Abstract: PURPOSE: The dual-task paradigm is widely used in studying the interaction between visual search and working memory. Lots of studies showed that holding items in working memory delays the overall response time (RT) in visual search, but not affect the search efficiency. Some researchers proposed that the difference in overall RT may be caused by factors that only affect the response selection process, while others argued it may reflect the involvement of working memory in visual search. The present study examined the two competing hypotheses by measuring the threshold stimulus exposure duration (TSED) for successfully fulfilling a search task. Since the TSED is not affected by the factors of response selection stage, it provides an excellent way to examine whether the delayed response selection account or the working memory account is correct.
METHODS: In the TSED paradigm, the exposure duration of the search array was adjusted according to a 1-up 3-down staircase procedure. Experiment 1 replicated the procedure of Woodman et al. (2001, Exp. 2) with both the RT and TSED paradigm. Participants were instructed to performing a visual search task while holding items in visual working memory. The procedure of Experiment 2 was similar to that of Experiment 1, except for a mixed-design of visual working memory load and a longer consolidation time for memory items.
RESULTS: Experiment 1 replicated the substantial overall RT difference with the RT method but only found a small (though reliable) overall TSED difference with the TSED method. Experiment 2, with better controls, found no TSED difference when manipulating visual working memory load.
CONCLUSIONS: Virtually no overall TSED difference was observed when manipulating visual working memory load, which suggested the difference of overall RT in visual search was due to factors only affecting response selection but not the involvement of visual working memory.

Key words: Visual search, Visual working memory, Response time, Threshold stimulus exposure duration