Throughout ascending and descending fixed-ratio (FR) sequences, rats were allowed to terminate the FR stimulus control by pressing a time-out (TO) lever. To minimize chance or accidental responses on this second lever, three presses were required to produce the 30-sec S(Delta) period. As FR performance became more "strained," there was an increased predisposition to escape from the time-in stimulus complex. The generality of this finding was extended by obtaining recoverability (independent of the direction of stimulus change) of the FR-TO function in the descending series. Typically, escapes were produced only during the post-reinforcement pause; however, under a mixed FR FR schedule, their occurrence shifted to a point within the inter-reinforcement interval corresponding to the unreinforced completion of the lower ratio component. It appears that the point where the rat can discriminate the size of the ratio requirement will be the place where TOs are imposed. This inference was supported by a substantial increase in TO frequency accompanying a shift from CRF to extinction on the FR lever. Finally, the escape lever was placed on a progressively increasing FR schedule and later extinguished to demonstrate that the TO condition was in fact reinforcing.

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This study provided a systematic replication of the tlFR-TO effect": the finding that escapes from afixedratio SD increase as the work requirement is made more demanding. It made little difference whether the session was based on a constant number of reinforcements or on a constant period of time. ProblemUnder certain conditions, an organism will acquire and maintain a response which removes a stimulus associated with po sit i v e reinforcement. Specifically it has been demonstrated that rats and pigeons, working on fixed-ratio (FR) schedules, will occaSionally respond on a second manipulandum to produce a "time-out" (TO) condition, during which extinction and its correlated stimulus (S lI ) are in effect (Azrin, 1961;Appel, 1963;Thompson, 1964). It has also been found that frequency (Thompson, 1964) and duration (Azrin, 1961) of selfimposed TO are increasing monotonic functions of the FR requirement. This finding (tlFR-TO effect"), taken together with the observation that TOs are typically restricted to the post-reinforcement pause, suggests that the TO response can be viewed as an escape from some aversive aspect of the FR schedule; for example, effort and/or delay of reinforcement.The main purpose of the present investigation was to provide a systematic replication (Sidman, 1960) of the FR-TO effect. A procedure (fixed-duration TO) previously used with rats (Thompson, 1964) was employed with pigeons in order to establish interspecies generality. Also of interest was the comparisonoftwo different bases for terminating the session: fixed time vs. fixed number of reinforcements . Zimmerman & Ferster (1964) recently suggested that this variable may be critical in obtaining the FR-TO effect. SubjectsTwo adult, male, White Carneaux pigeons (P-1 and P-2) served as subjects. Both had previously been used in an experiment involving different colored key-lights and variable-interval schedules of reinforcement. The birds were maintained at 80% of their free-feeding weights, and were run on alternate days. Water was available at all times. ApparatusA pigeon box of the type described in detail elsewhere (Ferster & Skinner, 1957) was modified to contain two response keys: the FR key (blue), located above the food hopper on the front wall, and the TO key (white), at the same height situated on the side wall opposite the door. A white houselight was mounted on the back wall Psychon. Sci., 1%5, VoL 2.

Pigeons obtained food by making four responses on three keys in a specified sequence, e.g., left, right, center, right. Under the "tandem-learning" condition, all three keys were the same color throughout the response sequence, and the sequence was changed from session to session. After total errors per session (overall accuracy) and within-session error reduction (learning) had stabilized, the effects of varying doses phenobarbital and chlordiazepoxide were assessed. For comparison, the drug tests were also conducted under a "tandem-performance" condition, in which the response sequence was the same from session to session, and under corresponding "chain-learning" and "chain-performance" conditions, where different colored keylights were associated with the response sequence. Under all four baseline conditions, the largest dose of each drug impaired overall accuracy. Under the two learning conditions, the error rate decreased across trials within each session, but the degree of negative acceleration was less in the drug sessions than in the control sessions. In contrast, under the two performance conditions, the error rate was relatively constant across trials, but was higher in the drug sessions than in the control sessions. Of the four baselines, the chain-learning condition was the most sensitive to the drug effects.

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