Effects of inflation on the subjective value of delayed and probabilistic rewards

In the years prior to 1994, there were very high rates of inflation in Poland, and the zloty depreciated relative to the U.S. dollar. However, the new zloty, introduced in 1995, was associated with greatly decreased rates of inflation and provided a more stable currency. We report a series of three experiments that take advantage of these changes to examine the effects of inflation on the subjective value of delayed and probabilistic rewards. Subjects were Polish citizens familiar with both zlotys and dollars. The first two experiments, conducted in 1994, used dollars and old zlotys, and the third experiment, conducted in 1996, used dollars and new zlotys. In all three experiments, the dollar and zloty rewards were of equivalent worth, according to the then current exchange rates. In Experiment 1, subjects chose between immediate and delayed rewards and, in Experiment 2, chose between certain and probabilistic rewards. The subjective value of a delayed reward was greater when its amount was specified in dollars than when it was specified in old zlotys. In contrast, the currency in which a reward was specified had no effect on the subjective value of probabilistic rewards. The results of these two experiments suggest a selective effect of inflation on decisions involving delayed rewards. This was verified in the third experiment, in which, using new zlotys, no differences in discounting were observed between the two currencies with either probabilistic or delayed rewards. Importantly, in all three experiments, the discounting of both delayed and probabilistic rewards was well described by the same simple mathematical model, suggesting that similar decision-making processes underlie both phenomena. However, the present results argue against a single-process theory in which the discounting of probabilistic rewards is derived from the discounting of delayed rewards.     

The information-loss model: a mathematical theory of age-related cognitive slowing

A model of cognitive slowing is proposed with the following assumption: Information is lost during processing, processing occurs in discrete steps with step duration inversely related to the amount of information currently available, and the effect of aging is to increase the proportion of information lost per step. This model correctly predicts a positively accelerated relation between latencies of older and younger adults and provides a unified account of the effects of task complexity, practice, speed-accuracy tradeoffs, and fluctuations in individual performance. Strong support for the thesis that cognitive slowing is global, and not localized in specific age-sensitive components, is provided by the fact that the model accurately predicts the latencies of older adults on the basis of those of younger adults, without regard to the nature of the task, across a latency range of nearly 2 orders of magnitude.     

Short-term and long-term effects of reinforcers on choice

The relation between molar and molecular aspects of time allocation was studied in pigeons on concurrent variable-time variable-time schedules of reinforcement. Fifteen-minute reinforcer-free periods were inserted in the middle of every third session. Generalized molar matching of time ratios to reinforcer ratios was observed during concurrent reinforcement. Contrary to melioration theory, preference was unchanged during the reinforcer-free periods as well as in extinction. In addition to this long-term effect of reinforcement, short-term effects were observed: Reinforcers increased the duration of the stays during which they were delivered but had little consistent effect either on the immediately following stay in the same schedule or on the immediately following stay in the alternative schedule. Thus, an orderly effect of reinforcer delivery on molecular aspects of time allocation was observed, but because of its short-term nature, this effect cannot account for the matching observed at the molar level.     

Differential Decline of Verbal and Visuospatial Processing Speed Across the Adult Life Span

The present study compared the age-related decline in verbal and visuospatial processing speed in 131 participants aged 18 to 90 years. Participants performed four verbal and four visuospatial tasks. Age differences in processing speed were compared at the group and individual levels. For the group-level analyses, participants were divided into a young adult group and six older groups subdivided by decade. The mean verbal and visuospatial response times (RTs) for each group were regressed on the corresponding RTs of the young adult group. The slope of the visuospatial regression was greater than that for the verbal regression at all ages, and the difference between the visuospatial and verbal slopes increased with each decade. For the individual-level analyses, a verbal and a visuospatial processing-time coefficient (Hale & Jansen, 1994) was obtained for each individual, and these values were then regressed on age. Verbal processing time increased linearly by approximately 50% while visuospatial processing time increased exponentially by approximately 500% from 18 to 90 years. Taken together, the results at both the group and the individual level demonstrate that aging affects visuospatial processing to a much greater extent than verbal processing.     

Pigeons' discounting of probabilistic and delayed reinforcers

Pigeons' discounting of probabilistic and delayed food reinforcers was studied using adjusting-amount procedures. In the probability discounting conditions, pigeons chose between an adjusting number of food pellets contingent on a single key peck and a larger, fixed number of pellets contingent on completion of a variable-ratio schedule. In the delay discounting conditions, pigeons chose between an adjusting number of pellets delivered immediately and a larger, fixed number of pellets delivered after a delay. Probability discounting (i.e., subjective value as a function of the odds against reinforcement) was as well described by a hyperboloid function as delay discounting was (i.e., subjective value as a function of the time until reinforcement). As in humans, the exponents of the hyperboloid function when it was fitted to the probability discounting data were lower than the exponents of the hyperboloid function when it was fitted to the delay discounting data. The subjective values of probabilistic reinforcers were strongly correlated with predictions based on simply substituting the average delay to their receipt in each probabilistic reinforcement condition into the hyperboloid discounting function. However, the subjective values were systematically underestimated using this approach. Using the discounting function proposed by Mazur (1989), which takes into account the variability in the delay to the probabilistic reinforcers, the accuracy with which their subjective values could be predicted was increased. Taken together, the present findings are consistent with Rachlin's (Rachlin, 1990; Rachlin, Logue, Gibbon, & Frankel, 1986) hypothesis that choice involving repeated gambles may be interpreted in terms of the delays to the probabilistic reinforcers.     

Delay discounting of qualitatively different reinforcers in rats

Humans discount larger delayed rewards less steeply than smaller rewards, whereas no such magnitude effect has been observed in rats (and pigeons). It remains possible that rats' discounting is sensitive to differences in the quality of the delayed reinforcer even though it is not sensitive to amount. To evaluate this possibility, Experiment 1 examined discounting of qualitatively different food reinforcers: highly preferred versus nonpreferred food pellets. Similarly, Experiment 2 examined discounting of highly preferred versus nonpreferred liquid reinforcers. In both experiments, an adjusting-amount procedure was used to determine the amount of immediate reinforcer that was judged to be of equal subjective value to the delayed reinforcer. The amount and quality of the delayed reinforcer were varied across conditions. Discounting was well described by a hyperbolic function, but no systematic effects of the quantity or the quality of the delayed reinforcer were observed.     

Individual differences, intelligence, and behavior analysis

Despite its avowed goal of understanding individual behavior, the field of behavior analysis has largely ignored the determinants of consistent differences in level of performance among individuals. The present article discusses major findings in the study of individual differences in intelligence from the conceptual framework of a functional analysis of behavior. In addition to general intelligence, we discuss three other major aspects of behavior in which individuals differ: speed of processing, working memory, and the learning of three-term contingencies. Despite recent progress in our understanding of the relations among these aspects of behavior, numerous issues remain unresolved. Researchers need to determine which learning tasks predict individual differences in intelligence and which do not, and then identify the specific characteristics of these tasks that make such prediction possible.     

Age differences in proactive interference, working memory, and abstract reasoning

It has been hypothesized that older adults are especially susceptible to proactive interference (PI) and that this may contribute to age differences in working memory performance. In young adults, individual differences in PI affect both working memory and reasoning ability, but the relations between PI, working memory, and reasoning in older adults have not been examined. In the current study, young, old, and very old adults performed a modified operation span task that induced several cycles of PI buildup and release as well as two tests of abstract reasoning ability. Age differences in working memory scores increased as PI built up, consistent with the hypothesis that older adults are more susceptible to PI, but both young and older adults showed complete release from PI. Young adults' reasoning ability was best predicted by working memory performance under high PI conditions, replicating M. Bunting (2006). In contrast, older adults' reasoning ability was best predicted by their working memory performance under low PI conditions, thereby raising questions regarding the general role of susceptibility to PI in differences in higher cognitive function among older adults. (PsycINFO Database Record (c) 2008 APA, all rights reserved).     

Assessing executive abilities following acute stroke with the trail making test and digit span

The Trail Making Test and Digit Span are neuropsychological tests widely used to assess executive abilities following stroke. The Trails B and Digits Backward conditions of these tests are thought to be more sensitive to executive impairment related to frontal lobe dysfunction than the Trails A and Digits Forward conditions. Trails B and Digits Backward are also thought to be more sensitive to brain damage in general. Data from the Stroke and Lesion Registry maintained by the Washington University Cognitive Rehabilitation Research Group were analyzed to compare the effects of frontal versus nonfrontal strokes and to assess the effects of stroke severity. Results showed that the performance of patients with frontal and nonfrontal strokes was comparable in each condition of both the Trail Making Test and Digit Span, providing no support for the widely held belief that Trails B and Digits Backward are more sensitive to frontal lobe damage. Further, Trails A was as strongly correlated with stroke severity as Trails B, whereas Digits Backward was more strongly correlated with stroke severity than Digits Forward. Overall, the Trail Making Test and Digit Span are sensitive to brain damage but do not differentiate between patients with frontal versus nonfrontal stroke.