—_ experiment 1, serum A contains no neutralizing poliovirus antibody. When placed on filter paper, it did not prevent the rise of virus; all of the ten spaces which became wet were shown to contain virus when placed in tissue cul- ture. In contrast, no virus could be detected above space 4 on the paper that had been treated with serum B, which schedule of reinforcement on the right key was designed to generate a high, sustained rate of pressing, whereas the schedule on the left key was designed to generate a low rate. The amountof independence between the performances on ture. For practical purposes, the placing of the two keys could be assessed because of the contrasting rates of key pressing. Any “confusion” between the two keys would result in high rates of pressing on the key normally producing low rates, and vice versa. The chimpanzee, with its semierect posture and good hand dexterity, was of special interest for this type of experiment because it could operate the two keys simultaneously. Most subprimates would have to alternate between the two virus antibody. This is true of space 7, forth between the keys would interfere with the characteristic performance under the single schedule of reinforcement. had a neutralizing antibodytiter of 1/64. Experiment 2 shows that the “blocking” action is still present when serum B is diluted. Experiment 4 shows that the “blocking” action was demonstrable with as little as 0.025 mi of serum B, a quantity readily obtainable by finger punca single paper space into a single tissueculture tube gave correct information regarding the presence or absence of poliofor example, in each of the 52 successive tests performed. When a pool of types 1, 2, and 3 polioviruses is tested against a serum, only that type against which there is no specific antibody in the serum can be detected high on the paper. This method requires only one tissucculture tube and a quantity of blood which is small enough to be obtained readily by finger puncture. The method may be, therefore, a valuable screening test for distinguishing immune from nonimmune persons in a poliomyelitis vac- keys. The time spent changing back and The specific experimental conditions were similar to those already described for the pigeon (2). The experiment began with only one key and a schedule in which the magazine operated after a fixed number of responses. This is called a fixed-ratio schedule: “ratio” refers to the ratio of presses to reinforcements (3). This schedule generates a high, sus- tained rate of responding except when the numberof responses required for reinforcement is large. Then, a pause de- velops following each reinforcement; but when the animal again starts pressing the key, it begins immediately at the prevailing high rate. In general, moderate rates or smooth transitions from one rate to another are absent under this schedule. If the animal operates the key at all, it tends to do so at the prevailing high rate. After a stable performance had developed on thefirst key, a second key was added 6 inches to theleft. Presses of the second key were reinforced on the basis of elapsed time rather than number of presses. The first press after a given interval operated the magazine; but the interval varied from reinforcementto reinforcement, ranging from 3 seconds to 8 minutes, with a mean value of 4 minutes. This schedule, which is called varia- ble-interval reinforcement, produces a moderate rate of responding (3). The random spacing of the reinforcements produces a constant rate of responding and prevents pauses from developing after reinforcements. Changes in rate, when they do occur, seldom are abrupt, as they are in the fixed-ratio schedule. The variable-interval performancestabilized quickly. The number of responses required for reinforcement on the right (fixed-ratio) key was then increased to 120 over 27 expcrimental sessions. The larger number of responses required for cination ._program, Horace L. Hopes, Heren D. Zepp, Wacter L. HeNiry, RutH Bercer Mount Sinai Hospital, New York, New York References and Notes 1, H. L. Hodes and H. D. Zepp, Am. J. Diseases Children 88, 787 (1954). . 2, This work supported by grant from National Foundation for Infantile Paralysis. 26 February 1957 Concurrent Schedules of Reinforcement in the Chimpanzee This report (J) describes a technique for establishing two behavioral repertoires simultaneously in a single animal subject. This was done by training chimpanzees that had been reduced to about 80 percent of their normal weight to press either or both of two keys that were 300 RESPONSES mounted 6 inches apart. The animals pressed the keys because occasional presses operated a food magazine that delivered 40-kcal. portions of food (re- inforcement). The schedule by which the key presses are reinforced determines the rate at which the animal presses the key. Different rates of pressing were estab- lished on the two keys by using two schedules of operation of the food magazine (schedule of reinforcement). The 1090 sin x. 4. VO. MINUTES ; . Fig. 1. Gumulative curves of responses on the two keys. Record A, responses on the right- hand key; reinforcement was on a fixed-ratio schedule. Record B, responses on the left- hand key; reinforcement was on a variable-interval schedule. SCIENCE, VOL 125