Visual-memory search: an integrative perspective

A large, single-frame, visual-memory search experiment is reported in which memory and display loads of 1, 2, and 4 alphanumeric characters were factorially combined. In addition to the usual Consistent Mapping and Varied Mapping conditions, the experiment also involved a Categorical Varied Mapping condition in which different sets of stimuli switched roles as targets and distractors over trials. The stimuli used in these various mapping conditions were either digits, letters, or digits and letters. Analyses of the response time means obtained early and late in training suggest that the presence of categorical distinctions among the stimuli is the most important determinant of search efficiency. Comparison of the load effects on the response time means and on their standard deviations revealed a fairly constant ratio throughout the experimental conditions, which suggests that similar search processes may have been involved. A feature-based comparison model is indeed shown to account for the response time means obtained after extensive training under just about all training conditions, as well as for the ratios of load effects on means and standard deviations. According to the model, improvement in search efficiency results from a reduction in the number of features considered. The model's performance questions the necessity to postulate qualitative differences between controlled and automatic processing, while the experiment forces a reassessment of the importance of the consistent mapping that underlies dual-process theories.