Laser-based Assessment of A-delta Fiber Activation

Infrared Laser stimuli were administered at the back of the left hand using a thulium solid-state laser (Themis®, StarMedTec GmbH, Starnberg, Germany) at a wavelength of 1.96 µm. The stimuli were short (1 ms) pulses with a power of 150–600 mJ and a beam diameter of 5 mm. The high power of a laser stimulus produces a very fast heat ramp, which generally activates the terminals of both A_δ and C fibers [13] , [14] . However, since A_δ fiber elicited first pain precedes the C fiber elicited second pain due to the different conduction velocities of the two afferents (∼10 m/s for A_δ and ∼1 m/s for C fibers), it can be easily distinguished when the stimulus is applied at a remote location such as the back of the hand [15] (link). Furthermore, “first pain” is more salient than “second pain” [16] (link). Therefore, in the present study, subjects were asked to rate and choose descriptors for the first sensation they experienced when being stimulated by the laser, which set the focus on the A_δ component.