In a series of elegant experiments, Saul Sternberg tested three proposals for how working memory scanning might function. Each makes different predictions for how response time should vary as a function of the size of the memory set, and whether the target was in the memory set. In parallel search, each item in memory is probed in parallel.
Response time increases with set size, and the slope is steep for both target and foil trials.
Incorrect trials were also removed. Each participant performed four trials for each combination of set size and target presence. The mean data for the group are shown in the figure below. Error bars one standard errorsof the mean. The results clearly favor the serial exhaustive search proposal.