Role of the preprobe delay in memory-scanning tasks.

In a variation of Sternberg's (1966, 1969) memory-scanning task not requiring an explicit negative response, Diener (1988) found that a preprobe delay was necessary to produce the usual set-size effect. In Experiment 1 of the present study, the effect of the preprobe delay was investigated in the typical two-response task. In the absence of a preprobe delay, the function relating response latency to set size was virtually flat for negative responses, but was described by a slope of about 18 msec/item for positive responses. Further research suggested that the reduced set-size effect in the absence of a preprobe delay is the result of expectancy effects usually controlled by the preprobe delay. Informing the subject of the size of the memory set before it was presented (Experiment 2) produced a set-size effect of the usual magnitude in the absence of a preprobe delay. Experiment 3 was designed to assess the effects of expectancy in the absence of a memory search. A task similar in stimulus arrangement to the memory-scanning task but requiring the subject to indicate whether the last digit in the set was odd or even produced a decrease in response latency with set size of 29 msec/item in the absence of a preprobe delay.