Estimation of the time fluctuation of polysynaptic responses evoked by constant spinal cord stimulation

Abstract

Polysynaptic activity is necessary to engage the neural circuitry that controls the motor behaviour and is crucial to the recovery after spinal cord injury (SCI) [1]. Polysynaptic responses can be elicited with spinal cord stimulation, and there are few reports on these types of responses in human electrophysiology, most of them describing them as constant to fixed stimuli. However, we have observed that the responses fluctuate in amplitude and time. Amplitude variations can be analysed with statistical methods. However, time fluctuations are more complex due to the multiple parameters involved. This work describes a methodology to analyse the time fluctuation in the responses of each pulse. It is based on the calculation of the signal temporal centroid, representing the whole activity, weighted by its latency, in a single time value. This value is then used to model the temporal fluctuation with linear regression. The methodology was verified with an electromyography dataset from a discomplete spinal cord injured patient with spinal cord stimulation. The parameter is able to follow small changes in the responses' distribution. Examples of how the temporal centroid and linear model identify the fluctuations are presented. Once fitted in a linear model, the fluctuation coefficient describes time fluctuations in the interneuron processing and, together with amplitude metrics, can characterise changes in polysynaptic responses during the application of fixed stimulation parameters.