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Cognition presents evolutionary research with one of its greatest challenges. Cognitive evolution has been explained at the proximate level by shifts in absolute and relative brain volume and at the ultimate level by differences in social and dietary complexity. However, no study has integrated the experimental and phylogenetic approach at the scale required to rigorously test these explanations. Instead, previous research has largely relied on various measures of brain size as proxies for cognitive abilities. We experimentally evaluated these major evolutionary explanations by quantitatively comparing the cognitive performance of 567 individuals representing 36 species on two problem-solving tasks measuring self-control. Phylogenetic analysis revealed that absolute brain volume best predicted performance across species and accounted for considerably more variance than brain volume controlling for body mass. This result corroborates recent advances in evolutionary neurobiology and illustrates the cognitive consequences of cortical reorganization through increases in brain volume. Within primates, dietary breadth but not social group size was a strong predictor of species differences in self-control. Our results implicate robust evolutionary relationships between dietary breadth, absolute brain volume, and self-control. These findings provide a significant first step toward quantifying the primate cognitive phenome and explaining the process of cognitive evolution.

Cognitive mapping implies the development of an internal representation of the spatial relationships among objects in the environment. One can assess the development of a cognitive map by demonstrating that an animal can select, when appropriate, a novel path to reach a goal in the absence of landmarks and when path integration does not provide an adequate account. Rats were trained to find reinforcement in two of three goal boxes in a three-arm maze. In test, the rats were given a choice between two novel paths, one that led to the goal box that had been baited during training, the other that led to the goal box that had been unbaited during training. When they were trained in the absence of distinctive intramaze cues (Experiment 1), no preference was found; however, when distinctive intramaze alley cues were present during training but were unavailable as directional cues during testing (Experiment 2), the rats demonstrated a significant preference for the correct novel path. These results suggest that under appropriate conditions rats are able to form simple cognitive maps of their environment.

Contrast refers to a comparison between two conditions of reward such that the relation between them is magnified (relative to an appropriate control condition). It is an effect that is opposite in direction from generalization (which is a form of averaging). Three kinds of contrast have received substantial empirical attention: Incentive contrast in which a sudden change in reward (either an increase or a decrease) results in an overreaction to the change, relative to a control condition; anticipatory contrast in which an anticipated improvement in reward results in less consumption of an initial reward; and differential or behavioral contrast in which a change in reward associated with one stimulus results in a change in behavior associated with a second stimulus in the opposite direction. Here I discuss a fourth kind of contrast that I call within-trial contrast. In this form of contrast a discriminative stimulus is preferred when it follows a less appetitive event (effort, delay, or the absence of reward). A model of this kind of contrast is proposed that is based on a presumed change in the hedonic state of the organism between the end of the less appetitive event and the reward (or stimulus signaling the reward). It is distinguished from an account based on the relative reduction in delay to reinforcement. Finally, I suggest that a general form of this within-trial contrast may help to explain various complex human social phenomena including cognitive dissonance, justification of effort, the effect of extrinsic rewards on behavior that is maintained by intrinsic rewards, and learned industriousness.

Pavlovian associative processes appear to be intimately involved in the acquisition of simultaneous discriminations by pigeons. We have found evidence that in a simultaneous discrimination, value transfers from the positive stimulus (S+) to the negative stimulus (S-) and the basis of that transfer appears to be the higher-order association of the S- with the reinforcer, by way of the S+. Furthermore, the association between the S+ and the S- appears to be bidirectional, occurring in the form of a within event association. In addition, it appears that when pigeons have extended experience with the consequences of responding to the Sstimulus, contrast (the opposite of value transfer) develops between the two (e.g., increasing the value of one, decreases the value of the other). Finally, I suggest that versions of simultaneous discriminations may provide a useful model of several Pavlovian conditioning phenomena including, higher-order conditioning, withinevent conditioning, postconditioning devaluation effects, inhibitory conditioning, potentiation, and perhaps also overshadowing.

When behavior suggests that the value of a reinforcer depends inversely on the value of the events that precede or follow it, the behavior has been described as a "contrast" effect. Three major forms of contrast have been studied: "incentive contrast," in which a downward (or upward) shift in the magnitude of reinforcement produces a relatively stronger downward (or upward) shift in the vigor of a response; "anticipatory contrast," in which a forthcoming improvement in reinforcement results in a relative reduction in consummatory response; and "behavioral contrast," in which a decrease in the probability of reinforcement in one component of a multiple schedule results in an increase in responding in an unchanged component of the schedule. Here we discuss a possible fourth kind of contrast that we call "within-trial contrast" because within a discrete trial, the relative value of an event has an inverse effect on the relative value of the reinforcer that follows. We show that greater effort, longer delay to reinforcement, or the absence of food all result in an increase in the preference for positive discriminative stimuli that follow (relative to less effort, shorter delay, or the presence of food). We further distinguish this within-trial contrast effect from the effects of delay reduction. A general model of this form of contrast is proposed in which the value of a primary or conditioned reinforcer depends on the change in value from the value of the event that precedes it. (Contains 10 figures.)

Vasconcelos, Urcuioli, and Lionello-DeNolf (2007) report the results of five experiments that fail to replicate the results of our within-trial contrast study (Clement, Feltus, Kaiser, & Zentall, 2000) and suggest that our results may represent a Type I Error. We believe that this conclusion is not warranted because (a) there is considerable evidence in support of the effect and (b) the amount of training that they gave to their pigeons prior to test may not have been sufficient to observe the effect reliably. We suggest that when sufficient training is provided, reliable contrast can be found. (Contains 1 table.)

Several types of contrast effects have been identified including incentive contrast, anticipatory contrast, and behavioral contrast. Clement, Feltus, Kaiser, and Zentall (2000) proposed a type of contrast that appears to be different from these others and called it within-trial contrast. In this form of contrast the relative value of a reinforcer depends on the events that occur immediately prior to the reinforcer. Reinforcers that follow relatively aversive events are preferred over those that follow less aversive events. In many cases the delay reduction hypothesis proposed by Fantino (1969) also can account for such effects. The current experiments provide a direct test of the delay reduction and contrast hypotheses by manipulating the schedule of reinforcement while holding trial duration constant. In Experiment 1, preference for fixed-interval (FI) versus differential-reinforcement-of-other-behavior (DRO) schedules of reinforcement was assessed. Some pigeons preferred one schedule over the other while others demonstrated a position (side) preference. Thus, no systematic preference was found. In Experiment 2, a simultaneous color discrimination followed the FI or DRO schedule, and following training, preference was assessed by presenting the two positive stimuli simultaneously. Consistent with the contrast hypothesis, pigeons showed a significant preference for the positive stimulus that in training had followed their "less" preferred schedule. (Contains 1 table and 7 figures.)

Research on pigeons' suggest a relationship between response rates and length of event duration. An evaluation of Staddon and Higa's research on trace-strength theory, and issues of memory loss are presented.

Experimental studies of forget-cue trials in pigeons confirm that performance losses on forget-cue probe trials can be accounted for by reinforced responding subsequent to forget cue. In the presence of reinforcement, the response pattern during and subsequent to a forget cue contributes to directed forgetting. Decreases in accuracy on these trials are observed upon omission of reinforcement.

Categorization and concept learning encompass some of the most important aspects of behavior, but historically they are not central topics in the experimental analysis of behavior. A brief introduction to some of the problems that have concerned investigators of categorization and concept learning are presented.