Schedule of Upcoming Defenses
Plan to attend the next defense!
Thesis & Dissertation Defenses for March and April
EFFECTS OF VARIABILITY IN DURATION AND DELAY OF REINFORCEMENT ON FOOD-RESPONDING IN RATSMonday, March 3
2:30 to 4:00
Understanding the variables that maintain reinforcer effectiveness has important implications for basic research to inform applied behavior analysis.The goal of the current study was to investigate the combined effects of rate of reinforcement and variability in duration and delay on within-session changes in operant responding in two experiments.In each experiment, Wistar rats lever pressed for liquid sucrose on three fixed interval (FI) schedules of reinforcement.In Experiment 1, subjects lever pressed for liquid sucrose by either a constant or varied duration of access to reinforcement. In Experiment 2, subjects were exposed to a constant or variable delay of reinforcement.Each experiment was a 2 (Condition: constant or variable reinforcement) X 3 (Rate of reinforcement: FI 8-s, 16-s, and 32-s) X 12 (2.5 min interval) counterbalanced design.Results showed two fundamental properties of behavior undergoing habituation: stimulus rate and variety effects.First, within-session decreases in responding were steeper (greater habituation) at higher rates (e.g., FI 8-s) than lower rates (e.g., FI 32-s).Second, within-session rates of responding declined slower (slower habituation) when access to the reinforcer was presented in a variable versus constant manner.Because habituation occurs for both ingestive and noningestive stimuli, the present study adds to the body of literature on habituation and can inform clinical practice on the variables that sustain operant behavior through varying the delivery of the reinforcer.
Friday, March 7, 2014
Title: Seasonal accumulation of anthocyanin pigmentation is too late to protect photosynthesis or nitrogen resorption in three widespread high-latitude plants
Abstract: While red-senescing plants are common in arctic, boreal and alpine regions, the adaptive advantage of the anthocyanin pigments in senescing leaves is disputed. I investigated three competing hypotheses: foliar anthocyanin protects photosynthesis by screening excess light; it aids nitrogen resorption, as an antioxidant for toxic photosynthetic catabolites; or it is repressed by nitrogen, accumulating and persisting after nitrogen resorption is complete but serving no protective role. I studied Arctous alpina, Chamerion angustifolium and Cornus canadensis. Anthocyanin concentrations were estimated by color indices using spectral reflectance data and pixel data from digital scanner images of leaves. To examine protection of photosynthesis, I measured carbon dioxide assimilation in senescing leaves. Photosynthetic rates declined steeply as anthocyanin concentrations increased. To see if anthocyanin aided resorption, I compared the timing of nitrogen loss and anthocyanin accumulation through a season. Anthocyanin concentrations peaked and persisted after nitrogen resorption. Nitrogen and anthocyanin concentrations were negatively correlated (-0.831), providing no support to the resorption protection hypothesis. The adaptive advantage offered by anthocyanin in senescing leaves in these species, if any, does not appear to be protection of photosynthesis or nitrogen resorption, because it occurs after the two processes are essentially complete, as the nitrogen repression hypothesis predicts. Because of this timing, red leaf pigmentation in senescence is a useful phenological marker for the end of nitrogen remobilization.