Melzack, R.&Wall, P. D. The Challenge of Pain. (Penguin, 1996).

Kupers, R. C, Konings, H., Adriaensen, H.&Gybels, J. M. 'Morphine differentially affects the sensory and affective pain ratings in neurogenic and idiopathic forms of pain'. Pain, 47, 5–12 (1991).