Spectral and spatial dependence of
diffuse optical signals in response to
peripheral nerve stimulation

Abstract

Using non-invasive, near-infrared spectroscopy we have previously reported optical signals measured at or around peripheral nerves in response to their stimulation. Such optical signals featured amplitudes on the order of 0.1% and peaked about 100 ms after peripheral nerve stimulation in human subjects. Here, we report a study of the spatial and spectral dependence of the optical signals induced by stimulation of the human median and sural nerves, and observe that these optical signals are: (1) unlikely due to either dilation or constriction of blood vessels, (2) not associated with capillary bed hemoglobin, (3) likely due to blood vessel(s) displacement, and (4) unlikely due to fiber-skin optical coupling effects. We conclude that the most probable origin of the optical response to peripheral nerve stimulation is from displacement of blood vessels within the optically probed volume, as a result of muscle twitch in adjacent areas.

Electrical stimulation of peripheral nerves induces optical responses via skeletal muscle kinematics

“…This finding is in agreement with signals generated in human subjects both in this and previous work 6 . In all 5 animals, a 0.2 mg/kg intra-arterial dose of Cisatracurium Besylate dramatically reduced, or completely eliminated the stimulation-induced signals, Fig.…”

“…The results are summarized in table 1 and detailed in sections 3.2 – 3.7 and elsewhere 5, 6 . In the animal model, optical signals were completely eliminated following both pharmacological and surgical separation of the nerve from the muscles it innervates.…”

“…The timescale of the signals and their wavelength dependence suggested that changes in absorption, rather than scattering, associated with tissue hemodynamic events were likely to underlie those changes. In the present work, we report further studies on the spatial and spectral dependence of these signals, as well as the contribution of stimulus-induced motion and the effect of vascular occlusion, the results of which have been recently published 5, 6 . We also discuss ongoing work aimed at understanding the usefulness of these signals for non-invasively assessing the health of both nerve and muscle.…”

“…A detailed description of the data processing has been described elsewhere 5, 6 . Briefly, the average change in optical irradiance over N nerve stimulations is written as <δ I stim ( t )/ I ( t 0 )>.…”

Reproducibility of parameters of postocclusive reactive hyperemia measured by diffuse optical tomography

“…The heterogeneity of tissue is evident, and this has been reported in similar studies involving two-dimensional images of venous occlusion tests. 48 Furthermore, spatial dependence of the hemodynamic responses in the forearm has also been reported in nerve stimulation studies 49 and in exercising hand in the reflectance 50 and transmittance 51 modes. This variability that occurs in all three spatial directions may contribute to diminished repeatability if the probe location is not consistent across examinations.…”