Human participants received unsupervised exposure to difficult-to-discriminate stimuli (e.g., A and A'), created with a morphing procedure from photographs of faces, before learning a discrimination between them. Experiments 1 and 2 demonstrated that prior exposure enhanced later discrimination and that intermixed exposure (A, A', A, A'...) resulted in better subsequent discrimination than blocked exposure (B, B, ...B', B'...). Experiments 3 and 4 showed that simultaneous exposure to 2 similar stimuli facilitated the later acquisition of both a simultaneous and a successive discrimination, and this effect was observed even though simultaneous exposure to 2 stimuli fostered the development of an excitatory association between them (Experiment 5). The findings of Experiments 1 and 2 revealed a perceptual learning effect with pictures of faces, and the findings of Experiments 3-5 are difficult to reconcile with associative analyses of perceptual learning.

Affective processes are a key determinant of behaviour: At its simplest, liked stimuli are approached while disliked stimuli are avoided. Although assessing hedonic responses in nonverbal animals can be difficult, one relatively tractable approach relies on detailed analyses of rodents' consummatory behaviour. Rodents typically produce rhythmic sets of licks that can be grouped into clusters on the basis of the intervals between licks. The mean number of licks in a cluster (cluster size) is directly related to the concentration of palatable and unpalatable solutions. These relationships suggest that lick cluster size might be a useful index of an animal's hedonic reaction to the solution being consumed. I begin by reviewing studies of conditioned flavour preference and aversion that support the idea that lick cluster size can provide useful information about rats' hedonic reactions. I then describe how this methodology has been used to address previously intractable issues in the investigation of contrast effects as well as revealing an analogue of effort justification effects that, in humans, are commonly explained in terms of cognitive dissonance reduction. Finally, I consider how lick analysis might provide information about hedonic responses in animal models of human psychiatric disorders. In all these cases, how an animal did something was particularly informative about why it was doing it.

Laboratory rats can exhibit marked, qualitative individual differences in the form of acquired behaviors. For example, when exposed to a signal-reinforcer relationship some rats show marked and consistent changes in sign-tracking (interacting with the signal; e.g., a lever) and others show marked and consistent changes in goal-tracking (interacting with the location of the predicted reinforcer; e.g., the food well). Here, stable individual differences in rats’ sign-tracking and goal-tracking emerged over the course of training, but these differences did not generalize across different signal-reinforcer relationships (Experiment 1). This selectivity suggests that individual differences in sign- and goal-tracking reflect differences in the value placed on individual reinforcers. Two findings provide direct support for this interpretation: the palatability of a reinforcer (as measured by an analysis of lick-cluster size) was positively correlated with goal-tracking (and negatively correlated with sign-tracking); and sating rats with a reinforcer affected goal-tracking but not sign-tracking (Experiment 2). These results indicate that the observed individual differences in sign- and goal-tracking behavior arise from the interaction between the palatability or value of the reinforcer and processes of association as opposed to dispositional differences (e.g., in sensory processes, “temperament,” or response repertoire).

It has recently been argued that rats engage in causal reasoning and they do so in a way that is consistent with Bayes net theories . This argument was based upon the finding that the tendency of cues to elicit approach to a food-well was reduced when their presentation was contingent on lever pressing. There is, however, an alternative interpretation of the critical experimental findings that is based on the simple principle of response competition: wherein lever pressing interferes with the tendency to approach the food well. Here the authors replicated Experiments 1 and 2a of and found reciprocal patterns of lever pressing and food well approach during the critical cues. These results lend direct support for an interpretation in terms of response competition while providing evidence contrary to Bayes net theories, and are readily interpreted within the theoretical framework provided by traditional associative learning theory. Keywords: Bayes casual nets, response competition, Lloyd MorganThe enduring debate within the field of comparative psychology concerns the degree to which the behavior of human and nonhuman animals is underpinned by the same cognitive mechanisms. Recent interest has focused on whether seemingly complex human behaviors can be understood in terms of simple (e.g., associative) processes that are ubiquitous in the animal kingdom, and whether nonhuman animals exhibit evidence of mechanisms considered to be uniquely human (e.g., causal reasoning). Recently, Blaisdell, Sawa, Leising, and Waldmann (2006a) have claimed that rats engage in causal reasoning-reasoning that is underpinned by the use of causal Bayes net models (see also Blaisdell, Sawa, Leising, & Waldmann, 2006b, 2007Leising, Wong, Waldmann, & Blaisdell, 2008). Evidence that is consistent with the suggestion that causal reasoning in humans is underpinned by causal Bayes nets comes from some simple observations that are analogous to those upon which Blaisdell et al. (2006a) made their claim.In humans who are aware that a single cause (e.g., raindrops) can have two separate effects (water droplets on window panes and dry clothes becoming damp on a washing line), the observation of one effect (wet windows) can lead to the inference that the other effect (damp clothes) has also occurred (this example is taken from Clayton & Dickinson, 2006). This inference presumably reflects the assumption that the observed effect (wet windows) must be the result of its usual cause (rain), which should also result in its second effect (damp clothes). However, this type of reasoning does not hold if there is an alternative cause (e.g., turning on a water sprinkler near to the window, but far away from the washing line) for the observed, first effect. The intervention, of turning on the sprinkler, provides an alternative cause for the first effect and the presence of the second effect is no longer inferred; and the response (e.g., removing the clothes from the line) is not required. The different patterns of inferences that obtain when an effect is simpl...

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