The Visuo-Spatial Sketchpad

An Overview of the Visuo-Spatial Sketchpad

Whereas the phonological loop Opens in new window handles the temporary storage of verbal and acoustic information, the visuo-spatial sketchpad controls short-term processing and retention of visuo-spatial material.

In a study by Baddeley et al. (1975), participants heard the locations of digits within a matrix described by an auditory message that was either easily visualized or rather hard to visualize. They then reproduced the matrix. When this task was combined with a task called the pursuit rotor (i.e., tracking a light moving around a circular track), performance on the easily visualized message was greatly impaired, but there was no adverse effect on the non-visualizable message.

The most obvious interpretation of these findings is that the pursuit rotor involves visual perception, and thus interferes with performance on the visualizable message.

However, Baddeley and Lieberman (1980) found that a specifically visual concurrent task (making brightness judgments) actually disrupted performance more on the non-visualizable message.

The results were very different when a spatial task with no visual input was performed while the message was being presented. This involved subjects trying to point at a moving pendulum while blindfolded, with auditory feedback being provided.

This spatial tracking task greatly reduced recall of the visualizable messages, but had little effect on the non-visualizable messages.

Thus, recall of visualizable messages of the kind used by Baddeley et al. (1975) and by Baddeley and Lieberman (1980) is interfered with by spatial rather than by visual tasks, implying that processing of such messages relies mainly on spatial coding.

Components of Visuo-Spatial Sketchpad

Like the phonological loop, the sketchpad is said to have separable components, controlling visual, spatial, and possibly kinesthetic information (Baddeley, 2000; Baddeley & Logie, 1999).

According to Logie (1995) the visuo-spatial sketchpad consists in two components:

1. The visual cache, which is responsible for storage of information about visual form and color.
2. The inner scribe, which deals with spatial and movement information. It rehearses information in the visual cache, transfers information from the visual cache to the central executive, and is also involved in the planning and execution of body and limb movements.

Support for the distinction between a visual cache and an inner scribe was reported by Quinn and McConnell (1996). Their participants learned word lists in two different ways:

1. the method of loci Opens in new window, with each word being associated with a different familiar location; and
2. the pegword technique Opens in new window, in which each word was associated to easily memorized items or pegs based on the rhyme “one is a bun, two is a shoe, three is a tree, four is a door…”.

Mental images were formed by associating the first list word with a bun, the second word with a shoe, and so on. It was assumed that the method of loci mainly requires visual processing, whereas the pegword technique requires spatial and visual processing.

Quinn and McConnell (1996) obtained evidence favouring the above assumption by using two interfering tasks:

One was a spatial task, in which a dot had to be monitored as it moved through a sequence of locations. The other was a visual task involving the presentation of dynamic visual noise (a meaningless display of dots that changed continuously).

Memory performance based on the method of loci Opens in new window was disrupted by the visual task but not by the spatial task, suggesting that learning primarily depended on the visual cache rather than the inner scribe.

In contrast, memory performance based on the pegword technique Opens in new window was adversely affected by both interference tasks, suggesting that learning with this technique required both components of the visuo-spatial sketchpad.

Evidence consistent with Logie’s theory was reported by Beschin, Cocchini, Della Sala, and Logie (1997). They studied a man, NL, who had suffered a stroke. He found it very hard to describe details from the left side of scenes in visual imagery, a condition known as unilateral representational neglect.

However, NL had no problems with perceiving the left side of scenes, so his visual perceptual system was essentially intact. He performed very poorly on tasks thought to require use of the visuo-spatial sketchpad, unless stimulus support in the form of a drawing or other physical stimulus was available.

According to Beschin et al. (1997), NL may have sustained damage to the visual cache, so he could only create impoverished mental representations of objects and scenes. Stimulus support allowed him to use his intact visual perceptual skills to compensate for the deficient internal representations.

Farah et al. (1988) reported on a patient, LH. He performed much better on tasks involving spatial processing much better on tasks involving spatial processing than on tasks involving the visual aspects of imagery (e.g., judging the relative sizes of animals). Presumable LH’s brain damage affected the visual cache rather than the inner scribe.

Smith and Jonides (1997) carried out an ingenious study in which two visual stimuli were presented together, followed by a probe stimulus. The participants had to decide either whether the probe was in the same location as one of the initial stimuli (spatial task) or whether it had the same form (visual task).

The stimuli were identical in the two tasks, but there were clear differences in brain activity as revealed by PET. Regions in the right hemisphere (prefrontal cortex, premotor cortex, occipital cortex, and parietal cortex) became active during the spatial task. In contrast, the visual task produced activation in the left hemisphere, especially the parietal cortex and the inferoteporal cortex.

Several other studies have indicated that different brain areas are activated during visual and spatial working-memory tasks. In essence, the ventral prefrontal cortex (e.g., the inferior and middle frontal gyri) is generally activated more during visual working memory tasks than spatial working-memory tasks. In contrast, more dorsal prefrontal cortex (especially an area of the superior prefrontal sulcus) tends to be more activated during spatial working memory tasks than visual working memory tasks.

It should be noted that this separation between visual and spatial processing is entirely consistent with evidence that rather separate pathways are involved in visual and spatial perceptual processing.

How useful is the visuo-spatial sketchpad in everyday life?

According to Baddeley (1997, p.82). “The spatial system is important for geographical orientation, and for planning spatial tasks. Indeed, tasks involving visuo-spatial manipulation… have tended to be used as selection tools for professions … such as engineering and architecture.”

The visuospatial sketchpad may also serve an important function during reading, as it visually encodes printed letters and words while maintaining a visuospatial frame of reference that allows the reader to backtrack and keep his or her place in the text (Baddeley, 1986).

Evaluation

The notion that the visuo-spatial sketchpad consists of somewhat separate visual (visual cache) and spatial (inner scribe) components is supported by various kinds of evidence.

• First, when a visual task and a spatial task are performed together, there is often little interference between them (e.g., Baddeley & Lieberman, 1980; Quinn & McConnell, 1996).

• Second, some brain-damaged patients seem to have damage to the visual component but not to the spatial component (e.g., Beschin et al., 1997).
• Third, brain-imaging data suggest that the two components of the visuo-spatial sketchpad are located in different brain regions (e.g., Smith & Jonides, 1997; Sala et al., 2003).

In spite of the evidence for separate visual and spatial components, many tasks require both components to be used in combination. What remains for the future is to understand more fully how processing and information from the two components are combined and integrated on such tasks.