Ch 11: Learning, Memory & Amnesia

Ch 11: Learning, Memory & Amnesia Part 2

Amnesia after Concussion • Concussions are the #1 cause of amnesia • Posttraumatic amnesia (PTA) • Coma: pathological state of unconsciousness • Can last seconds to weeks… • After regaining consciousness, there is a period of confusion • Usually there is permanent retrograde amnesia for events just prior to the blow to the head & permanent retrograde amnesia for the subsequent period of confusion • With amnesia, there are sometimes surviving memories for isolated events: islands of memory

Memory Consolidation • The fact that concussions disrupt recent memories more than older ones suggests that the older ones have been strengthened • Hebb’s theory of consolidation • Memories are stored in STM by reverberating in closed circuits • If this is disrupted ex: by a blow to the head, the memory can be lost • But normally the recirculating STM will cause structural changes in the synapse, providing stable place to store it as LTM • Electroconvulsive shock (ECS) used to study consolidation by interrupting this reverberation • Mixed results; doesn’t explain long gradients of amnesia; consolidation can continue potentially indefinitely

Hippocampus & Consolidation • Standard consolidation theory • Memories are temporarily stored in the hippocampus until they are transferred to cortical storage • Multiple-trace theory • Hippocampus & other memory storage areas store the memories as long as they exist but over time, they become more resistant to disruption • Each time a similar experience occurs, the original memory is recalled & creates a new engram(change in the brain that stores a memory), which is linked to the original engram, making it stronger • Accounts for long gradients of retrograde amnesia

Reconsolidation • Theory that each time a memory is retrieved from LTM, it is temporarily held in STM and is once again susceptible to amnesia until it is reconsolidated • Some evidence suggests not all types of memories are susceptible to reconsolidation

Neuroanatomy of Object-Recognition Memory • Lesion studies to determine neuroanatomy of memory • Object-recognition memory tested with delayed nonmatch-to-sample test • Rhinal cortex: cortical area adjacent to the hipppocampus & amygdala; one of the 3 major temporal lobe structures • Lesions of this area caused serious deficit in object recognition memory • 2 areas: entorhinal cortex & perirhinal cortex • Hippocampal lesions only caused moderate deficits • Pyramidal cells of the CA1 region seem to be the key to hippocampal involvement in this kind of memory

Hippocampus & Spatial Memory • While hippocampal lesions have little/no effect on general memory, they consistently impair memory of spatial location • Many hippocampal neurons are place cells • Neurons that respond only when a subject is in specific locations • Grid cells: neurons in the rhinal cortex that have an array of evenly spaced place fields

Hippocampus & Spatial Memory • Birds that cache food have larger hippocampi than other species • Cognitive map theory • There are several systems in the brain each specialized in memory for different kinds of info; the hippocampus is for spatial memory • There are criticisms of this theory

Where are memories stored? • While the hippocampus & rhinal cortex are involved in memory, memories are not necessarily stored there • Memories are stored diffusely in the brain • Can survive the destruction of one specific area • Memories become more resistant to disruption over time • Memories appear to be stored in the brain areas that participated in the original experience that created the memory

Where are memories stored? • So far we’ve discussed 4 brain areas with some memory storage • Hippocampus (spatial) • Perirhinal cortex (object) • Mediodorsal nucleus (Korsakoff’s) • Basal forebrain (Alzheimer’s) • 5 more to discuss • Inferotemporal cortex • Amygdala • Prefrontal cortex • Cerebellum • Striatum

Inferotemporal Cortex • Involved in visual perception of objects & thought to participate with the perirhinal cortex in storing memories of visual patterns • In general, areas of 2ndary sensory cortex play a role in storing sensory memories

Amygdala • Role in memory for the emotions tied to an experience • Ex: the fear that goes along with a foot shock • Doesn’t seem to store the memory itself • It is involved in strengthening emotionally significant memories stored elsewhere • Explains why you are better at remembering emotion-provoking experiences

Prefrontal Cortex • Damage to this area does not cause major amnesia & patients often perform well on traditional memory tests • However, damage to the PFC does seem to impair 2 episodic memory abilities • The order of events • Working memory (ability to maintain relevant memories while a task is being completed)

Cerebellum & Striatum • Implicit memories of sensorimotor learning presumed to be stored in the sensorimotor circuit associated with first experiencing them • Cerebellum involved in Pavlovian conditioning • Striatum stores memories of consistent relationships between stimuli & responses; habit formation • Both also seem to be involved in non-motor memories

Synaptic Mechanisms of L&M • Long-term potentiation (LTP) • Based on Hebb’s hypothesis about reverberation & STM to LTM • Facilitation of synaptic transmission following high-frequency electrical stimulation applied to presynaptic neurons • Can last for a long time (months) after multiple stimulations • Only develops if both the pre & postsynaptic neurons fire • One of the most widely studied phenomena in neuroscience

LTP • 3 part process • Induction • High-frequency stimulation induces LTP (learning) • Maintenance • Changes responsible for storing LTP (memory) • Expression • Changes that allow it to be expressed during the test (recall) • NMDA receptors (glutamate) play a large role in LTP • Still more questions than answers

Infantile Amnesia • We remember essentially nothing from our infancy • But we do show there are implicit memories for experiences from this time • When kids are later shown pictures of themselves with preschool classmates, they remember almost none of them; however, they do exhibit a skin conductance response associated with seeing familiar individuals

Ch 11: Learning, Memory & Amnesia