Abstract
Background/ObjectiveTranscutaneous electrical stimulation has been proven to modulate nervous system activity, leading to changes in pain perception, via the peripheral sensory system, in a bottom up approach. We tested whether different sensory behavioral tasks induce significant effects in pain processing and whether these changes correlate with cortical plasticity.Methodology/Principal FindingsThis randomized parallel designed experiment included forty healthy right-handed males. Three different somatosensory tasks, including learning tasks with and without visual feedback and simple somatosensory input, were tested on pressure pain threshold and motor cortex excitability using transcranial magnetic stimulation (TMS). Sensory tasks induced hand-specific pain modulation effects. They increased pain thresholds of the left hand (which was the target to the sensory tasks) and decreased them in the right hand. TMS showed that somatosensory input decreased cortical excitability, as indexed by reduced MEP amplitudes and increased SICI. Although somatosensory tasks similarly altered pain thresholds and cortical excitability, there was no significant correlation between these variables and only the visual feedback task showed significant somatosensory learning.Conclusions/SignificanceLack of correlation between cortical excitability and pain thresholds and lack of differential effects across tasks, but significant changes in pain thresholds suggest that analgesic effects of somatosensory tasks are not primarily associated with motor cortical neural mechanisms, thus, suggesting that subcortical neural circuits and/or spinal cord are involved with the observed effects. Identifying the neural mechanisms of somatosensory stimulation on pain may open novel possibilities for combining different targeted therapies for pain control.
Highlights
Pain perception can be influenced by several factors such as expectation, drugs, attention and emotional state
This study demonstrated that sensory behavioral tasks induced laterality specific effects as they increased pressure pain thresholds of the ipsilateral left hand - decreasing perception of pain – and, in contrast, they had the opposite effect in the contralateral right hand, increasing perception of pain
transcranial magnetic stimulation (TMS) measurements showed that somatosensory input (SSI) generally decreased motor cortex excitability over time indexed by significantly reduced amplitudes of Motor-evoked potentials (MEP) and a trend for increased short intracortical inhibition (SICI)
Summary
Pain perception can be influenced by several factors such as expectation, drugs, attention and emotional state. In this context methods to induce changes in the pain-related neural network can alter pain perception. Transcutaneous electrical stimulation (TENS) is an example of an intervention that targets the peripheral sensory system leading to changes in pain perception. Pain modulation with TENS is hypothesized to induce effects at spinal and thalamic levels [1]. Extensive data from TENS studies have confirmed the notion that modulation of somatosensory system at peripheral level leads to a change in pain perception, one important question is whether behavioral tasks involved with somatosensory processing would have significant effects in pain processing
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