Emotions can bias human decisions- for example depressed or anxious people tend to make pessimistic judgements while those in positive affective states are often more optimistic. Several studies have reported that affect contingent judgement biases can also be produced in animals. The animals, however, cannot self-report; therefore, the valence of their emotions, to date, could only be assumed. Here we present the results of an experiment where the affect-contingent judgement bias has been produced by objectively measured positive emotions. We trained rats in operant Skinner boxes to press one lever in response to one tone to receive a food reward and to press another lever in response to a different tone to avoid punishment by electric foot shock. After attaining a stable level of discrimination performance, the animals were subjected to either handling or playful, experimenter-administered manual stimulation – tickling. This procedure has been confirmed to induce a positive affective state in rats, and the 50-kHz ultrasonic vocalisations (rat laughter) emitted by animals in response to tickling have been postulated to index positive emotions akin to human joy. During the tickling and handling sessions, the numbers of emitted high-frequency 50-kHz calls were scored. Immediately after tickling or handling, the animals were tested for their responses to a tone of intermediate frequency, and the pattern of their responses to this ambiguous cue was taken as an indicator of the animals' optimism. Our findings indicate that tickling induced positive emotions which are directly indexed in rats by laughter, can make animals more optimistic. We demonstrate for the first time a link between the directly measured positive affective state and decision making under uncertainty in an animal model. We also introduce innovative tandem-approach for studying emotional-cognitive interplay in animals, which may be of great value for understanding the emotional-cognitive changes associated with mood disorders.

The discrimination reversal paradigm is commonly used to measure a subject's ability to adapt their behavior according to changes in stimulus–reward contingencies. Human functional neuroimaging studies have demonstrated activations in the lateral orbitofrontal cortex (OFC) and the inferior frontal gyrus (IFG) in subjects performing this task. Excitotoxic lesions of analogous regions in marmosets have revealed, however, that although the OFC is indeed critical for reversal learning, ventrolateral prefrontal cortex (VLPFC) (analogous to IFG) is not, contributing instead to higher order processing, such as that required in attentional set-shifting and strategy transfer. One major difference between the marmoset and human studies has been the level of training subjects received in reversal learning, being far greater in the latter. Since exposure to repeated contingency changes, as occurs in serial reversal learning, is likely to trigger the development of higher order, rule-based strategies, we hypothesized a critical role of the marmoset VLPFC in performance of a serial reversal learning paradigm. After extensive training in reversal learning, marmosets received an excitotoxic lesion of the VLPFC, OFC, or a sham control procedure. In agreement with our prediction, postsurgery, VLPFC lesioned animals were impaired in performing a series of discrimination reversals, but only when novel visual stimuli were introduced. In contrast, OFC lesioned animals were impaired regardless of whether the visual stimuli were the same or different from those used during presurgery training. Together, these data demonstrate the heterogeneous but interrelated involvement of primate OFC and VLPFC in the performance of serial reversal learning.