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Subjects performed a 90-min. watchkeeping task requiring the detection of a plus sign which occasionally appeared in a matrix of solid circles. Independent variables were: (1) number of stimulus elements in the display; (2) location of those elements relative to the strong contours produced by the physical boundary of the viewingfie1d;and (3) distance of the observer from the display. Average detection time for successive 100-trial blocks revealed the commonly observed vigilance decrement. Detection times were consistently shorter when a wider expanse of surround separated the stimulus array and the physical boundary of the display. They were not influenced by either stimulus density or viewing distance. ,Accuracy of response was found to deteriorate with an increase in stimulus density but was unaffected by the proximity of the display contours to the stimulus array or, again, by viewing distance.

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