Abstract
Previous work suggests that masks fail to produce impairment when observers attend the location of the target 200 ms or more in advance of the mask. We present here a new form of masking which persists beyond this limit.
Observers were asked to detect a change in alternating displays containing an array of six line segments, one of which changed orientation by 45° on half the trials. The duration of each display was 60 ms and the inter-stimulus interval (ISI) was 420 ms. During the ISI, a square mask appeared at the location of each item for 100 ms, at a fixed stimulus onset asynchrony (SOA). Twelve observers were tested in each condition, with response times compared with those for the no-mask equivalent.
A set of SOAs was tested, ranging from 60 to 320 ms. In all cases, the mask increased response times by over 1000 ms, with no significant effect on accuracy. This effect was greatest for an SOA of 220-260 ms; it remained strong even when the display contained only two elements.
A variety of masks was then examined. Little difference was found between the effect of solid squares, four-dot patterns, two-dot patterns, a single dot, and a single dot that was blurred. However, masking decreased greatly for long lines extending across the display, and stimuli without local boundaries, such as a large-scale random field. Evidently, the effect required the mask to be a localized item, but was otherwise independent of its visual characteristics.
The all-or-nothing character of this effect and its sensitivity to mask structure suggests that it may be due to "knock-out" – the displacement of an item from visual short-term memory (vSTM) by the localized mask. If so, knock-out may be an interesting new way to investigate the contents of vSTM.
Meeting abstract presented at VSS 2012