Pigeons were trained on a concurrent-chains procedure in which the initial link associated with the shorter terminal-link delay to food changed unpredictably across sessions. In the minimal-variation condition, delays were always 10 s and 20 s, whereas in the maximal-variation condition delays were generated pseudorandomly for each session. On some terminal links, food was withheld to obtain measures of temporal control. Measures of choice (log initial-link response ratios) and timing (start and stop times on no-food trials) showed temporal control and stabilized within the 1st half of each session. In the maximal-variation condition, choice was a nonlinear function of the log delay ratio, consistent with a categorical discrimination but contrary to models based on the matching law. Residuals from separate regressions of log response and log start and stop time ratios on log delay ratios were positively correlated. Overall, results support cognitive models that assume that initial-link choice is based on an all-or-none decision process, and that choice and timing are mediated by a common representation of delay.

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Overall characteristics of both the family and child predicted a TBI event. An increased understanding of risks associated with TBI in childhood will provide an avenue to prevent these injuries by targeting at-risk families and aiding the development of appropriate intervention strategies.

In fixed-interval (FI) and response-initiated fixed-interval (RIFI) schedules of reinforcement, a response is required after an interval has elapsed for delivery of reinforcement. In RIFI schedules, a response is required to initiate each interval as well. The objective of this experiment was a systematic comparison of performance in the two schedule types over a range of interval durations. Four pigeons were exposed to FI and RIFI schedules of 15, 30, 60, 120 and 240 s. Interfood intervals were longer and more variable in RIFI than corresponding FI schedules. In addition, response rates early in the RIFI schedules were higher than in corresponding FI schedules. However, the distribution of first-response latencies, mean breakpoints, and normalized response gradients suggest that temporal discrimination was equivalent in the two schedules.