Panksepp, J., Burgdorf, J. 2003. Laughing rats and the evolutionary antecedents of human joy? Physiology and Behavior 79, 25-38.

Rapid finger movements across the nape, where rats usually solicit play, followed by tickling of the bellly simulates the rat's own playful interaction. Recent affective neuroscience research has yielded the discovery of play- and tickle-induced ultrasonic vocalization patterns ( approximately 50-kHz chirps) in rats may have more than a passing resemblance to primitive human laughter. In this paper, we summarize a dozen reasons for the working hypothesis that such rat vocalizations reflect a type of positive affect that may have evolutionary relations to the joyfulness of human childhood laughter commonly accompanying social play. The neurobiological nature of human laughter is discussed, and the relevance of such ludic processes for understanding clinical disorders such as attention deficit hyperactivity disorders (ADHD), addictive urges and mood imbalances are discussed.