Panksepp، J؛ Burgdorf, J. (أكتوبر 2000). "50k-Hz chirping (laughter?) in response to conditioned and unconditioned tickle-induced reward in rats: effects of social housing and genetic variables". Behavioural Brain Research. ج. 115 ع. 1: 25–38. DOI:10.1016/s0166-4328(00)00238-2. PMID:10996405.
Panksepp، J؛ Burgdorf, J. (أكتوبر 2000). "50k-Hz chirping (laughter?) in response to conditioned and unconditioned tickle-induced reward in rats: effects of social housing and genetic variables". Behavioural Brain Research. ج. 115 ع. 1: 25–38. DOI:10.1016/s0166-4328(00)00238-2. PMID:10996405.