DEMAND EQUATIONS FOR QUALITATIVELY DIFFERENT FOODS UNDER FIXED‐RATIO SCHEDULES: A COMPARISON OF THREE DATA CONVERSIONS

Concurrent schedules were used to establish 6 hens' preferences for three foods. The resulting biases suggested wheat was preferred over honey‐puffed and puffed wheat, and puffed wheat was the least preferred food. The hens then responded under fixed‐ratio schedules for each food in 40‐min (excluding reinforcer time) sessions, with the response requirement doubling each session until no reinforcers were received. At the smaller ratios, the less preferred the food, the faster the hens' overall response rates (mainly as a result of shorter postreinforcement pauses) and the more reinforcers they received. The relations between the logarithms of the number of reinforcers obtained (consumption) and the response ratio (price) were well fitted by curvilinear demand functions. Wheat produced the smallest initial consumption (ln L), followed by honey‐puffed and puffed wheat, respectively. The response requirement at which the demand functions predicted maximal responding (P_max) were larger for wheat than for the other foods. Normalizing consumption and price, as suggested by Hursh and Winger (1995), moved the data for the three foods towards a single demand function; however, the P_max values were generally largest for puffed wheat. The results of normalization, as suggested by Hursh and Silberberg (2008), depended on the k value used. The parameter k is related to the range of the data, and the same k value needs to be used for all data sets that are compared. A k value of 8.0 gave significantly higher essential values (smaller α values) for puffed wheat as compared to honey‐puffed wheat and wheat, and the P_max values, in normalized standard price units, were largest for puffed wheat. Normalizing demand by converting the puffed and honey‐puffed wheat reinforcers to wheat equivalents (by applying the bias parameter from the concurrent‐schedules procedure) maintained separate demand functions for the foods. Those for wheat had the smallest rates of change in elasticity (a) and, in contrast to the other analyses, the largest P_max values. Normalizing demand in terms of concurrent‐schedule preference appears to have some advantages and to merit further investigation.     

Contingency tracking during unsignaled delayed reinforcement: Effects of delay duration and d‐amphetamine

In two experiments, the role of the response–reinforcer relation in maintaining low‐rate responding under unsignaled delay conditions was investigated. In both experiments pecking by pigeons on one response key, denoted the relevant key, was reinforced under an unsignaled delay‐of‐reinforcement procedure (defined as tandem variable‐interval (VI) differential‐reinforcement‐of‐other behavior [DRO] schedule). Responding on a second key, denoted the irrelevant key, had no programmed consequences. Between sessions, the location of the relevant key varied (after one, two, or three sessions) pseudorandomly. In Experiment 1, the delay (DRO) duration was manipulated parametrically. Overall, proportional relevant‐key response rates (relevant‐key response rates / [relevant‐key response rates + irrelevant key response rates]) increased across 3‐session sequences in which the relevant key remained in the same location and decreased as the DRO duration was changed systematically (2, 5, and 10 s). In Experiment 2, acute administration of d‐amphetamine increased proportional relevant‐key response rates during 1‐day sequences for only the DRO 5‐s duration, and results over 3‐day sequences, once a discrimination had already been established, were inconsistent. Results support that the response–reinforcer relation is the primary determinant of responding, and such discriminations are relatively resistant to disruption or potentiation by behaviorally active doses of d‐amphetamine.     

RESISTANCE TO CHANGE AND FREQUENCY OF RESPONSE‐DEPENDENT STIMULI UNCORRELATED WITH REINFORCEMENT

Stimuli uncorrelated with reinforcement have been shown to enhance response rates and resistance to disruption; however, the effects of different rates of stimulus presentations have not been assessed. In two experiments, we assessed the effects of adding different rates of response‐dependent brief stimuli uncorrelated with primary reinforcement on relative response rates and resistance to change. In both experiments, pigeons responded on variable‐interval 60‐s schedules of food reinforcement in two components of a multiple schedule, and brief response‐dependent keylight‐color changes were added to one or both components. Although relative response rates were not systematically affected in either experiment, relative resistance to presession feeding and extinction were. In Experiment 1, adding stimuli on a variable‐interval schedule to one component of a multiple schedule either at a low rate (1 per min) for one group or at a high rate (4 per min) for another group similarly increased resistance to disruption in the components with added stimuli. When high and low rates of stimuli were presented across components (i.e., within subjects) in Experiment 2, however, relative resistance to disruption was greater in the component presenting stimuli at a lower rate. These results suggest that stimuli uncorrelated with food reinforcement do not strengthen responding in the same way as primary reinforcers.     

RISKY CHOICE IN PIGEONS AND HUMANS: A CROSS‐SPECIES COMPARISON

Pigeon and human subjects were given repeated choices between variable and adjusting delays to token reinforcement that titrated in relation to a subject's recent choice patterns. Indifference curves were generated under two different procedures: immediate exchange, in which a token earned during each trial was exchanged immediately for access to the terminal reinforcer (food for pigeons, video clips for humans), and delayed exchange, in which tokens accumulated and were exchanged after 11 trials. The former was designed as an analogue of procedures typically used with nonhuman subjects, the latter as an analogue to procedures typically used with human participants. Under both procedure types, different variable‐delay schedules were manipulated systematically across conditions in ways that altered the reinforcer immediacy of the risky option. Under immediate‐exchange conditions, both humans and pigeons consistently preferred the variable delay, and indifference points were generally ordered in relation to relative reinforcer immediacies. Such risk sensitivity was greatly reduced under delayed‐exchange conditions. Choice and trial‐initiation response latencies varied directly with indifference points, suggesting that local analyses may provide useful ancillary measures of reinforcer value. On the whole, the results indicate that modifying procedural features brings choices of pigeons and humans into better accord, and that human—nonhuman differences on risky choice procedures reported in the literature may be at least partly a product of procedural differences.