Memory

The sensory buffer is only technically a part of memory: it might make more sense to regard it as part of the perceptual system. Information may stay there for about 1/15 of a second, while the brain assembles it to “make sense”. We are familiar with the illusion by which a succession of still pictures presented rapidly enough appear to be moving: it is the basis of all cinematography. Once the frame rate drops below about 16 frames per second, however, we may well become conscious of the flicker or jumps from one still image to another. Similarly, we do not hear a succession of phonemes (or speech sounds), but complete words or phrases: it is as if the brain waits to assemble a meaningful sound before passing it on to the next stage.

Which is Short-term Memory (if the information makes it that far). The research suggests that STM deals best with sounds rather than visual stimuli, but that may perhaps be a reflection of the problem of devising experiments which do justice to the fact that visual stimuli are taken in all at once, whereas sounds are processed in a linear fashion—over time. In any event, the STM:

holds material for about 15-30 seconds, although this can be expanded by practice. This is much shorter than most of us think—a lot of people seem to think that it lasts for ten minutes or so. You can see, however, how disabling damage to the STM is, as in the case of dementia.
  
has a capacity of seven items (plus or minus two). However, “items” are defined by meaning rather than size, so it may be difficult to remember telephone numbers of more than seven digits, but if “01234” is remembered as “Bedford STD code” it becomes just one item, and remembering “7,9,3,1,5,6” after it becomes simpler. If that too is “chunked” as “my work 'phone number” it is even easier.

And so to Long-term Memory. Theoretically, LTM has infinite capacity and lasts for the rest of your life.

Tulving (1985) has suggested the useful distinction between three components of LTM:

• Semantic memory stores concepts and ideas
     
• Episodic (sometimes referred to as “autobiographical”) memory contains memories of events.
     
• Procedural memory concerns skills and “know-how” rather than “know-that” knowledge.

A few moments' thought will make it clear that these are fundamentally different. People with amnesia, for example, typically lose episodic memory, but other memories may be relatively intact. Episodic and semantic memory are more prone to distortion than procedural memory, which is more robust: a skill lost through lack of practice typically comes back rapidly when called upon, and without significant degradation. However, semantic and episodic memories are more amenable to linguistic description and communication.

I have depicted the three forms as overlapping. Apart from the fact that they are of course simply constructs imposed on the blooming buzzing confusion of reality, the relations between them raise interesting issues for facilitators of learning:

• The relationship between knowing that and knowing how
   
• “learning from experience” as the creation of semantic memory from episodic memory, and conversely
   
• challenging semantic memories (prejudices, for example) which are based solely on episodic memory.

(David Silk, 1962: based on Mark 4:3-20)

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