A novel dual-wavelength laser stimulator to elicit transient and tonic nociceptive stimulation

Abstract

This study aimed to develop a new laser stimulator to elicit both transient and sustained heat stimulation with a dual-wavelength laser system as a tool for the investigation of both transient and tonic experimental models of pain. The laser stimulator used a 980-nm pulsed laser to generate transient heat stimulation and a 1940-nm continuous-wave (CW) laser to provide sustained heat stimulation. The laser with 980-nm wavelength can elicit transient pain with less thermal injury, while the 1940-nm CW laser can effectively stimulate both superficial and deep nociceptors to elicit tonic pain. A proportional integral-derivative (PID) temperature feedback control system was implemented to ensure constancy of temperature during heat stimulation. The performance of this stimulator was evaluated by in vitro and in vivo animal experiments. In vitro experiments on totally 120 specimens fresh pig skin included transient heat stimulation by 980-nm laser (1.5 J, 10 ms), sustained heat stimulation by 1940-nm laser (50-55 °C temperature control mode or 1.5 W, 5 min continuous power supply), and the combination of transient/sustained heat stimulation by dual lasers (1.5 J, 10 ms, 980-nm pulse laser, and 1940-nm laser with 50-55 °C temperature control mode). Hemoglobin brushing and wind-cooling methods were tested to find better stimulation model. A classic tail-flick latency (TFL) experiment with 20 Wistar rats was used to evaluate the in vivo efficacy of transient and tonic pain stimulation with 15 J, 100 ms 980-nm single laser pulse, and 1.5 W constant 1940-nm laser power. Ideal stimulation parameters to generate transient pain were found to be a 26.6 °C peak temperature rise and 0.67 s pain duration. In our model of tonic pain, 5 min of tonic stimulation produced a temperature change of 53.7 ± 1.3 °C with 1.6 ± 0.2% variation. When the transient and tonic stimulation protocols were combined, no significant difference was observed depending on the order of stimuli. Obvious tail-flick movements were observed. The TFL value of transient pain was 3.0 ± 0.8 s, and it was 4.4 ± 1.8 s for tonic pain stimulation. This study shows that our novel design can provide effective stimulation of transient pain and stable tonic pain. Furthermore, it can also provide a reliable combination of transient and consistent stimulations for basic studies of pain perception.

Low Back Pain Assessment Based on Alpha Oscillation Changes in Spontaneous Electroencephalogram (EEG)

“…It has been reported that experimentally induced transient pain elicits a decrease in alpha power [ 9 , 11 , 21 ]. Compared with transient noxious painful experiences, findings regarding the effect of tonic pain on alpha oscillatory activity have been inconsistent: some studies have indicated that alpha rhythm induced by tonic pain is suppressed in frontal-central or parietal-occipital regions [ 15 , 24 – 26 ], whereas others have reported that alpha oscillations are enhanced over these cortical regions [ 14 , 27 ]. Although there is no clear consensus regarding how alpha power changes as sustained stimuli are processed, alpha band oscillations are generally considered to be correlated with different levels of pain [ 18 ].…”

“…This previously reported relationship between alpha oscillations and subjective perceived pain encouraged us to investigate whether alpha oscillations obtained from continuous resting EEG are associated with subjective reports of perceived pain in chronic pain patients. However, our previous studies were limited in that the subjects were healthy adults, so tonic noxious stimuli were used to induce pain and elicit the EEG oscillations [ 15 , 26 ]. Although experimentally induced tonic pain stimuli better resemble the sensory experience in a clinical setting than brief stimuli, they may not involve the same neurological responses as pain [ 18 , 19 , 28 ].…”

A novel temperature-controlled laser system to uniformly activate cutaneous thermal receptors during movable thermal stimulation

“…Additionally, there were only very few studies in the literature that compared both open-loop and closed-loop laser stimulation systems. One example, however, was a study where in vitro, stationary stimulations were delivered to pieces of fresh pig skin using a 1940 nm Tm:YAG laser [32], in which closed-loop control (PID controller) was used to maintain the stimulation temperature between 50 and 55 • C, and in open-loop control, constant power of 1.5 W was used during 5 min laser stimulation for both control modes. Their results showed that in closed-loop control, the mean temperature was 53.7 ± 1.3 • C with a temperature variation of 1.6 ± 0.2%, and a continuous temperature increment was observed during open-loop control with a final temperature up to 60 • C at the end of stimulation.…”

“…Similar to the findings of the current study, their results showed that temperature control is superior during closed-loop control. Additionally, it must be noted that [32] used temperature control during stationary stimulations, which lasted a relatively long stimulation time (5 min) and their target temperatures were not a single fixed point, but a simply larger range between 50 • C and 55 • C. In contrast, in the present study, movable stimulations were delivered to the human skin in vivo to attain fixed target temperatures (42 • C and 46 • C) with different and relatively shorter stimulation durations (1.66-20 s), because of using different displacement lengths and velocities, that entirely achieved better performance than what was reported in [32] in terms of temperature control for all stimulation conditions (figure 7 and table 4).…”

“…With this type of system, skin temperature is not controlled during the stimulation, instead laser power is set at the beginning and kept constant. To have low-temperature variations during the laser stimulation, closed-loop control systems have been implemented [24,25,[30][31][32] that purely involved stationary stimuli. This type of control involves skin temperature feedback to adjust the laser power dynamically.…”