Memory Systems Chapter 23 Friday, December 5, 2003

What is Memory?  Memory is a change in the synapses of the neurons comprising a network.  Memories are not static “records” but products of a change in the connections among nerve cells.  Memories extract and store the meanings of events, not their exact details. Sensory aspects are reexperienced not recalled. Recall is reconstructed, not just retrieved.

The Search for the Engram  Engram – the physical memory trace.  Lashley couldn’t find one: “This series of experiments…has discovered nothing directly of the real nature of the engram. I sometimes feel in reviewing the evidence on the localization of the memory trace, that the necessary conclusion is that learning just is not possible.”pg 276 of text

Where is Memory?  Both cortical and subcortical structures are involved in creating memories.  Although memories appear to be stored in the cortex, they are: Distributed – spread out in a network Redundant – represented in multiple ways  Hebbian cell assemblies (loops) permit access at various points in a network.

Stages of Memory  Sensory buffers – one for each sense: Iconic – for visual information Echoic – for auditory information  Working memory/short term memory: Limited capacity – 7 plus or minus 2 Capacity can be expanded by chunking Executive functions  Long term memory – permanent.

Amnesia  Retrograde amnesia – inability to recall previously stored memories of one’s life.  Anterograde amnesia – inability to form new memories of events in one’s life. H.M. -- surgery removed parts of the temporal lobe, including hippocampus.  With amnesia, some kinds of memory are spared, others impaired, suggesting there are different kinds of memory.

Working Memory  Unimpaired in amnesics.  In humans, localized to frontal lobes.  Delayed development of frontal lobes in humans and monkeys: “A not B” task Can’t tolerate delays (8-10 sec) until 1-1/2 yrs old

Infantile Amnesia  Inability to remember events of one’s childhood before ages year olds couldn’t remember preschool classmates Attributed to immaturity of memory systems in children.

Long Term Memory  Localized to temporal lobes: Hippocampus – memory formation and consolidation Entorhinal cortex (adjacent to hippocampus)  During memory storage the temporal region forms a connection between the relevant networked areas of the brain.

Procedural vs Declarative  Procedural memory – memory for how to do things, skills.  Declarative memory – memory for abstract knowledge, facts and events of one’s life.  Only memory for events of one’s life is affected by amnesia – not procedural memory or memory for facts.

Explicit vs Implicit  Implicit memory – does not involve conscious awareness – unimpaired in amnesics. Procedural memory Priming, recognition tasks Classical conditioning, habituation Semantic memory  Explicit memory – involves conscious attention – impaired in amnesics. Episodic and autobiographical memory, recall

Conditioning  A form of learning: Classical (Pavlovian) conditioning – present at birth, unimpaired in amnesics. Operant (instrumental) conditioning.  How are learned associations maintained? Amygdala (and emotional arousal) mediates activity of the hippocampus to form neural connections.

Types of Amnesia  Damage to Temporal Lobe – H.M.  Damage to Thalamic Structures – N.A. Most impaired with verbal material.  Korsakoff’s Syndrome Chronic alcoholics with thiamine deficiency Retrograde and anterograde amnesia.  Electroconvulsive Shock Therapy (ECT)

Amnesia and Forgetting  Forget at normal rate – thalamic (diencephalon): N.A. Korsakoff’s patients Monkeys with medial thalamic lesions  Forget rapidly – hippocampus: H.M. Bilateral ECT Monkeys with amygdala/hippocampus lesions

Semantic Memory  Unimpaired by anterograde amnesia. Children with amnesia develop normal semantic memory. Intact entorhinal cortex may be used to store semantic memories despite hippocampal damage.  Context-rich, episodic memory impaired.

Cellular Mechanisms  Habituation involved suppression of action potentials through decreased release of neurotransmitter – change is at the synapse. Sensitization – increased neurotransmitter release  Explicit memory involves long-term potentiation, LTP (a physical change to neurons) in hippocampus and neocortex. See Chapter 24 for details