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.

Mice are the most widely used model species for drug discovery and scientific research. Consequently, it is important to refine laboratory procedures and practices to ensure high standards of welfare and scientific data quality. Recent studies have identified that the standard practice of handling laboratory mice by their tails increases behaviours indicative of anxiety, which can be overcome by handling mice using a tunnel. However, despite clear negative effects on mice’s behaviour, tunnel handling has yet to be widely implemented. In this study, we provide the first evidence that tail handling also reduces mice’s responses to reward. Anhedonia is a core symptom of clinical depression, and is measured in rodents by assessing how they consume a sucrose solution: depressed mice consume less sucrose and the size of their licking bouts when drinking (their ‘lick cluster sizes’) also tend to be smaller. We found that tail handled mice showed more anhedonic responses in both measures compared to tunnel handled mice, indicative of a decreased responsiveness to reward and potentially a more depressive-like state. Our findings have significant implications for the welfare of laboratory mice as well as the design and interpretation of scientific studies, particularly those investigating or involving reward.

It is debated whether subregions within the medial temporal lobe (MTL), in particular the hippocampus (HC) and perirhinal cortex (PrC), play domain-sensitive roles in learning. In the present study, two patients with differing degrees of MTL damage were first exposed to pairs of highly similar scenes, faces, and dot patterns and then asked to make repeated same/different decisions to preexposed and nonexposed (novel) pairs from the three categories (Experiment 1). We measured whether patients would show a benefit of prior exposure (preexposed > nonexposed) and whether repetition of nonexposed (and preexposed) pairs at test would benefit discrimination accuracy. Although selective HC damage impaired learning of scenes, but not faces and dot patterns, broader MTL damage involving the HC and PrC compromised discrimination learning of scenes and faces but left dot pattern learning unaffected. In Experiment 2, a similar task was run in healthy young participants in the MRI scanner. Functional region-of-interest analyses revealed that posterior HC and posterior parahippocampal gyrus showed greater activity during scene pattern learning, but not face and dot pattern learning, whereas PrC, anterior HC, and posterior fusiform gyrus were recruited during discrimination learning for faces, but not scenes and dot pattern learning. Critically, activity in posterior HC and PrC, but not the other functional region-of-interest analyses, was modulated by accuracy (correct > incorrect within a preferred category). Therefore, both approaches revealed a key role for the HC and PrC in discrimination learning, which is consistent with representational accounts in which subregions in these MTL structures store complex spatial and object representations, respectively.

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.