Short-Term Memory: In short-term memory, this pattern of activity is maintained through attention and rehearsal.

  3. Long-Term Memory: The pattern of activity is stored through long-term potentiation but fades once attention shifts.

  4. Retrieval: To recall the memory, a partial representation is activated (e.g., thinking "what does a cow look like?").

  5. Reactivation: If learning has occurred (through LTP), the partial representation can trigger the reactivation of the full pattern of activity, allowing you to remember the stimulus.

The strength of the memory determines how well the full pattern of activity can be reactivated.

Demonstrations of Retrieval

Giving more information helps retrieval:

By giving a partial representation of the stimulus this in turn helps the brain reactivate the neural pattern, hence making it easier to retrieve the memory.

Recognition

Recognition is easier than recall because it provides a partial representation of the memory, which the brain can then use to complete the full representation.

Using cues and prompts can aid memory retrieval by giving the brain more information to work with.

Explicit vs. Implicit Long Term Memory

Long-term memory is divided into two main categories:

  • Explicit (Declarative) Memory: Consciously stored and remembered information.

  • Implicit (Nondeclarative) Memory: Unconsciously and effortlessly remembered information.

Explicit memory is more vulnerable to brain disorders and natural aging, while implicit memory is more resilient.

Types of Implicit Memory

  1. Classical Conditioning: Associating an involuntary response with a new stimulus.

    • Example: Pavlov's dogs associating the sound of a bell with food, leading to salivation.

    • Brain Regions: Hippocampus and cerebellum.

  2. Operant Conditioning: Associating a voluntary behavior with a consequence (reward or punishment).

    • Example: Skinner's rats learning to press a lever for a reward.

    • Brain Regions: Amygdala and orbitofrontal prefrontal cortex.

  3. Motor Learning: Improvement in motor performance through practice.

    • Examples: Learning to play an instrument, ride a bike, or tie shoelaces.

    • Brain Regions: Primary motor cortex (M1), supplementary motor area (SMA), basal ganglia, and cerebellum.

  4. Priming: Exposure to one stimulus influences the response to a subsequent stimulus.

    • Example: Being more likely to fill in "s o
      p" with "u" as "soup" after reading the words bread, juice, milk.

    • Mechanism: Decreasing the threshold for reactivation of particular neural activity patterns.

Types of Explicit Memory

Explicit memory is subdivided into:

  1. Semantic Memory: General knowledge, facts, and information about the world.

    • Example: Knowing that cows are brown, have horns, and live on farms.

    • Lacks experiential context (no connection to a specific event or experience).

  2. Episodic Memory: Recall of events, experiences, and episodes from your life.

    • Example: Remembering when you fed a specific cow on the farm last week, noting its color, horns, and the carrot you gave it.

    • Contains experiential content (connected to a specific event or experience).

Semantic Memory

Semantic memory is a knowledge base containing facts, details about objects, vocabulary, social rules, and legal rules. It lacks experiential content and is not tied to specific events.

Example: Patient KC retained the ability to use his knowledge but could not remember where he acquired it.

Semantic Dementia

Semantic dementia is a type of frontotemporal dementia characterized by degeneration in the lateral temporal lobe, leading to progressive language impairment.

Symptoms include difficulty understanding others, trouble naming objects, and using incorrect words due to loss of semantic knowledge.

Episodic Memory

Episodic memory involves recalling what happened, when it happened, and where it happened. It is closely tied to autonoetic consciousness, the ability to relive the episode and know that it happened to you.

Brain Regions Involved in Episodic Memory

  • Hippocampal Complex: Hippocampus and parahippocampal areas are crucial for storing and retrieving information.

  • Diencephalon: Includes the thalamus, which has projections to the hippocampus.

  • Cortex: Anterior and posterior brain regions contribute to episodic memory.

  • Left Hemisphere: More involved in verbal memory.

  • Right Hemisphere: More involved in visual memory.

Encoding and Retrieval in Episodic Memory

The same processes of encoding and reactivation are used in episodic memory. The hippocampus acts as a central hub that binds information together across the cortex.

  1. Encoding: A stimulus generates a pattern of activity in the brain, which feeds forward to the hippocampus.

  2. Storage: The hippocampus stores a representation of the activity pattern.

  3. Retrieval: A partial representation of the stimulus (retrieval cue) activates the hippocampus.

  4. Reactivation: The hippocampus reinstates the full representation of the stimulus by activating the previously formed connections.

Role of the Hippocampus

The hippocampus binds and stores memory traces across the cortex and is critical for both storage and retrieval of episodic memories.

Dysfunction of the hippocampus, as seen in conditions like epilepsy, traumatic brain injury, and Alzheimer's, leads to significant memory impairment.

Successful retrieval is dependent on the level of activity of the hippocampus, therefore more hippocampal involvement creates a stronger memory.

Role of the Frontal Lobes

The frontal lobes are important for complex thinking skills and play a crucial role in the complex aspects of memory, such as:

  • Organizing information.

  • Sequencing events.

  • Providing source context.

  • Applying strategies to encoding information.

Memory Access

The frontal cortex is important for retrieving information without cueing. It is involved in rifling through the filing cabinet to find the information.

Patients with frontal lobe damage struggle to recall information without cues, even though the information is still there. Frontal lobes are responsible for the organisational process where the hippocampus is responsible for the feed forward loop for the partial representation.

Compared to hippocampal damage, frontal lobe damage has less of an effect on encoding memory and more prevalent in memory retrieval.