View source for Visual short term memory

==Capacity limits in visual short term memory==
[[Image:VSTM_fig5.jpg|thumb|300px|right|F5|Stimuli and results from the study of Luck and Vogel (1997).  In one condition, observers were instructed to remember only the colors of the items because only color could change.  In a second condition, observers were instructed to remember only the orientations of the items because only orientation could change.  In a third condition, observers were instructed to remember both the colors and the orientations because either could change.]]
Early studies of VSTM using alphanumeric characters suggested a capacity limit of 4-5 items (e.g., Sperling, 1960), but it was not clear whether the items were being stored visually or verbally.  Experiments using the change-detection task have estimated a capacity of 3-4 objects using basic visual features combined with an interference task to limit contributions from verbal short term memory (Luck & Vogel, 1997).  As shown in Figure 3, for example, observers are highly accurate for arrays containing 1-3 simple objects, and performance declines systematically as the number of items increases.  Quantitative estimates of capacity using the Pashler/Cowan K equation (Cowan et al., 2005; Pashler, 1988) typically lead to estimates of 3-4 items, which might reflect a broad limit on active memory maintenance (Cowan, 2001).

However, it is not yet clear whether VSTM is limited to a set of 3-4 high-resolution representations or "slots" or whether the limits are due to the amount of information rather than the number of objects.  The first view proposes that VSTM consists of a small number of fixed-resolution slots, and Luck and Vogel (1997) proposed that the capacity of VSTM is limited by the number of objects rather than the number features that must be remembered.  That is, objects are the fundamental storage unit for VSTM.  As shown in Figure 5, they demonstrated that observers could remember the colors and orientations of four objects just as well as they could remember only the colors or only the orientations.  They further showed that objects defined by four features could be remembered as well as objects defined by a single feature.  An alternative possibility is that each feature dimension is represented in a separate memory store (Magnussen, Greenlee, & Thomas, 1996).  Subsequent research has shown that features can be stored more efficiently when they form an object than when they do not (Xu 2002a, 2002b) and that observers can detect differences between arrays that contain the same features but in different combinations (Johnson, Hollingworth, & Luck, in press; Wheeler & Treisman, 2002).  Yet, such research also shows that performance is worse when multiple features are drawn from the same feature dimension (e.g, objects are composed of two colors that could change independently rather than one color and one orientation).

The second view proposes that capacity limits on VSTM result not from the number of objects or a fixed number of slots, but from the amount of information in the display.  This view, often called the “resource” hypothesis, proposes that a fixed pool of resources is divided among the available items, with the resolution of the representation reduced as the number of items is increased.  In this view, all of the objects may be stored, but with decreased resolution as the amount of information increases (Alvarez & Cavanagh, 2004; Vogel et al., 2001; Wilken & Ma, 2004).  Alvarez and Cavanagh (2004) found that estimated capacity decreased as a function of the difficulty of discriminating different items increased.  By extrapolating to a case in which the items were maximally discriminable with minimal effort, they also estimated a maximum capacity of approximately 4.5 items for the simplest items.

The difference between these views rests on whether the fundamental units of visual memory are discrete objects that are stored in fixed-resolution slots or whether the determining factor in the capacity of VSTM is the amount of information to be stored, independent of the number of objects.