In 1975, Mor and Carmon introduced infrared laser stimulators as tools to explore nociception in humans. Allowing brief, synchronous, and selective activation of cutaneous Aδ- and C-fibre nociceptors, laser heat stimulators are now used extensively to study nociception in humans.
Infrared laser stimulation to elicit time-locked responses related to the selective activation of heat-sensitive skin nociceptors.
Laser stimuli which concomitantly activate Aδ- and C-fibre nociceptors produce a characteristic double sensation, reminiscent of the ‘first’ and ‘second’ pain described by Lewis and Ponchin (1937). First pain is often reported as a localized and short-lasting ‘pricking’ sensation while second pain is often reported as a ‘burning’ sensation which spreading beyond the spatial and temporal limits of the stimulus.
Telethermographic recording of skin temperature following CO2 laser stimulation of the hand dorsum.
Selective activation of C-fibre nociceptors
Several methods allow narrowing the selectivity of the laser stimulator such as to activate C-fibres selectively (Plaghki and Mouraux 2002).
A first method takes advantage of the lower thermal activation threshold of C-nociceptors. Therefore, it is possible to devise a thermal stimulus that will be above the threshold of C-nociceptors, but still below the threshold of Aδ-nociceptors. A second method takes advantage of the higher distribution density of C-nociceptors in the skin. Therefore, it is possible to devise a thermal stimulus whose surface area is so small that the probability that the stimulus activates Aδ-nociceptors is very low, while the probability of activating C-nociceptors remains relatively high.
Follow this link for a short review of the different methods to activate C-fibres selectively.
Temperature-controlled laser stimulation
Recently, we have collaborated to the development of a novel CO2 laser stimulator whose power is regulated using a feedback control based on an online measurement of skin temperature at the site of stimulation (Laser Stimulation Device, SIFEC, Belgium). Conception of the laser was inspired by a similar feedback-controlled device developed by Meyer et al. (1976). Both devices are based on a closed-loop control of laser power by an online monitoring of skin temperature performed using a radiometer collinear with the laser beam, enabling to produce temperature steps with rise rates greater than 350°C/s and pulse durations from 10 ms to 12 s. The heat source is a 25 W radio-frequency excited CO2 laser (Synrad 48-2; Synrad, USA). Power control is achieved by pulse width modulation at 5 KHz clock frequency. The stimuli are delivered through a 10 m optical fibre. By vibrating this fibre at some distance from the source, a quasi-uniform spatial distribution of radiative power within the stimulated area is obtained. At the end of the fibre, optics are used to collimate the beam.
As compared to other devices, the system offers the following advantages :
- MRI-compatible optical fiber and fiber head
- Temperature-controlled stimulation through an online adjustment of laser power output
- Uniform spatial distribution of radiative power within the entire beam area
This simple experiment illustrates the advantage of temperature-controlled laser stimulation. Here, skin temperature is measured while delivering a constant-power laser stimulus to the hand dorsum using two different beam incidences (0°: laser beam orthogonal to the skin surface; 30° : leaser beam tilted 30° relative to the skin surface). Note how the change in power density markedly changes the increase in skin temperature generated by the laser beam.
Temporally-dissociated activation of A-delta and C-fibers using temperature-controlled laser stimulation. The online adjustment of laser power allows to generate two separate heat pulses, the first activating C fibers selectively (at time=4 s), the second activating A-delta fibers selectively (at time=7 s).