The study of any sensory system requires the availability of a quantifiable stimulus that activates selectively the sensory system under investigation. Until recently, in contrast with other sensory systems (e.g., auditory, visual), specific nociceptive stimulators had not been available. Transcutaneous electrical stimuli are easy to control, and activate the peripheral afferents in a highly synchronous manner. However, the large diameter of Aβ non-nociceptive afferents results in an activation threshold that is lower than that of Aδ and C nociceptive afferents. Thus, when the intensity of electrical stimulation is above the threshold of nociceptive afferents, the coactivation of mechanoreceptors is unavoidable. For this reason, the use of nociceptive heat stimuli, which activate a molecular transduction mechanism that is specific for nociception (Julius & Basbaum, 2001), has received great attention. The introduction of high power, radiant heat stimulators (lasers) in sensory physiology in the ‘70s (Mor & Carmon, 1975) has revolutionized the study of the nociceptive system. Indeed, nociceptive free nerve endings belonging to Aδ and C fibres are mostly located in the epidermis, whereas non-nociceptive ﬁbers terminate more deeply, in the dermis. Laser pulses produce a transient heating of the most superficial skin layers. Thus, laser pulses are nociceptive-specific both from a physical perspective (i.e., they use a nociceptive-specific transduction mechanism) and from a spatial perspective (i.e., they only heat the most superficial skin layers, where nociceptors are located). Thus, they activate Aδ and/or C skin nociceptors selectively, without co-activating deeper, tactile mechanoreceptors (Plaghki & Mouraux. 2003). In addition, due to their physical properties (monochromatic nature, high spectral density and fast energy transfer), laser pulses activate peripheral afferents in a synchronous fashion, making this technique optimal to elicit time-locked responses in the ongoing EEG.
For all these reasons laser stimulators are now recognized as the most accurate way to elicit phasic pain sensation in human experiments (Bromm & Meier, 1984).