Time-dependent contrast effects in a multiple schedule of food reinforcement

Four rats were rewarded for running in a wheel under two alternating conditions of food reinforcement. These periods of frequent and infrequent reinforcement, each accompanied by a particular stimulus, were presented a number of times in each daily session. Following shifts from high to low frequency of reinforcement, responding decreased suddenly and markedly, and then recovered within the next few minutes. The magnitude of this temporary depression was an increasing function of the duration of the immediately preceding component of high-frequency reinforcement. A transient elevation in performance, which did not vary with the duration of the prior component, was noted in two subjects following shifts from low to high frequency of reinforcement. The elevation and depression effects did not appear simultaneously during the 48 experimental sessions. A possible relation between the difficulty of the discrimination and the extent of contrast effects is discussed.     

Molar versus local reinforcement probability as determinants of stimulus value.

During one component of a multiple schedule, pigeons were trained on a discrete-trial concurrent variable-interval variable-interval schedule in which one alternative had a high scheduled rate of reinforcement and the other a low scheduled rate of reinforcement. When the choice proportion between the alternatives matched their respective relative reinforcement frequencies, the obtained probabilities of reinforcement (reinforcer per peck) were approximately equal. In alternate components of the multiple schedule, a single response alternative was presented with an intermediate scheduled rate of reinforcement. During probe trials, each alternative of the concurrent schedule was paired with the constant alternative. The stimulus correlated with the high reinforcement rate was preferred over that with the intermediate rate, whereas the stimulus correlated with the intermediate rate of reinforcement was preferred over that correlated with the low rate of reinforcement. Preference on probe tests was thus determined by the scheduled rate of reinforcement. Other subjects were presented all three alternatives individually, but with a distribution of trial frequency and reinforcement probability similar to that produced by the choice patterns of the original subjects. Here, preferences on probe tests were determined by the obtained probabilities of reinforcement. Comparison of the two sets of results indicates that the availability of a choice alternative, even when not responded to, affects the preference for that alternative. The results imply that models of choice that invoke only obtained probability of reinforcement as the controlling variable (e.g., melioration) are inadequate.     

Body weight and response acquisition with delayed reinforcement.

The relation between body weight and responding established with unsignaled delayed reinforcement was investigated. In three experiments, naive rats were deprived to either 70%, 80%, or 90% of ad libitum weight and were then exposed to tandem variable-interval 15-s differential-reinforcement-of-other-behavior 30-s schedules. The tandem schedule defined a resetting unsignaled delay-of-reinforcement procedure. In the first experiment, speed of magazine training, acquisition of lever pressing, and final rate of lever pressing were related to body weight. In the next experiment, lever pressing was established and maintained in rats that were magazine trained at 70% of ad libitum weight but that were then exposed to the delay procedure at 90% of ad libitum weight. Responding did not change consistently either across or within subjects in subsequent conditions in which body weight was manipulated. In the final experiment, lever pressing was established and maintained with delayed reinforcement in the absence of magazine training for each of 2 rats at 70% and for 1 of 2 rats at 90% of ad libitum weight. The results further illuminate the conditions under which responding can be established in the absence of training and when such responses are reinforced only following an unsignaled delay period.     

Visual perception and gaze control in judging versus producing phase relations

We studied visual perception and gaze control in nine participants while they judged the relative phase between two oscillating stimuli (Experiment 1), and while they moved their hand--and therewith a concurrent feedback signal--in-phase or in antiphase with an oscillating stimulus (Experiment 2). As in previous studies, the mean relative phase judgements in Experiment 1 corresponded to the presented phase relations (0 degree, 45 degrees, 90 degrees, 135 degrees, and 180 degrees), whereas their standard deviations followed an inverted U-function of relative phase. The relative phase judgements were hardly affected by the degree of visibility (fully visible, inner parts occluded, outer parts occluded) and the amplitude (5 degrees, 10 degrees, and 20 degrees) of the stimuli. Stimulus-gaze coupling decreased as relative phase increased, and its variability correlated with that of the relative phase judgements. Taken together, task performance and gaze behaviour suggested that the judgement of relative phase might be flexibly based on different variables, rather than a single variable like relative direction of motion. In Experiment 2, the production of the antiphase relation was less stable than that of the in-phase relation. Performance deteriorated when the outer parts of the signals were occluded and when their amplitudes were reduced. Stimulus-gaze coupling was stronger during in-phase than during antiphase tracking and weaker when the signals were partially occluded and when their amplitudes were reduced. Stimulus-gaze coupling at 0 degrees and 180 degrees was stronger in Experiment 2 than in Experiment 1, suggesting that the visual perception of relative phase may benefit from its active production. Overall, the results clearly indicated that visual perception of relative phase and the corresponding gaze control are strongly task-dependent.     

Signal duration and suppression of operant responding by free reinforcement

Pigeons were trained on a VI (variable interval) schedule of food presentation with a superimposed schedule of response-independent food. Substantial suppression of the operant response rate occurred when the free food was presented without a signal. When the free food was preceded by a short (4 sec) signal, the degree of suppression was similar to that with unsignaled free food. But when the signal was lengthened to 12 sec, the degree of suppression was substantially reduced. Experiment 3 assessed the effect of signal duration using a baseline schedule of delayed reinforcement, in which contingent reinforcers were themselves preceded by a signal. The signal preceding the free reinforcers was then either the same as or different from this contingent signal. Signal duration effects occurred only when the two types of signals were different. These differences as a function of signal duration have implications for both “context-blocking” and “comparator” interpretations of the effects of noncontingent reinforcement in both Pavlovian and operant procedures.     

Response-dependent prechoice effects on foraging-related choice

Three experiments explored the influence of prechoice events on pigeons' preference. In two of three studies, a fixed-interval 200-second prechoice period preceded the initial links of a concurrent chain in which outcomes differed either (a) in terms of the delay to food or (b) in terms of amount of food and delay to food. In Experiment 3, the prechoice period preceded the initial links that provided a choice between a small single food presentation and two identical, more delayed food presentations. In all three cases, obtained choice proportions did not vary as a function of prechoice duration. These results suggest that a local-contextual view adequately describes the foraging context; they also have implications for the appropriate formulation of the delay-reduction theory of conditioned reinforcement and rate-maximizing views of optimal foraging theory.     

Confusion in philosophy: A comment on Williams (1992)

Patricia Williams made a number of claims concerning the methods and practise of cladistic analysis and classification. Her argument rests upon the distinction of two kinds of hierarchy: a divisional hierarchy depicting evolutionary descent and the Linnean hierarchy describing taxonomic groups in a classification. Williams goes on to outline five problems with cladistics that lead her to the conclusion that systematists should eliminate cladism as a school of biological taxonomy and to replace it either with something that is philosophically coherent or to replace it with pure methodology, untainted by theory (Williams 1992, 151). Williams makes a number of points which she feels collectively add up to insurmountable problems for cladistics. We examine Williams' views concerning the two hierarchies and consider what cladists currently understand about the status of ancestors. We will demonstrate that Williams has seriously misunderstood many modern commentators on this subject and all of her five persistent problems are derivable from this misunderstanding.

Nelson (1989, 67).