Figure 2.

Concurrent object discrimination task. Subjects learned 11 discrimination problems, in which the number of objects was held constant, but the degree of feature ambiguity was varied systematically. Each discrimination problem consisted of four objects, presented in pairs. Two of the four objects were designated correct (targets), and two were designated incorrect (nontargets). In each pair, only one object was a target (shown here on the left). The pairs of objects were presented continuously in a pseudorandom order until the subject selected the target objects for eight consecutive trials. Stimuli, Each object consisted of the conjunction of the following two stimulus features: barcode components (a; individual features shown as letters for illustrative purposes) and bug parts (b; body and legs). Shape and fill were the manipulated features for the blobs stimulus set; body plan and coat pattern were used for the beast stimulus set. For examples of these stimuli, see Figure 3. Ambiguity, There were three feature ambiguity conditions: minimum ambiguity, in which no features were explicitly ambiguous (i.e., each feature was consistently either part of a target or a nontarget); intermediate ambiguity, in which one feature in each object (e.g., legs) was ambiguous; and maximum ambiguity, in which all features were ambiguous (i.e., each feature was simultaneously present in a target and nontarget object).