LOCAL AND GLOBAL FACTORS OF SIMILARITY IN VISUAL-SEARCH

Effects of the similarity between target and distractors in a visual s earch task were investigated in several experiments. Both familiar (nu merals and letters) and unfamiliar (connected figures in a 5 x 5 matri x) stimuli were used. The observer had to report on the presence or ab sence of a target among a variable number of homogeneous distractors a s fast and as accurately as possible. It was found that physical diffe rence had the same clear effect on processing time for familiar and fo r unfamiliar stimuli: processing time decreased monotonically with inc reasing physical difference. Distractors unrelated to the target and t hose related to the target by a simple transformation (180-degrees rot ation, horizontal or vertical reflection) were also compared, while th e physical difference was kept constant. For familiar stimuli, transfo rmational relatedness increased processing time in comparison with tha t for unrelated stimulus pairs. It was further shown in a scaling expe riment that this effect could be accounted for by the amount of percei ved similarity of the target-distractor pairs. For unfamiliar stimuli, transformational relatedness did have a smaller and less pronounced e ffect. Various comparable unrelated distractors resulted in a full ran ge of processing times. Results from a similarity scaling experiment c orrelated well with the outcome of the experiments with unfamiliar sti muli. These results are interpreted in terms of an underlying continuu m of perceived similarity as the basis of the speed of visual search, rather than a dichotomy of parallel versus serial processing.