Neuroevolutionary sources of laughter and social joy: modeling primal human laughter in laboratory rats (2007)

Abstract

Rats make abundant 50 kHz ultrasonic vocalizations (USVs) when they play and exhibit other positive social interactions. This response can be dramatically increased by tickling animals, especially when directed toward bodily areas toward which animals direct their own play solicitations (e.g., nape of the neck). The analysis of this system indicates that the response largely occurs in positive, playful social situations, and may index willingness for social engagement, similar to human infantile laughter, which may mature into productive adult socio-sexual behaviors. There are now enough formal similarities between rat 50 kHz USVs and human laughter, to realistically hypothesize that they are neurally and functionally homologous at the subcortical level of brain organization. To help contrast this behavior with human laughter, the available evidence concerning neural organization of human laughter is summarized from brain imaging and neuropsychological perspectives.