Panksepp, J; Burgdorf, J. (octubre de 2000). «50k-Hz chirping (laughter?) in response to conditioned and unconditioned tickle-induced reward in rats: effects of social housing and genetic variables». Behavioral Brain Research115 (1): 25-38. PMID10996405. doi:10.1016/s0166-4328(00)00238-2.
Panksepp, J; Burgdorf, J. (octubre de 2000). «50k-Hz chirping (laughter?) in response to conditioned and unconditioned tickle-induced reward in rats: effects of social housing and genetic variables». Behavioral Brain Research115 (1): 25-38. PMID10996405. doi:10.1016/s0166-4328(00)00238-2.