The dependent variable is the average time it takes to scan one line. This is calculated by dividing reaction time by the line number at which the target appears. Predictions Hierarchical Levels For a task in which subjects search for a single letter, Neisser (1964) suggests a Pandemonium something like that shown in Figure 1. Only one cognitive demon is present, the cognitive demons for all letters but the target letter being effectively turned off. This implies that the processing of a letter needs to go only as far as the feature extraction level until the target letter is found. lf, for example, the search is for a Q, processing of each letter terminates as soon as features not associated with a Q are extracted. The recognition process is completed (i.e., the Q cognitive demon and the decision demon are activated) only when features associated with a Q are extracted. When searching for the absence of a letter (e.g., not-Q), the structure of the Pandemonium is similar to that used in searching for the presence of a letter in that the cognitive demons for all letters but the critical letter (e.g., Q) are turned off. As before, this means that the recognition process is completed only for the critical letter. However, since the critical letter is present in all lines but one, the recognition process must be completed at least once in each line. Since completion of the recognition process (activation of the cognitive and decision demons) requires more time than processing to only the feature extraction level, it should take longer to search for the absence than for the presence of a letter. One prediction derived from the idea of hierarchical levels of processing, then, is that scanning time per line should be greater when the search is for not-Q than for Q, and for not-Z than for Z. Referring again to Figure 1, it is seen that there are three levels of feature extraction. Level 1 extracts the crudest features, the simple presence or absence of lines, angles and curves. At level 2, the feature extraction is more detailed, producing information regarding the presence or absence of categories of lines (vertical, horizontal and oblique), angles (right and acute), and curves (continuous and discontinuous). Level 3 produces the most detailed information, going beyond the mere presence or absence of the feature categories of level 2 to indicate the number of features in each category. lt is assumed that these three levels of feature extraction form a hierarchy, level 1 extraction occurring before level 2, and level 2 extraction occurring before level 3. lt is further assumed that the cognitive demon evaluates features in the order of their extraction, level 1 features being evaluated first, then level 2 features, and finally level 3 features. The cognitive demon evaluates the features extracted by level 2 and level 3 only if the features extracted by the preceding level, levels 1 and 2, respectively, indicate that the letter being processed could be the target letter. Since more processing time will be required when more feature extraction levels must be evaluated, and since more feature extraction levels must be evaluated with greater similarity of target-letter and context-letter features, a second prediction derived from the idea of hierarchical levels of processing is that scanning time per line should be greater for a Q-search than for a Z-search in the round context and greater for a Z-search in the angular context. Parallel Processing Searching for two target letters, e.g., searching for either a Q or a Z, requires the addition of a cognitive demon to the Pandemonium shown in Figure 1. In general, there will be one cognitive demon for each target letter. Since demons at a particular level of the Pandemonium operate in parallel, however, the addition of a cognitive demon will have no effect on processing time. Accordingly, there should be no difference in scanning time per line in searching for a Q, a Z, or either a Q or a Z. To be more specific, it should take no longer to search for a Q when a Z is also a possible target than to Search for a Q as a single target, and vice versa. USING THE PROGRAM The program has five running options: (1) to run a subject and the simulation or run the simulation alone; (2) a choice of four targets (Q, Z, Q or Z, or any letter of the alphabet); (3) a goal of detecting the presence of the target in a line or a goal of detecting its absence; (4) a choice of three contexts (round -C, D, G, O, R and U; angular - X, I, M, V, W and X; or any six letters of the alphabet); and (5) to have the Pandemonium search procedure printed or omitted. lt is suggested that a student should run himself as a subject in conjunction with the simulation before running his own experiments on other students and the simulation. The choice of target, goal (presence or absence of the target), and context will be determined by the experimental condition under which the program is to be run. For the first run, this should probably be determined for each student by the instructor in order to ensure an approximately equal number of subjects in each experimental condition. The printout of the Pandemonium search procedure lists the feature extraction levels used in processing each target and context letter. Examining the procedure for each of the experimental conditions should help the student to understand the structure of the Pandemonium and how the experimental effects predicted above are produced by this structure. The search procedure should be printed whenever the student runs a new experimental condition.