The Episodic Buffer

An Overview of the Episodic Buffer

The episodic buffer is basically a “backup” store that communicates with or receives input from both the working memory slave systems (phonological loop Opens in new window and visuo-spatial sketchpad Opens in new window) and long-term memory Opens in new window.

The episodic buffer, recently added by Baddeley (2000a) as additional component to the working memory model Opens in new window, is assumed to be controlled by the central executive Opens in new window which is capable of retrieving information from the store in the form of conscious awareness, of reflecting on that information and, where necessary, to manipulate it.

Baddeley (2000) outlines a number of principal features of the proposed episodic buffer component:

• It has a limited capacity, and provides temporary storage of representations using some form of multimodal code.

• It possess the ability to bind information from the slave systems and long-term memory into a unitary episodic representation.
• The buffer is controlled by the central executive component, and representations are retrieved from it in the form of conscious awareness.

The episodic buffer is so called buffer in the sense of acting as an intermediary between the subsystems (phonological loop Opens in new window and visuo-spatial sketchpad Opens in new window) which use different codes, combining them into a unitary multi-dimensional representation.

The buffer is episodic in the sense that it holds episodes of the information that is being manipulated across space and time. In this respect, it resembles Tulving’s concept of episodic memory Opens in new window.

Thus, this new model of working memory Opens in new window comprises a limited capacity system that provides a temporary storage of information held in a multi-modal code, which is capable of binding information from the subsidiary systems and from long-term memory Opens in new window into a unitary episodic representation. Conscious awareness is assumed to be the principal mode of retrieval from the buffer.

This revised model differs from the old principally in focusing attention on the processes of integrating information rather than on the isolation of the subsystems. Therefore, this new model of working memory provides a better basis for talking the more complex aspects of executive control in working memory.

Why did Baddeley (2000) feel it necessary to add the episodic buffer to the working memory model about 25 years after the other three components were identified?

In essence, the phonological loop Opens in new window and the visuo-spatial sketchpad Opens in new window permit the processing and temporary storage of specific kinds of information only, and the central executive is involved in general processing but has no storage capacity.

There seems to be something missing, since none of these three components can be regarded as a general storage system that can combine several kinds of information. It is precisely this gap that the episodic buffer is designed fill.

Evidence

There are various findings that are hard to account for in terms of previous versions of the working memory model which lacked an episodic buffer. For example, Chincotta, Underwood, Abd Ghani, Papadopoulou, and Wresinksi (1999) studied memory span for Arabic numerals and digit words, finding that the participants used both verbal and visual coding while performing the task.

This suggests that verbal information processed within the phonological loop and visual information processed within the visuo-spatial sketchpad must be combined and stored somewhere within working memory Opens in new window.

It was rather mysterious where this could be within earlier versions of the working memory model Opens in new window, but the natural location within the current model is clearly the episodic buffer.

Additional reasons for postulating an episodic buffer can be seen if we consider the finding that immediate memory span for unrelated words is about 5 words, whereas the immediate span for sentences is about 15 or 16 words (e.g., Baddeley, Vallar, & Wilson, 1987).

Before the introduction of the episodic buffer, it would have been natural to explain this difference on the basis that long-term memory plays a much larger role in recalling words within sentences.

If that account were correct, then brain-damaged patients with impaired short-term phonological memory should be able to use long-term memory Opens in new window to produce a respectable sentence span. In fact, however, their sentence span is only about 5 words (Baddeley et al., 1987).

The findings can be explained if we assume that sentence span depends on the episodic buffer, and the episodic buffer can only function effectively when the other components of working memory are intact.

Stronger evidence in support of the notion of an episodic buffer was reported by Baddeley and Wilson (2002). They argued that high levels of immediate prose recall depend on two factors:

1. the capacity of the episodic buffer; and
2. an efficiently functioning central executive which facilitates the creation and maintenance of information in the buffer.

According to this argument, even severely amnesic patients Opens in new window (with very impaired long-term memory) should have good immediate prose recall provided that they have an efficient central executive Opens in new window.

As predicted, immediate prose recall was much better in amnesics having little deficit in executive functioning than in those with a severe executive deficit.

There is relatively little information concerning the location of the episodic buffer within the brain, and Baddeley (2001) doubted whether it is actually located in any single location. Relevant evidence was reported by Prabhakaran, Narayanan, Zhao, and Gabrieli (2000, p. 89), who carried out an fMRI study and concluded that, “The present fMRI results provide evidence for another buffer, namely one that allows for temporary retention of integrated information.” The brain activation associated with this (episodic) buffer was within the frontal lobes.

Evaluation

Several findings that were hard to explain within previous versions of the working memory model are relatively easy to account for by assuming that people have an episodic buffer.

This suggests that the episodic buffer is a valuable addition to the model, and increases its ability to predict behavior in many situations. It remains for future research to clarify the processes determining what information is stored in the episodic buffer and how different kinds of information are integrated within the buffer.