Human Memory Encoding, Storage, Retention, and Retrieval

Memory is retention of information over a period of time. Ebbinghaus studied memories by teaching himself lists of nonsense words and then studying his retention of these lists over periods of hours to days. This was one of the earliest studies of memory in psychology.

Contents:

  1. Short Term Memory
  2. Working Memory
  3. Long Term Memory
  4. Spreading Activation Model
  5. Practice and Strength
  6. Depth of Processing
  7. Elaborative Processing and Text
  8. Forgetting: Gone or Inaccessible?
  9. Forgetting: Decay or Interference?
  10. Retrieval and Inference
  11. Other Facts about Memory

Short Term Memory

While Ebbinghaus studied retention over long intervals, later experiments studied memory loss over periods of seconds to minutes. Short term memory was postulated to explain temporary retention of information as distinct from long term retention of information . Short term memory acts to also store current sensory information and to rehearse new information from sensory buffers. It has limited capacity (Miller's 7 plus or minus 2). The probability of encoding in Long term memory has been directly related to time in short term memory.

It is now believed that the loss of information stored in short term memory has the same characteristics as loss of information stored in long term memory. It happens quicker because it involves information that is not learned as well. What we call the learning process is transferring information from short term to long term memory and is a physiological process. The shape of the memory loss curves are the same. Hence we don't need to postulate a special type of memory. Instead, we need a theory of: 

  • Why we can rehearse only a limited amount of information at a time.
  • How different memories get different strengths (and so are forgotten at different rates).

Working Memory

Here we address why we can rehearse only limited information at a time.

Articulatory Loop Rehearsal limitations are due to limits in how long it takes verbal material to decay, not how many items we can store. Hence, the faster we can rehearse, the more we can store (Baddeley, 1986). Experimental support: word length effect. How long it takes to read words predicts how many words will be remembered. Articulatory loop is called the phonological loop due to evidence that it involves speech. We can rehearse about 1.5 seconds of verbal material before it decays. Time in the loop is not related to probability of coding in long term memory. Baddeley's model  proposes that we have a visual/spatial sketchpad as well as the phonological loop. These hold information for use by a central executive. There is evidence that a particular area of the frontal cortex is involved in working memory.

Long Term Memory

A simple observation: we often need to recall information that we learned long ago.How quickly and reliably we recall it depends on:

  • Activation: How long since we last used the information.
  • Strength: How well we have practiced it.

Experimental Evidence: (Anderson 1976) - Subjects learn some sentences. Some sentences are studied twice as long as others. Subjects must discriminate sentences they learned from distracters. They are tested for each sentence more than once, with varying intervening sentences. Results: Both amount of study and how recently the information was accessed affect speed of response. However amount of study matters only if the information was not recently accessed (an interaction effect).

 

Delay (number of intervening items) Degree of Study
Less Study More Study

Short (0-2)

1.11 seconds 1.10 seconds
Long (3 or more) 1.53 seconds 1.38 seconds

Spreading Activation Model

When information becomes easier to access as a result of having been used recently, we say it is more activated. This activation spreads between semantically related concepts.

Empirical Evidence:

  • Subjects are faster at confirming that a pair of words are both words if the second word is an associate of the first, for example, bread and butter (Meyer and Schvaneveldt 1971).
  • Given a word, subjects are asked to give an associated word. Their response is faster if subjects have responded with an associated word on a previous trial (Perlmutter and Anderson, figure 6.8).
  • Speed of activation seems to be about 200ms (as measured by Ratcliff and McKoon, 1981).
Implication: Text is easier to read if semantically related words are used.

Practice and Strength

We've seen that speed of recall of information from long term memory depends in part on how recently that information has been activated. However, what about the fact that speed of recall also depends on amount of practice? Activation changes quickly over time. The effect of practice decays much more slowly over time (witness Ebbinghaus, the alphabet). Thus these are believed to be distinct processes.

Power Law of Learning

A very robust result: the effect of practice in a wide range of different tasks fits a power law 

Reaction Time equals C * Practice Time K where C and K are constants that depend on the task. 

Practice helps a lot at first, then provides decreasing gains as you reach the limits of your performance ability.

Long-Term Potentiation - There appears to be a neural basis for this law of learning. Neural pathways in the hippocampus (known to be involved in learning) become increasingly sensitive when stimulated. The change in sensitivity follows a power law relationship.

Depth of Processing

Craik and Lockhart (1972) proposed that strength of memory depends on how deeply information is processed, not on how long it is processed

Experimental support:  Memory for words not improved by merely repeating them for a longer period of time (Glenberg et al. 1977). A large number of studies support the depth of processing conclusion. It applies to subject matter learning as well as laboratory situations. Subsequent work focused on what constitutes deep processing.

Processing Meaning:

Some lab studies compare tasks that require processing meaning of words versus form (e.g., what letters do they have).

Elaborative Processing and Text

Studies show benefits of connecting the items to be remembered to other related information (e.g., elaborating on sentences to be remembered, or rhyming). Intention does not matter. Subjects in deeper processing conditions do better regardless of whether they know they will need to remember the processed items.

Implications for study habits and method.

  1. Preview the material
  2. Make up questions
  3. Read, trying to answer the questions
  4. Reflect while you read. think of examples, relate it to what you know.
  5. Recite the information in each section after you've read it. Re-read what
  6. you can't recall.
  7. Review the major points and the answers to your questions at the end.
  • Question generation is at least as beneficial as question answering.
  • Questions generated before the material rather than after may be more beneficial

Forgetting: Gone, or Inaccessible

Do we forget because the information is gone, or do we forget because we can't access information that is still there? It is difficult to distinguish the two. However, there is evidence that we retain more than we can retrieve.

Experiment: (Nelson 1971) - Learn paired associates (numbers to nouns). Tested 2 weeks later to see which were remembered. Then given new material to learn that had some of the forgotten numbers, both with and without their original nouns.

Results: Subjects relearned the original associations faster (in spite of the fact that they could not recall them). Subjects relearned the original associations faster (in spite of the fact that they could not recall them). This suggests that some associative information was retained. One possible interpretation: strength of memories decay gradually. If these strengths fall below a certain threshold, we can't recall the information, but the remaining memory trace is still there to facilitate relearning.

Forgetting: Decay or Interference?

Is forgetting due to decay of unused information, or to interference of new information with old information? Different kinds of evidence are offered for each position.

A survey of forgetting research concluded that the rate at which we forget information usually conforms to a power law: we forget a lot at first, but over time the rate of forgetting diminishes. 

Decrease in long-term potentiation follows a similar power law. These facts are interpreted by some as evidence for a physiologically determined decay rate.

Interference Experiments Typical Experiment (A-D C-D paradigm):

  1. Subjects all learn A-B association (between items on list A and items on list B).
  2. Experimental subjects learn A-D associations (which use the same stimuli items as the A-B associations), while control subjects learn C-D association.
  3. Everyone is tested on A-B associations.

Typical Results: Experimental subjects take longer to learn their second set of associations than controls, and make more errors on the A-B test. Experimental subjects take longer to learn their second set of associations than controls, and make more errors on the A-B test. These results are interpreted as evidence that learning new associations to stimuli causes forgetting of old associations. However, interference does not happen with factual material when the additional facts are redundant with (e.g., causally related to) the original facts.

Fan Effect (a model) - Interference effects can be modeled as weakening of spreading activation over multiple links in a propositional network.

Stimulus activates concept nodes.- Fixed (limited) amount of activation spreads from activated nodes over associative links, divided equally between links. (Hence the more links, the less activation per link.) Activation converges at propositional nodes (candidate responses) until one emerges as the answer. Time to identify the response is inversely related to level of activation. 

Decay or Interference? Some claim that interference can produce the appearance of decay although it appears, both mechanisms are involved in forgetting or memory loss.

Retrieval and Inference

It is well established that people make inferences during retrieval, and believe that they saw or heard things that they in fact did not. People are more likely to erroneously think they read a sentence if it is an implication of something they read.

Effect of Prior Knowledge - People add other knowledge they have about the material studied.

Effect of Question Wording - Subjects shown film of automobile accident. Subjects asked: Did you see a broken headlight? or Did you see the broken headlight? (There was actually none.) Results: Subjects more likely to respond yes to  the broken headlight. Implications for courtroom testimony!

Other Facts about Memory

Organization of Material

Retrieval of information is better if the information is organized in some manner supporting systematic search, such as in hierarchies.

Method of Loci

The ancients remembered things by imagining taking a familiar walk, and placing the things to be remembered at locations along the way. This method works because it organizes the material to be remembered and it encourages elaborative processing and memorable imagery.

Context-Dependent Learning

Physical and emotional context may be inadvertently coded as retrieval cues, along with the intended cues.

Consistent with this idea, various studies show that recall is better when tested in the same context (physical or emotional) as in which learning took place. Some benefit has been found studying for important exams in the same room as they will be taken.  However these results are variable.

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