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A. INTRODUCTIONSolution by sub-human primates of spatial delayed response problems has been frequently demonstrated. In recent years considerable interest has been shown in the utilization by monkeys and apes of non-spatial cues in bridging delay intervals. There is ample evidence that this type of task can be solved by sub-human primates (2), ( 8), ( 9), ( 10), ( 11), (12).A logical extension of this type of investigation is the determination of the proficiency of a monkey in the retention of separate aspects or attributes of a given stimulus during a delay interval. Successful results on this type of problem, without a delay, have been reported by Harlow (4) using the matchingfrom-sample technique. The tests were described as tests involving the Weigl principle.The purpose of the present study was to determine the performance of rhesus monkeys in delayed response to either the form attribute or the color attribute of a given stimulus-object. This is a non-spatial delayed response problem, with the additional complication that the animal is required to attend, retain, and respond differentially to each of two characteristics of a single stimulus. B. SUBJECTSTwo young adult female rhesus monkeys, Nos. 51 and 53, were used in this experiment. Both of these animals had been trained in the solution of the Weigl principle using the matching-from-sample method without delay interval (4), and in Weigl generalization tests (6). They had been trained earlier on a series of discrimination and discrimination reversal problems (5) and on a series of problems in which responses were made to stimuli having multiple sign values (3). These thoroughly tame and emotionally stable monkeys were adapted to the experimenter (M. M. S.) over a three week period before training on this particular problem began.Subject 53 had been run on Weigl delayed reaction tests eighteen and again eight months before the beginning of the present experiment. 2 1 This work was supported in part by grants from the Special Research Fund of the University of Wisconsin for 1942-43 and 1943-44. * In the earlier tests 4 choice-objects were used (see figure 1), one matching the sample for color, one matching for form, and the remaining two not matching the sample in either form or color. • A non-correction method was used. The delayed reactions averaged about IS seconds in length. A one way vision screen was interposed between monkey and experi-211 212 M. M. SIMPSON AND H. F. HA.RLOW C. APPARATUS

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