Computer-Based Experiments in Cognitive Psychology William L. Bewley Computer Technology Program Northwest Regional Educational Laboratory Portland, Oregon 97204 Author is formerly of Dept. of Psychology at Lawrence University in Appleton, Wisconsin EXPERIMENT 1: PATTERN RECOGNITION This paper describes the first of six computer-based experiments contained in Cognitive Psychology: A Computer-Oriented Laboratory Manual, a product of Dartmouth College's project COMPUTe. The six experiments were designed for use in any introductory or advanced undergraduate psychology course either partially or totally concerned with human cognition. Each experiment is run on a time-sharing system using a terminal as the input/output device. The computer programs which run the experiments require 8K storage and are written in Dartmouth BASIC. Versions which run on DEC'S RSTS system are also available, and the programs are presently being modified to run in HP BASIC. Each program performs two functions: (1) it runs the student as a subject in an experimental task, e.g., visual search, continuous memory, paired-associate learning, concept learning, a game similar to the Prisoner's Dilemna, and the Missionaries and Cannibals problem; (2) it runs a simulation of an information-processing model on the task, e.g., Pandemonium, the Atkinson and Shiffrin buffer model, Hintzman's discrimination-net model, Levine's focusing model, a modification of Messick's social motives model, and the Newell and Simon General Problem Solver. The student is asked to compare his performance on the task with that of the model and to compare what he thinks he did in the task with what the model says he did. The programs are fairly flexible in that some features of the task and model can be changed by the student so that he can run his own experiments on other students and the model. Background According to the Pandemonium model of Selfridge (1959), pattern recognition is a hierarchical process in which information from input patterns is fed through a succession of analyzers or "demons," the output of one demon serving as the input of the next. At the lowest level are image demons which simply form an image or unprocessed copy of the input stimulus. This image is then processed by a set of feature extraction demons which, as their name implies, extract from the image such features as straight lines, curved lines and angles. Cognitive demons then evaluate the extracted features to determine the likelihood that one of the several alternative patterns is the input pattern. These likelihood estimates are then sent to a decision demon which makes the recognition response, deciding that the most likely alternative is the input pattern. There are two strong implications of this model. The first is that there are hierarchical levels of processing in pattern recognition. The second is that the processing at each level is parallel, i.e., all image demons process the input pattern simultaneously, all feature extraction demons process the images simultaneously, and all cognitive demons evaluate the extracted features simultaneously. The purpose of this experiment was to test these two implications. The Experiment This experiment is a partial replication of the visual search experiment of Neisser (1963). Each student is shown 20 lists of letters, one list at a time. Each list is composed of 50 lines, 6 letters per line, arranged in 5 rows with 10 lines per row. The student's task is to search each list for a particular target letter, scanning the list from left to right within each row starting with the top row and working down (i.e., the scanning pattern used in reading). A question mark is typed immediately below each list to indicate that the student may begin his search. When he is ready to begin he presses the carriage return button again. The computer then prints the line number at which the target occurred and the student's search time in seconds. The simulation of the Pandemonium model then scans the same list for the same target, and its search time, in arbitrary units, is printed. Following this, the computer types the next list. This procedure is repeated until a search time is obtained for each of the 20 lists. There are two independent variables: the target to be found and the context in which it is embedded. The five targets are Q, Z, not-Q, not-Z, and Q or Z. If the target is Q, only one of the 50 lines in the list contains a Q. If the target is Z, only one of the 50 lines contains a Z. If the target is not-Q, only one of the 50 lines does not contain a Q. If the target is not-Z, only one of the 50 lines does not contain a Z. lf the target is Q or Z, one of the 50 lines contains either a Q or a Z. ln every case, the subject is searching for the line containing the target. The context in which the target is embedded is either angular or round. For the angular.context, the non-target letters are drawn from the set E, I, M, V, W and X. For the round context, the non-target letters are drawn from the set C, D, G, O, R and U. Since there are five targets and two contexts, there are ten experimental conditions. The 20 lists shown to each subject represent only one experimental condition. That is, the 20 lists have the same target and context. The only difference among the lists presented to each subject is the line at which the target occurs.