Journal of the experimental analysis of behavior
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A discrimination was established between two fixed-ratio schedules of reinforcement. In one, fixed ratio 25, the reinforcer was delivered on the twenty-fifth response; on the other, fixed ratio 50, the fiftieth response was reinforced. In the first component of a chain, either fixed ratio 25 or fixed ratio 50 was randomly programmed on the center key of a three-key pigeon box. ⋯ The loss of discrimination was primarily due to errors after fixed ratio 50 was completed. The timeout appears to weaken the control over the choice response by the response-produced stimuli which preceded the timeout. The results are consistent with the interpretation that the discrimination between fixed ratio 25 and fixed ratio 50 is maintained by chaining of response-produced stimuli within the ratio cycle.
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Extinction of Sidman avoidance behavior by eliminating the noxious stimulus was studied in Sprague-Dawley rats with bar-pressing as the response. Each of three subjects was trained and extinguished on each of the following schedules in a different order: nondiscriminated, response-shock interval = 20 sec, shock-shock interval = 5 sec; nondiscriminated, response-shock interval = 40 sec, shock-shock interval = 5 sec; discriminated, response-white noise interval = 15 sec, noise-shock interval = 5 sec, shock-shock interval = 5 sec. ⋯ Subjects did not, however, respond to avoid the signal. Only small differences in extinction were found after training on different schedules with no warning signal.
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The introduction of a warning signal that preceded a scheduled shock modified the temporal distribution of free-operant avoidance responses. With response-shock and shock-shock intervals held constant, response rates increased only slightly when the response-signal interval was reduced. The result is consistent with Sidman's (1955) findings under different conditions, but at variance with Ulrich, Holz, and Azrin's (1964) findings under similar conditions. ⋯ Drug treatment may have disrupted a "temporal discrimination" formed within the signal-shock interval. More simply, methylphenidate influenced response rates by increasing short response latencies after signal onset; this effect was more prominent than the drug's tendency to increase the frequency of pre-signal responses. When signal-onset preceded shock by 2 sec, individual differences in performance were marked; methylphenidate suppressed responding in one rat as a function of increasing dose levels to a greater degree than in a second animal, but both subjects received more shocks than under control conditions.
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Two temporal parameters of free operant or Sidman avoidance behavior are the interval by which responses postpone shocks (Response-Shock interval) and the interval between shocks when no responses occur (Shock-Shock interval). Avoidance behavior was examined in three white rats under conditions where the Response-Shock and Shock-Shock intervals were always equal. With intervals from 10 to 60 sec response rates and shock rates were similar, decreasing, negatively accelerated functions of increasing Response-Shock=Shock-Shock interval. ⋯ It was shown, however, that this relation cannot hold at extremely long intervals. Both the ratio of responses emitted to shocks received and the percentage of shocks possible which were avoided increased at long Response-Shock=Shock-Shock intervals. These findings may be related to the fact that long intervals provide optimal conditions for conditioning avoidance behavior in the rat.
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After exposure to an avoidance schedule which included a warning signal, a rat was placed on a multiple schedule in which the first component was the same as before, i.e., a single response reset the response-shock interval, delaying shock, and the second component differed only in that four bar-presses were required to postpone shock. A fixed ratio requirement of four responses (FR 4) generated behavior resembling a fixed ratio requirement of one response (FR 1) since responding was controlled by the warning signal but more shocks were received. ⋯ Response distributions showed that methylphenidate increased response rates in the absence of the warning signal, i.e., stimulus control of ratio-avoidance behavior was impaired although the increased response rates reduced shock frequency. One hour later responses again occurred more frequently during the signal than in its absence but shocks were less frequent than during control (non-drug) periods.