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    Lecture 5 slides Lecture 5 slides Presentation Transcript

    • Neural Basis of Cognition
      Lecture 5
      Learning and Memory
    • The Famous Patient, H.M.
      Severe epilepsy can be debilitating and even lead to death. In extreme cases, surgery is used to treat epilepsy
      In 1953, a patient called H.M. (to preserve anonymity) with severe epilepsy was treated surgically via removal of portions of his left and right medial temporal lobes
      Removed: two thirds of his hippocampus, parahippocampalgyrus, and amygdala
      Damaged/Atrophied: the remainder of his hippocampus, entorhinal cortex, some of his anterolateral temporal cortex
    • H.M.
      What happened?
      His epilepsy was successful brought under control.
      He developed severe memory deficits - amnesia.
      Why is his case so famous?
      His memory abilities were extensively studied prior to the operation, allowing extensive postoperative analysis.
      Today’s imaging techniques did not exist; the fact that damage was done during surgery meant that it was known exactly what parts of his brain were damaged.
    • H.M.
      What abilities have been destroyed?
      Creation of new short term memories – he cannot recall recent weather, the current date, where he currently lives, or even a conversation with someone if there is an interruption that lasts a few minutes
      Learning of new information
      He would misidentify current pictures of himself as pictures of his father
      He had no idea of his fame
    • H.M.
      What abilities were not destroyed?
      Linguistic ability, recall of memories formed before the operation, ability to reason - almost all abilities he had before the operation aside from those listed on the previous slide
      Learning of new skills
      Working memory (until distracted)
    • H.M.
      The extensive study of H.M. taught us much about what we now know about learning and memory in humans.
      He passed away in 2008, still unaware of his fame.
    • Memory
      What is memory?
      Memory is the storing of information related to day-to-day experiences for later retrieval.
      Are there different types of memory?
      Yes.
      Is memory stored in a single part of the brain?
      No.
    • How can amnesia occur?
      Damage to the medial temporal lobe
      Herpes simplex encephalitis, blockage of blood supply, hypoxic ischemia, trauma, Alzheimer’s disease
      Damage to the midline diencephalic region
      Korsakoff’sdiease (due to chronic alcohol abuse), blockage of blood supply, third ventricle tumors
    • Is amnesia always permanent?
      No.
      Closed head injury (such as in a motorcycle accident) can cause amnesia that almost completely disappears
      Electroconvulsive therapy (memory recovers over weeks to months)
      Seizures can temporarily induce amnesia that heals completely.
    • Memory impairment
      Anterograde amnesia: Impairment in forming new memories.
      Almost always associated with some retrograde amnesia.
      Retrograde amnesia: Impairment in memory for information acquired prior to amnesia-inducing event
      In closed head injury, retrograde amnesia extends back less than a week before the injury in 80% of patients, but can extend to years.
      H.M.’s retrograde amnesia extended 11 years; unclear how much of this was due to the seizure disorder, which began 11 years before surgery, and how much was due to the surgery
      Retrograde amnesia that extends back decades is often seen in progressive disorders such as Korsakoff’s, Alzheimer’s, Parkinson’s, or Huntington’s
      Temporal gradient (Ribot’s Law): generally, more recent memories are more severely affected, e.g. identifying Ronal Reagan
      “Flat” Gradients have been observed
    • Amnesia
      Modality general, including in H.M.’s case
      Therefore, amnesia is a deficit in memory functions rather than perceptual, linguistic, or other cognitive processes
      Unilateral hippocampal damage CAN be modality-specific
      Left hemisphere damage can lead to selective impairment for verbal material, right hemisphere damage for nonverbal material
    • Working Memory
      Digit span task: repeating back, in order, a string of digits
      H.M.: Normal performance (7 +/- 2 digits)
      Extended digit span task: once a patient’s digit span is known, the same string is iteratively lengthened by one digit until a limit is reached
      Normal performance: 20+ digits
      H.M.’s performance: no longer than the digit span task
    • Retained abilities
      Skill learning
      H.M. did exhibit some forms of learning, though he was unaware of having done so
      Mirror tracing task: tracing the outline of a figure by looking at it in a mirror
      Across sessions, H.M. began to perform this task more accurately and more quickly
      Rotary pursuit: tracking a circularly moving target
      With practice, amnesiacs become more adept
    • Retained abilities
      These are motor skills that are repeated over and over; are amnesiacs learning the specific instance of those tasks (such as drawing a particular figure) or the skill in general?
      Mirror-reading task: mirror images of word triplets shown
      Amnesiacs show the same improvement in this task – even with new words – that non-amnesiacs do, even if the patients cannot recall practicing
    • Retained abilities
      Repetition priming: performance is enhanced as a result of previous exposure to an item
      Gollin incomplete pictures task: patients are shown very degraded and incomplete line drawings of objects and are asked to name them
      The procedure is repeated with increasingly incomplete drawings
      Amnesiacs showed improvement on this task
    • Retained abilities
      Word-stem completion task
      Patient is given a list of words to study
      After a delay, memory is tested in two ways using three-word stems of these words:
      Patient is asked to recall the word from the study list that began with the given three-letter stem
      Amnesiacs perform poorly
      Patient is asked to give “the first word that comes to mind” when given a three-letter stem
      Amnesiacs performed at normal levels
    • Memory loss patterns
      Explicit memory:
      Conscious recollection of some prior event
      Implicit memory:
      Subconscious recollection of information about some prior event
      Does not depend on consciously remembering that event – “memory without awareness”
      Explicit but not implicit memory can be impaired or destroyed due to hippocampal damage
    • Eye-movement monitoring
      Amnesiac is shown an image with three points of interest
      Eye movements indicate he looks at each of the three points of interest
      After a delay, the amnesiac is shown the same image with one point of interest removed
      There are about as many eye movements to the now-absent third point of interest as there are to the other two points of interest, but if the patient is asked, he will not remember that there was a third point of interest
      This is a demonstration of implicit versus explicit memory impairment due to hippocampal damage
    • Neuroimaging evidence
      PET and fMRI provide converging evidence that the hippocampal system is associated with relational memory
      Higher activation when given a task that requires memory of relations among items
    • The role of the hippocampal area
      After damage to the hippocampal area, amnesia can develop
      Therefore, the hippocampal area is vital for the formation of new memory
      After damage, previously formed memories can still be recalled
      Therefore, the hippocampus neither stores memories nor is mandatory for memory recall
    • Where is memory stored?
      Imaging studies and cell recording studies show that declarative memory is distributed by modality, in the regions which initially process sensory input
      Visual information is stored in the visual system, auditory information is stored in the auditory system, and so on
      Similar studies show that procedural memory is stored in the parts of the brain involved in carrying out the related action
      For example, motor skills are learned and “stored” in the motor cortex
    • Integration of memory systems
      Unclear.
    • The memory system
      Encoding:
      Memories must be created and stored
      Consolidation:
      Memories may be “strengthened”
      Retrieval:
      Memories must be retrieved for later use
    • Hippocampus
      Encoding:
      Activation during encoding of faces, words, scenes, objects; degree of activity is proportional to degree of memorization
      fMRI studies show that the amount of activity at the time an item is first seen and encoded predicts how well that item is remembered later on
      Subsequent memory effect: subsequently remembered items are associated with greater activation at encoding time than items not subsequently remembered
    • Hippocampus
      Consolidation:
      Unclear mechanism
      Theory: reactivation of already stored memories during the time after learning plays a crucial role in consolidation
      There is some evidence tying such reactivation to activity in the hippocampus during sleep subsequent to a learning event
    • Hippocampus
      Retrieval:
      Hippocampal system activation occurs during retrieval of memory
      The purpose of this activation has not been determined
    • Next Lecture
      Other areas involved in encoding, consolidation, retrieval
      Mechanism of the forming of memories (learning)
      Computational and theoretical modeling of learning