Alan Silberberg

The Structure of Choice

Both Nevin (1969) and Shimp (1966) found on different choice procedures that pigeons equate (match) the proportion of their choices to the proportion of reinforcers each choice delivers. Their results differed in terms of the order of successive choices: Shimp found pigeons ordered successive choices so as to maximize the reinforcement rate, whereas Nevin found no evidence of such an ordering. Experiment 1 replicated both studies and found in both: (a) matching relations and (b) sequential dependencies of choice that corresponded with Shimps maximizing prediction. The next three experiments studied the order of choices in three other choice procedures: (a) concurrent variable-interval schedules with a changeover delay, (b) concurrent variable-interval schedules without a changeover delay, and (c) concurrent-chains schedules. In all of these procedures, control of choice at the level of the response sequence was evident. The major features of the data from all four experiments were attributed to two molecular processes: response perseveration and reinforcement maximization. This evidence for a microstructure of choice suggests that the molar matching law is not isomorphic with the molecular processes governing concurrent performances.

Does inequity aversion depend on a frustration effect? A test with capuchin monkeys (Cebus apella)

Brosnan and de Waal (Nature 425:297–299, 2003) reported that if a witness monkey saw a model monkey receive a high-value food, the witness was more inclined to reject a previously acceptable, but low-value food. Later work demonstrated that this alleged inequity aversion might be due to frustration induced by switching subjects from their role as models receiving a high-value food to the role of witnesses receiving a low-value food. In the present study, pairs of female capuchins exchanged a token for either a high- or a low-value food without switching their model–witness roles. Witnesses could exchange a token for a low-value food after an adjacent model had exchanged a token for the same food (Equity Condition) or for a high-value food (Inequity Condition). Failure- and latency-to-exchange measures showed that witnesses were unaffected by the food type offered to models (no inequity aversion). Moreover, models were unaffected by their history of food type offered (no frustration). These results join earlier work suggesting that alleged inequity effects depend on frustration-induction procedures. Furthermore, inequity effects sometimes fail to emerge because frustration induction in nonhuman primates is labile.

Olfactory learning and odor memory in the rat

Rats were trained on a series of 9 tasks, each of which required discrimination among 8 different and unique odors. Discrimination accuracy improved across successive problems and, by the end of training, most rats made few errors after their initial exposure to each new odor. Despite the number of stimuli to be discriminated, this acquisition of a learning set was not appreciably different from that demonstrated in an earlier study that used only 2 odors per task. In subsequent retention tests, most rats also showed excellent memory for odors used in prior problems.

Essential value of cocaine and food in rats: tests of the exponential model of demand

RationaleTo provide a prospective test of the predictive adequacy of the exponential model of demand (Hursh and Silberberg, Psych Rev 115(1):186–198, 2008).ObjectivesIn Experiment 1, to measure the ‘essential value’ (the propensity to defend consumption with changes in price) of cocaine and food in a demand analysis (functional relation between price and consumption) by means of the exponential model; in Experiment 2, to test whether the model’s systematic underestimation of cocaine consumption in Experiment 1 was due to weight loss; and in Experiment 3, to evaluate the effects of cocaine on the essential value of food.Materials and methodsIn Experiment 1, demand curves for food and cocaine were determined by measuring consumption of these goods in a multiple schedule over a range of fixed ratios; in Experiment 2, a demand curve for only cocaine was determined; and in Experiment 3, demand for food was determined in the absence of cocaine.ResultsIn Experiment 1, the exponential equation accommodated high portions of variance for both curves, but systematically underestimated cocaine demand; in Experiment 2, this predictive underestimation of the equation was eliminated; and in Experiment 3, the essential value of food was greater than in Experiment 1.ConclusionsThe exponential model of demand accommodated the data variance for all cocaine and food demand curves. Compared to food, cocaine is a good of lower essential value.

Odor matching and odor memory in the rat

To assess olfactory matching-to-sample learning, rats were exposed to two odors separated by a 1-s presentation of clean air. If, and only if, the odors were identical, a response produced a water reinforcer. High levels of performance were maintained over a series of 10 novel three-odor matching-to-sample problems on this conditional go/no-go discrimination procedure. In general, performance accuracy improved over problems and errorless or near errorless performance on many stimulus combinations, particularly near the end of training, indicated acquisition of a learning set. There was little decrement in performance when the interstimulus interval was increased gradually from 1 to 10 s and matching-to-sample was not disrupted when a novel odor was presented during the interstimulus interval. These results demonstrate that rats readily learn an olfactory matching-to-sample task, maintain high levels of performance even with delays of 10-s between stimuli, and can acquire a matching-to-sample learning set. The outcomes are in agreement with prior studies demonstrating exceptional learning of instrumental tasks by rats when they are provided with odor cues.

Demand for cocaine and food over time

When reinterpreted, data from Ahmed and Koob [Ahmed, S.H., Koob, G.F., Transition from moderate to excessive drug intake: Change in hedonic set point. Science 1998; 282:298-301.] show that the reinforcing strength of cocaine, an inessential good, increases with experience. However, no such effect obtains with a homeostatically regulated good such as food. The present study evaluated whether this difference could serve to distinguish abused drugs from biologically necessary goods. In Experiment 1, five rats from Christensen, Silberberg, Hursh, Huntsberry and Riley [Christensen, C.J., Silberberg, A., Hursh, S.R., Huntsberry, M.E., Riley, A.L., Essential value of cocaine and food in rats: tests of the exponential model of demand. Psychopharmacology 2008;198(2):221-229.] earned cocaine under a Fixed-Ratio 3 schedule for 7 sessions. Thereafter, in a demand procedure identical to that in Christensen et al., demand was re-assessed by measuring consumption at Fixed Ratios between 3 and 560. In Experiment 2, five different rats from Christensen et al. had their food demand curves re-determined using an identical procedure as the first. When fit with the exponential model, the second determination of cocaine demand in Experiment 1 showed greater essential value than the first, indicating that strength increased with cocaine exposure. In Experiment 2, the re-determined food demand curves showed no change from their initial determination. These results show that the strength of cocaine, but not food, increases with increased experience. Measures of time-based changes in essential value may serve as a basis for distinguishing addictive from non-addictive reinforcers.

Natural Choice in Nonhuman Primates

In 5 experiments, 4 monkeys and 1 ape chose between 2 food sources, each held in 1 of the experimenters hands while he stood in front of a cage. When choosing between 2 sources of the same food that differed in amount, preference for the larger amount decreased as the size of each good proportionately increased. A second finding was that subjects were indifferent between a 2-food mixture and a single food that was part of the mixture if the single food was the preferred food of the mixture, a result suggesting the less preferred food had no value. Experiment 6 replicated these effects in 4 additional monkeys. These effects may be incompatible with previous theorizing about animal choice and may reflect a cognitive difference between nonhuman primates and humans.

Demand for food and cocaine in Fischer and Lewis rats.

Fischer and Lewis rat strains often serve as animal vulnerability models for drug abuse and addiction. When these strains respond for drugs of abuse, several measures, including total drug intake, response rate and progressive-ratio breakpoints, have been reported to be strain-dependent, a result suggesting genetic differences in drug reactivity and vulnerability. The present study extends these strain comparisons to a previously untested measure--demand analysis. In Experiment 1, four Fischer and four Lewis rats earned their daily food ration by lever pressing under a fixed-ratio schedule, the size of which was increased every three sessions from 3 to 1,000 in logarithmic steps. Consumption was plotted as a function of ratio size, and modeled by the exponential-demand equation (Hursh & Silberberg, 2008). Experiment 2 replicated Experiment 1 except that different rats were used, and cocaine reinforced lever pressing. A between-experiment comparison showed a commodity-by-strain interaction: Fischer rats defended consumption with greater vigor when cocaine served as the reinforcer than did Lewis rats; for food, this relation was reversed. However, for both strains, defense of consumption of food exceeded that of cocaine.

ON LOSS AVERSION IN CAPUCHIN MONKEYS

Chen, Lakshminarayanan, and Santos (2006) claim to show in three choice experiments that monkeys react rationally to price and wealth shocks, but, when faced with gambles, display hallmark, human-like biases that include loss aversion. We present three experiments with monkeys and humans consistent with a reinterpretation of their data that attributes their results not to loss aversion, but to differences between choice alternatives in delay of reinforcement.

Olfaction and olfactory epithelium in mice treated with zinc gluconate

Objective: We assessed whether a nasal spray containing zinc gluconate (ZG) compromises the integrity of olfactory epithelium and olfactory function.