Single pulse and pulse train modulation of cutaneous electrical stimulation: a comparison of methods

Changing the amplitude of single rectangular pulse stimuli (SP) has the disadvantage of recruiting tactile and nociceptive fibers in a changing, unknown proportion. Keeping the amplitude constant, but applying a varying number of pulses in a train is another way of stimulus variation, keeping the proportion constant. So, pulse trains (PT) with a variable number of pulses (NoP) but fixed amplitude might be more suitable to study non-peripheral aspects of processing of stimuli. In this study, we compared the effects of PT and SP stimulation on subjective Numeric Rating Scale (NRS) scores of perceived stimulus strength and evoked potentials (EP). A total of 41 healthy subjects were electrically stimulated at the left forearm or left middle fingertip using SP and PT stimuli. NRS scores and EPs were averaged from 105 randomized stimuli at 5 stimulus amplitudes or NoP for each subject. The relationships between stimulus amplitudes or NoP, EP components and NRS scores differed depending on the stimulation method and stimulus location. Although the repeatedly reported NRS-EP (N150-P200) correlation was reproduced for SP at the fingertip, no significant correlation was found for SP stimulation at the forearm. For PT this correlation was found for both stimulus locations. These findings demonstrate that SP and PT involve different ways of processing. The two methods result in different NRS scores and EP components. Furthermore, PT stimulation is less dependent on stimulus location