We report our studies on the optical signals measured non-invasively on electrically stimulated peripheral nerves. The
stimulation consists of the delivery of 0.1 ms current pulses, below the threshold for triggering any visible motion, to a
peripheral nerve in human subjects (we have studied the sural nerve and the median nerve). In response to electrical
stimulation, we observe an optical signal that peaks at about 100 ms post-stimulus, on a much longer time scale than the
few milliseconds duration of the electrical response, or sensory nerve action potential (SNAP). While the 100 ms optical
signal we measured is not a direct optical signature of neural activation, it is nevertheless indicative of a mediated
response to neural activation. We argue that this may provide information useful for understanding the origin of the fast
optical signal (also on a 100 ms time scale) that has been measured non-invasively in the brain in response to cerebral
activation. Furthermore, the optical response to peripheral nerve activation may be developed into a diagnostic tool for
peripheral neuropathies, as suggested by the delayed optical signals (average peak time: 230 ms) measured in patients
with diabetic neuropathy with respect to normal subjects (average peak time: 160 ms).
We present a study of the near-infrared optical response to electrical stimulation of peripheral nerves. The sural nerve of six healthy subjects between the ages of 22 and 41 was stimulated with transcutaneous electrical pulses in a region located approximately 10 cm above the ankle. A two-wavelength (690 and 830 nm) tissue spectrometer was used to probe the same sural nerve below the ankle. We measured optical changes that peaked 60 to 160 ms after the electrical stimulus. On the basis of the strong wavelength dependence of these fast optical signals, we argue that their origin is mostly from absorption rather than scattering. From these absorption changes, we obtain oxy- and deoxy-hemoglobin concentration changes that describe a rapid hemodynamic response to electrical nerve activation. In five out of six subjects, this hemodynamic response is an increase in total (oxy+deoxy) hemoglobin concentration, consistent with a fast vasodilation. Our findings support the hypothesis that the peripheral nervous system undergoes neurovascular coupling, even though more data is needed to prove such hypothesis.
Near-infrared spectroscopy (NIRS) has been used for functional brain imaging by employing properly designed source-detector matrices. We demonstrate that by embedding a NIRS source-detector matrix within an electroencephalography (EEG) standard multi-channel cap, we can perform functional brain mapping of hemodynamic response and neuronal response simultaneously. In this study, the P300 endogenous evoked response was generated in human subjects using an auditory odd-ball paradigm while concurrently monitoring the hemodynamic response both spatially and temporally with NIRS. The electrical measurements showed the localization of evoked potential P300, which appeared around 320 ms after the odd-ball stimulus. The NIRS measurements demonstrate a hemodynamic change in the fronto-temporal cortex a few seconds after the appearance of P300.