Lecture 18: Memory

Memories  Memory 1: a lasting consequence of an event (a broken glass)  Memory 2: a trace of an event that needs recovery with a key (a recollection about a broken glass)

Hysteresis An isolated muscle is being stimulated at a constant rate and strength. Muscle force will depend on muscle length. During slow stretching and slow shortening, the muscle will display different dependences F(L). This behavior is called hysteresis. F, L Stim F L Curve 1 Curve 2

Memories Declarative (explicit)Nondeclarative (implicit) Facts and events Hippocampus, medial temporal lobe, diencephalon NonassociativeAssociative Reflexes Amygdala, cerebellum, BG, cortex Skills, habits

Memory  Habituation: learning not to respond to a stimulus following its multiple presentations (usually, when it is not very meaningful)  Sensitization: learning to respond to smaller magnitudes of a stimulus (usually, if it is very meaningful)

Skill  A motor program? Can it be applied to different effectors?  Are there control functions stored in the brain? In what variables?  A reflex?  Perception-action coupling? How different is it from a reflex?

Learning Mirror Writing

Three Stages  Encoding: putting an event into an internal code  Storage: maintaining the code over time  Retrieval: using a key (intrinsic or extrinsic) to recover the code/event

Short-Term Memory  Encoding: typically an acoustic, visual, or somatosensory code  Storage: limited capacity (7 ± 2 “pieces”); decay; followed by consolidation or loss of memory

A Reverberating Circuit The activity in this simple reverberating circuit will persist until some crucial substances are depleted. Open circles show excitatory synapses. In Out

Consolidation of Memory Processing a sensory stimulus may lead to creating a short-term memory trace in parallel with producing an effector (motor) output. Short-term memory can be consolidated into long-term memory. Input Receptor Central processing Effector Output Short-term memory Long-term memory Consolidation

Conditioning  Classical conditioning: associating a response with a stimulus based on repetitive presentations (e.g., ringing a bell is associated with getting food); the animal has no initiative  Operant conditioning: searching for an action that leads to a desired consequence; active exploration. Even monosynaptic reflexes can show it (as demonstrated by Jon Wolpaw)!

Conditioning  Classical conditioning: Pavlov’s experiments  The theory of conditioned reflexes starting from inborn reflexes  Towers of silence

Holography Holography creates an image (a memory) of an object on a photographic plate with the help of two light beams: the object beam and the key beam (A). One plate can store a number of images using different key beams. If the plate is illuminated by a key beam, an image of the corresponding object will emerge (B). Key beam Object beam Photo plate Key beam A B Image Object

Synapses as the Site for Memory  There are phenomena of LTP and LTD (cerebellum, hippocampus)  Where else?  Repetition without repetition  “Disposable synapses”  Lashley: Each neuron takes part in many memories; each memory is represented all over  LTP: too short, mostly animal studies  Importance of whole-brain (emotion) mechanisms  Eccles: use and disuse of synapses, spinal memory  Spinal memory experiments Pro: Contra:

Spinal Memory Experiments  Stage 1: extirpation of half of the cerebellum. This leads to asymmetrical monosynaptic reflexes.  Stage 2: spinalization. If enough time elapses between the two surgeries, the asymmetry persists. If not, it disappears.  Cooling experiments: MSRs appear, followed by their asymmetry.

Questions Without Answers  Which of the external events are being remembered?  Which of the brain processes that accompany the events are being fixed in memory?  Can all or only some of the neural elements fix memories?  What are the neural substrates of STM and LTM, and what are the mechanisms of exchange between them?  What happens in memory disorders? What is affected— whole-brain mechanisms, storage, attention, classification, retrieval, etc.?

Memory Disorders: Amnesia  Causes: brain injury, stroke, encephalitis, electric shock, etc.  Anterograde: affects the ability to recollect events that occurred after an injury  Retrograde: affects the ability to recollect events that occurred prior to an injury Amnesia is a partial loss of memory.

Memory Disorders Korsakoff’s syndrome:  Chronic alcohol abuse  Defective retrieval  Partial cues or prompts help Alzheimer’s:  Motor skills are not affected  Priming experiments show major defects