Modulation of perception and neurophysiological correlates of brief CO₂laser stimuli in humans using concurrent large fiber stimulation

Abstract

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.

Static mechanical allodynia (SMA) is a paradoxical painful hypo-aesthesia: Observations derived from neuropathic pain patients treated with somatosensory rehabilitation

“…Based on the precise timing of activities generated in these two separate systems of afferent fibres, a recent work suggested that such inhibition is more likely to occur at cortical than spinal level (Inui et al 2006). In line with this observation, another study using psychophysical and physiological measures in humans concluded that the inhibition by large diameter fibres takes place at supraspinal level (Nahra and Plaghki 2003). The evoked chronic pain in SMA is elicited by application of a non-noxious tactile stimulus within the SMA territory.…”

Pain relief by touch: A quantitative approach

“…First, segmental brushing has been found to reduce overall pain ratings and response biases, without modulating d ’ measures of sensitivity to the nociceptive stimulation [22]. However, our Experiments 1 and 2 question this finding, since we observed strong decreases in sensitivity to the laser energy following naturalistic touch.…”

Pain additivity, diffuse noxious inhibitory controls, and attention: A functional measurement analysis

“…Dowman and Zimmer (1996) discovered that painful heat can increase the perceived magnitude of an innocuous electrical stimulus when applied nearby but not contralaterally. Likewise, Nahra and Plaghki (2003), who presented subjects with brush strokes on the left arm or right foot while stimulating the left arm with brief CO 2 laser pulses, found that segmental brushing interfered with the detection of low amplitude laser pulses but that the extra-segmental interactions between the foot and arm were negligible.…”