Abstract
BackgroundClassic work on visual short-term memory (VSTM) suggests that people store a limited amount of items for subsequent report. However, when human observers are cued to shift attention to one item in VSTM during retention, it seems as if there is a much larger representation, which keeps additional items in a more fragile VSTM store. Thus far, it is not clear whether the capacity of this fragile VSTM store indeed exceeds the traditional capacity limits of VSTM. The current experiments address this issue and explore the capacity, stability, and duration of fragile VSTM representations.Methodology/Principal FindingsWe presented cues in a change-detection task either just after off-set of the memory array (iconic-cue), 1,000 ms after off-set of the memory array (retro-cue) or after on-set of the probe array (post-cue). We observed three stages in visual information processing 1) iconic memory with unlimited capacity, 2) a four seconds lasting fragile VSTM store with a capacity that is at least a factor of two higher than 3) the robust and capacity-limited form of VSTM. Iconic memory seemed to depend on the strength of the positive after-image resulting from the memory display and was virtually absent under conditions of isoluminance or when intervening light masks were presented. This suggests that iconic memory is driven by prolonged retinal activation beyond stimulus duration. Fragile VSTM representations were not affected by light masks, but were completely overwritten by irrelevant pattern masks that spatially overlapped the memory array.Conclusions/SignificanceWe find that immediately after a stimulus has disappeared from view, subjects can still access information from iconic memory because they can see an after-image of the display. After that period, human observers can still access a substantial, but somewhat more limited amount of information from a high-capacity, but fragile VSTM that is overwritten when new items are presented to the eyes. What is left after that is the traditional VSTM store, with a limit of about four objects. We conclude that human observers store more sustained representations than is evident from standard change detection tasks and that these representations can be accessed at will.
Highlights
Humans are constantly interacting with a complex and everchanging environment
Does this suggest that we build up a limited internal picture of the world? Or can it be that visual scenes are more fully represented on a neural level, but not completely transferred to a reportable stage [23,24,25]? To answer this question, we used a change detection task in which attention-directing cues are incorporated
We found that human observers can represent and access more objects than they can keep in traditional visual short-term memory (VSTM) up to four seconds after disappearance of the visual scene
Summary
Humans are constantly interacting with a complex and everchanging environment. Selectively orienting our attention to specific parts of the external world seems to be essential to efficiently process all available information. When human observers are cued to shift attention to one item in VSTM during retention, it seems as if there is a much larger representation, which keeps additional items in a more fragile VSTM store. Far, it is not clear whether the capacity of this fragile VSTM store exceeds the traditional capacity limits of VSTM.
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