Intensity modulation of cutaneous electrical stimulation: EPs and subjective ratings

Chronic pain research is increasingly focused on the neuroplastic mechanisms underlying subjective pain experience. The latter is often measured using reported pain intensity, e.g. using a numeric rating scale (NRS). Evoked Potentials (EPs) reflect the cortical representation of applied stimuli and are experimentally used for observation of neural processing. Both sensation and cortical representation depend on central modulation of applied stimuli, as described by theories like Diffuse Noxious Inhibitory Control and the Gate Control Theory. In multiple mechanisms both tactile and nociceptive activations interact, indicating that the distribution of activated afferents (Aβ, Aδ-fibers) is relevant for correct interpretation of results. From this point of view, the traditional use of amplitude modulated single electrical stimulation pulses (SP) might not be optimal for studying central processing of pain as the amplitude changes this distribution (depending on the local fiber densities). Pulse trains (PT) with a variable number of fixed amplitude pulses might be more appropriate stimuli as they resemble the coding of stimulus intensity by skin receptors. In this study we therefore compared NRS scores and EP components (P90, N150, P200, P300) obtained with SP and PT modulated intensity (I). A total of 30 healthy female subjects were electrically stimulated at the tip of the left middle finger or forearm. NRS scores and EPs were averaged from 105 randomized stimuli at 5 intensity levels. Both the I-EP (components) and the I-NRS relationships differed depending on the modulation method and stimulation site. Although the repeatedly reported NRS-EP correlation was reproduced for SP at the finger, no significant correlation was found with stimulation at the forearm. For PT however, the NRS-EP correlation was found for both stimulation sites. These findings support the view that SP and PT methods offer a different excitation of neural mechanisms, which might be fruitful for future observation of central mechanisms underlying chronification of pain.