Working memory has more layers than thought

April, 2011

A new study provides further support for a three-tier model of working memory, where the core only holds one item, the next layer holds up to three, and further items can be passively held ready.

Readers of my books and articles will know that working memory is something I get quite excited about. It’s hard to understate the importance of working memory in our lives. Now a new study tells us that working memory is in fact made up of three areas: a core focusing on one active item, a surrounding area holding at least three more active items (called the outer store), and a wider region containing passive items that have been tagged for later retrieval. Moreover, the core region (the “focus of attention”) has three roles (one more than thought) — it not only directs attention to an item and retrieves it, but it also updates it later, if required.

In two experiments, 49 participants were presented with up to four types of colored shapes on a computer screen, with particular types (eg a red square) confined to a particular column. Each colored shape was displayed in sequence at the beginning with a number from 1 to 4, and then instances of the shapes appeared sequentially one by one. The participants’ task was to keep a count of each shape. Different sequences involved only one shape, or two, three, or four shapes. Participants controlled how quickly the shapes appeared.

Unsurprisingly, participants were slower and less accurate as the set size (number of shape types) increased. There was a significant jump in response time when the set-size increased from one to two, and a steady increase in RT and decline in accuracy as set-size increased from 2 to 4. Responses were also notably slower when the stimulus changed and they had to change their focus from one type of shape to another (this is called the switch cost). Moreover, this switch cost increased linearly with set-size, at a rate of about 240ms/item.

Without getting into all the ins and outs of this experiment and the ones leading up to it, what the findings all point to is a picture of working memory in which:

  • the focus contains only one item,
  • the area outside the focus contains up to three items,
  • this outer store has to be searched before the item can be retrieved,
  • more recent items in the outer store are not found any more quickly than older items in the outer store,
  • focus-switch costs increase as a direct function of the number of items in the outer store,
  • there is (as earlier theorized) a third level of working memory, containing passive items, that is quite separate from the two areas of active storage,
  • that the number of passive items does not influence either response time or accuracy for recalling active items.

It is still unclear whether the passive third layer is really a part of working memory, or part of long-term memory.

The findings do point to the need to use active loads rather than passive ones, when conducting experiments that manipulate cognitive load (for example, requiring subjects to frequently update items in working memory, rather than simply hold some items in memory while carrying out another task).

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