Visual short-term memory

In the study of vision, visual short-term memory (VSTM) is one of three broad memory systems including iconic memory and long-term memory. VSTM is a type of short-term memory, but one limited to information within the visual domain.

The term VSTM refers in a theory-neutral manner to the non-permanent storage of visual information over an extended period of time. The visuospatial sketchpad is a VSTM subcomponent within the theoretical model of working memory proposed by Alan Baddeley; in which it is argued that a working memory aids in mental tasks like planning and comparison. Whereas iconic memories are fragile, decay rapidly, and are unable to be actively maintained, visual short-term memories are robust to subsequent stimuli and last over many seconds. VSTM is distinguished from long-term memory, on the other hand, primarily by its very limited capacity.

Overview

The introduction of stimuli which were hard to verbalize, and unlikely to be held in long-term memory, revolutionized the study of VSTM in the early 1970s. The basic experimental technique used required observers to indicate whether two matrices,). The probability that a suprathreshold change will be detected is simply the probability that the change element is encoded in VSTM (i.e., k/N). This capacity limit has been linked to the posterior parietal cortex, the activity of which initially increases with the number of stimuli in the arrays, but saturates at higher set-sizes. Although urn models are used commonly to describe performance limitations in VSTM, it is only recently that the actual structure of items stored has been considered. Luck and colleagues have reported a series of experiments designed specifically to elucidate the structure of information held in VSTM. This work provides evidence that items stored in VSTM are coherent objects, and not the more elementary features of which those objects are composed.

Noise models

An alternative framework has more been put forward by Wilken and Ma who suggest that apparent capacity limitations in VSTM are caused by a monotonic decline in the quality of the internal representations stored (i.e., monotonic increase in noise) as a function of set-size. In this conception capacity limitations in memory are not caused by a limit on the number of things that can be encoded, but by a decline in the quality of the representation of each thing as more things are added to memory. In their 2004 experiments, they varied color, spatial frequency, and orientation of objects stored in VSTM using a signal detection theory approach. The participants were asked to report differences between the visual stimuli presented to them in consecutive order. The investigators found that different stimuli were encoded independently and in parallel, and that the major factor limiting report performance was neuronal noise (which is a function of visual set-size).

Under this framework, the key limiting factor on working memory performance is the precision with which visual information can be stored, not the number of items that can be remembered.

Psychophysical models

Psychophysical experiments suggest that information is encoded in VSTM across multiple parallel channels, each channel associated with a particular perceptual attribute. Within this framework, a decrease in an observer's ability to detect a change with increasing set-size can be attributed to two different processes:

if decisions are made across different channels, decreases in performance are typically small, and consistent with decreases expected when making multiple independent decisions
if multiple decisions are made within the same channel, the decrease in performance is much greater than expected on the basis of increased decision-noise alone, and is attributed to interference caused by multiple decisions within the same perceptual channel.

However, the Greenlee-Thomas model are able to use this model to predict that greater interference will be found when dual decisions are made within the same perceptual dimension, rather than across different perceptual dimensions, this prediction lacks quantitative rigor, and is unable to accurately anticipate the size of the threshold increase, or give a detailed explanation of its underlying causes.

In addition to the Greenlee-Thomas model, there are two other prominent approaches for describing set-size effects in VSTM. These two approaches can be referred to as sample size models, and urn models. They differ from the Greenlee-Thomas model by:

ascribing the root cause of set-size effects to a stage prior to decision making
making no theoretical distinction between decisions made in the same, or across different, perceptual dimensions.

Intermediate visual store

There is some evidence of an intermediate visual store with characteristics of both iconic memory and VSTM. This intermediate store is proposed to have high capacity (up to 15 items) and prolonged memory trace duration (up to 4 seconds). It coexists with VSTM but unlike it visual stimuli can overwrite the contents of its visual store. Further studies suggests an involvement of visual area V4 in the retention of information about the color of the stimulus in visual working memory, and the role of the VO1 area for retaining information about its shape.

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Overview

Noise models

Psychophysical models

Intermediate visual store

See also

Notes

References

Sources