Lasers and other thermal stimulators for activation of skin nociceptors in humans

Pain can be induced by thermal, chemical, and mechanical stimulation in animals and man. Of the thermal stimulation modalities, heat is the most commonly used, as a variety of reliable stimulation techniques are available. Heat is a natural stimulus modality to evoke pain, and it has been used to study animal nociception and human pain perception for (a) examining the mechanisms of tissue injury and sensitisation and (b) quantifying the therapeutic effects of pharmacological, physical, and psychological interventions. This paper summarises the current understanding of the physiology and psychophysical response to painful heat stimulation in humans. By understanding the underlying mechanisms, new methods of heat stimulation may be developed for basic and clinical applications. Traditionally, contact heat, indirect thermal heat by focused light bulb, and laser pulses have been the methods used to induce heat pain in humans for experimental and clinical studies. The following lasers have been used in pain research: argon (488-515 nm), copper vapour (510-577 nm), semiconductor (e.g. 970 nm), neodymium-YAG (1064 nm), thulium-YAG (2000 nm), and CO(2) (10,600 nm).