Effects of stimulus pulse rate on somatosensory adaptation in the human cortex

BackgroundIntracortical microstimulation (ICMS) of the somatosensory cortex can restore sensation to people with neurological diseases. However, many aspects of ICMS are poorly understood, including the effect of continuous stimulation on percept intensity over time.ObjectiveHere, we evaluate how tactile percepts, evoked by ICMS in the somatosensory cortex of a human participant adapt over time.MethodsWe delivered continuous and intermittent ICMS to the somatosensory cortex and assessed the reported intensity of tactile percepts over time in a human participant. Experiments were conducted across approximately one year and linear mixed effects models were used to assess significance.ResultsContinuous stimulation at high frequencies led to rapid decreases in intensity, while low frequency stimulation maintained percept intensity for longer periods. Burst-modulated stimulation extended the time before the intensity began to decrease, but all protocols ultimately resulted in complete sensation loss within one minute. Intermittent stimulation paradigms with several seconds between stimulus trains also led to decreases in intensity on many electrodes, but never resulted in extinction of sensation after over three minutes of stimulation. Additionally, longer breaks between each pulse train resulted in some recovery of the stimulus-evoked percepts. For several electrodes, intermittent stimulation had almost no effect on the perceived intensity.ConclusionsIntermittent ICMS paradigms were more effective at maintaining percepts, and given that transient activity in the somatosensory cortex dominates the response to object contact, this stimulation method may mimic natural cortical activity and improve the perception of stimulation over time.

Evoked Electromyographic Fatigue Indices During Intermittent Stimulation Towards Dynamic Wrist Contractions

Functional electrical stimulation (FES) in stroke rehabilitation is hindered by early patient fatigue. Therefore the study of FES-elicited fatigue is needed to optimise its therapeutic impact. In this study, the reliability of evoked electromyography (EMG) median frequency (MDF) and peak-to-peak (PTP) fatigue indices were investigated during stimulation of the forearm. During isometric constant stimulation the MDF-time and PTP-time waveforms exhibited negative gradients in the ranges (−1.3 to −3.6) Hz · s−1 and (−0.013 to −0.016) V · s−1 respectively, most with coefficients of determination (R2) greater than 0.83. During vEMG-controlled stimulation the MDF-contraction waveforms exhibited negative gradients in the range (−2.4 to −9.0) Hz · Cont−1 while the PTP gradients ranged from (0.0070 to −0.019) V · Cont−1. For these datasets the MDF waveforms’ R2 > 0.62 and most PTP waveforms’ R2 < 0.35. These findings suggest that the MDF and PTP amplitude are suitable fatigue indices during isometric constant stimulation but the PTP-based index becomes unreliable during vEMG-controlled stimulation.