Short-Term and Long-Term Memory

Short-Term and Long-Term Memory

1. Early Memory Models

   • William James (1980): Primary memory and Secondary memory are temporally distinct. Primary memory contains info which is conscious at any given moment. Secondary memory encompasses all the knowledge that we have ever accumulated.
   • Waugh & Norman (1965): Rehearsal is necessary for info to pass from primary to secondary memory - or the info will be forgotten due to interference from other info which is processed - Primary and Secondary memory are separate stores.
     • Probe digit task: series of 16 digits which don't occur more than twice in a row. Ps to indicate for the last digit (which is repeated) which one was following it previously. This was easier when there was less space between the repetition - less interfering items.
   • Atkinson & Shiffrin (1968): Attention is needed to pass info from sensory register on to the STM store. Rehearsal is needed to pass info from the STM to the LTM store. There is a bidirectional connection between STM and LTM

2. Is forgetting from STM due to interference or decay?

   • Probe digit task suggests that forgetting from STM occurs due to interference from other items.
   • Debate between interference and decay is still unresolved

3. Patient H.M.

   • Became amnesic in 1953 after bilateral resection of medial temporal lobe as a treatment for epilepsy - Removal tissue included the hippocampus
   • Strongly impaired on episodic LTM tasks for events which took place after the operation - anterograde amnesia
   • Intact STM (e.g. digit span task)
   • Suggests that STM and LTM are separate memory stores

4. Patient K.F.

   • Intact LTM
   • Impaired STM - very short digit span of 2-3 items (verbal info only)
   • Cannot be explained with Atkinson & Shiffrin 1968 model - If one has STM problems, they should be impaired on LTM tasks because info gets transferred from STM to LTM
   • There are separate LTM and STM stores
   • Also shows difference between verbal and visual memory

5. Working Memory: Short-term storage and manipulation of info - we are doing something with the info. STM: Short-term storage of info.

6. Representations: symbols which stand for real events/ objects. Processes are using symbols which stand for real events/ objects

7. WM vs LTM:

   • WM
     • Holds active info for a short time
     • Allows us to work effectively on a current task
     • Short-term storage function & manipulation/ control function of the content of the store.
   • LTM
     • Info is encoded and stored (building memory traces)
     • info is not held in an active state as in WM
     • Retrieval mechanisms are needed to reinstate the memory trace.
   • Multistore Models
     • WM and. LTM are considered separate memory stores with their own set of representations
     • They Interact by means of bidirectional info transfer - LTM can also influence WM
   • Unitary store models
     • There is a single set of representations which can either be used for WM or LTM processes
     • Activated lTM representations form the content of WM

8. Ranganath & Blumenfeld (2005): Is the medial temporal lobe exclusively necessary for episodic LTM?

   • STM tasks often use highly familiar and over learned material (e.g. digits). This material is represented in cortical regions outside the medial temporal lobe.
   • It is better to use novel material to investigate if the medial temporal lobe is important for STM. There is evidence that the medial temporal lobe is also important for STM and perceptual tasks.
   • WM for novel material engages the hippocampus and predicts LTM performance
     • Line drawings of novel objects. Delayed match-to-sample task. Match/non0match decision for the probe. Working memory as the incidental encoding phase for a surprise recognition memory test (episodic LTM test)
     • There was more activation in WM task for items that were later on remember than the words that were later on forgotten, although p's did not know they were going to be testest

9. Are representations shared in WM and LTM?

   • Neuroimaging evidence for same representations (Ranganath et al 2014)
     • Category-sensitive brain regions in the inferior temporal cortex were activated both during WM retention and retrieval from episodic LTM
   • More evidence for shared representations
     • Univariate neuroimaging studies: level of activation in specific regions
     • Multivariate neuroimaging studies: patterns of brain activity

10. Summary

    • STM holds info active for seconds and has a limited capacity
    • Rehearsal is necessary for info to be passed from STM to LTM
    • Is forgetting due to passing of time or interference?
    • Double dissociations between STM and LTM in neuropsychological patients (H.M., K.F.)
    • Multi-store models assume STM and LTM are separate stores with separate representations
    • Unitary store models assume that a single set of representations is shared by STM and LTM
    • Results from brain imaging studies question the assumptions of multi-store models
      • Hippocampus is also recruited during some STM tasks and even perceptual tasks
      • The same domain-sensitive brain regions are involved during retention in STM and retrieval from LTM