This study examined the relative capacity of Adelta- and C-fibers to encode non-painful and painful brief CO(2) laser stimuli by comparing the effects of Adelta/C-fiber activation versus C-fiber activation alone. In nine normal subjects, brief CO(2) laser pulses of four different intensities (range 5.8-10.6mJ/mm(2)) were delivered at random on the first intermetacarpal zone of the dorsum of the hand. A-fiber pressure block of the superficial radial nerve was performed to fully isolate the activity of C-fibers. Quality and intensity (VAS) of percepts, reaction time (RT) and laser-evoked potentials (LEPs) were examined in baseline and A-fiber block conditions. During A-fiber block, absolute detection threshold increased dramatically from 4.8+/-1.8 to 10.9+/-4.8mJ/mm(2), proportion of detected stimuli decreased from 87% to 47% and proportion of pain reports from 39% to 10%. The quality of sensations became mainly 'light touch' and the 'pricking' sensation almost vanished. The stimulus-VAS curve shifted to the right and the slope was reduced. Signal Detection Theory analysis revealed that discrimination performance (P(A)) was significantly depressed and that response bias (B) evolved from a neutral towards a stoical attitude. Median RT increased from 492 to 1355ms. The late LEPs, attributed to the activation of Adelta-fibers, disappeared and ultra-late LEPs were recorded at Cz with a positivity peaking around 800ms. Collectively, these observations lead to the conclusion that Adelta-fibers are the main peripheral mediators for the perception of brief CO(2) laser stimuli and that they provide more sensory information than C-fibers.

The present study examined the influence of innocuous skin cooling on the perception and neurophysiological correlates of brief noxious CO2 laser stimuli. In nine normal subjects, brief CO2 laser pulses of four different intensities (duration 50 ms; diameter 5 mm; intensity range 5.8-10.6 mJ/mm2) were delivered at random every 5-10 s on the dorsum of the hand. Innocuous skin cooling was performed by a thermode (20 degrees C; 3x3 cm) with a central hole for the laser test stimuli. Quality and intensity (VAS) of perceptions, reaction times and laser evoked potentials (LEPs) were examined. Signal detection theory analysis was performed to evaluate discrimination performance and decision criterion. During innocuous skin cooling, detection threshold increased from 4.8+/-1.81 to 8.2+/-1.05 mJ/mm2 and pain threshold from 8.7+/-1.53 to 13.5+/-1.57 mJ/mm2. proportion of detected stimuli decreased from 87% to 48% and pain reports from 42% to 10%. The well localized 'pricking' sensation mediated by Adelta-nociceptors almost vanished. The intensity of sensations (VAS scores) was considerably reduced. Sensory discriminative performance was significantly depressed but decision criterion remained unchanged. Reaction times were delayed. The late-LEPs, correlates of Adelta-nociceptor activations, were also significantly depressed while the ultra-late LEPs, correlates of C-nociceptors, were not affected. Taken together, these results strongly suggest that innocuous skin cooling interfered with the sensory processing of laser heat stimuli and more prominently with those related to Adelta-nociceptive input.

It has been postulated that peripheral large fiber stimulation could modulate pain perception, probably by gating the input from AS-and C-fibers. The present study examined the effects of concurrent large fiber stimulation on the perception and neurophysiological correlates of brief CO2 laser stimuli known to activate A- and C-nociceptor endings selectively. Four test stimuli of brief non-painful and painful CO2 laser pulses (duration 50 ms; diameter 5 mm; intensity range 0.116-0.212J) were delivered at random every 5-10s on the dorsum of the left forearm of ten healthy subjects. Large fiber stimulation was performed by a dynamic soft brush applied either adjacently to test stimuli (segmental brush condition) or on the dorsum of the contra-lateral foot (extrasegmental brush condition). Perception, reaction time (RT) and laser-evoked potentials (EPs) were examined for conditions with brush and without brush (control condition). The signal detection theory (SDT) was used to evaluate the discrimination performance and the decision criterion. During extrasegmental brushing, these variables were unaffected as compared with control conditions. During segmental brushing, the absolute detection threshold increased, the probability of detection decreased and the RT increased. Interestingly, the stimulus-response curve of detected stimuli and late LTPs did not change significantly. SDT analysis showed that segmental brushing did not change the discrimination performance or sensitivity but increased significantly the subject's decision criterion for reporting sensation. It was concluded that segmental brushing acted primarily at supra-spinal levels and not by gating the input from small primary afferents activated selectively by brief CO2 laser stimuli.