50-kHz chirping (laughter?) in response to conditioned and unconditioned tickle-induced reward in rats: effects of social housing and genetic variables.
Behav Brain Res. 2000 Oct;115(1):25-38. doi: 10.1016/s0166-4328(00)00238-2.
Behav Brain Res. 2000.
PMID: 10996405