For IR block, 600-µm multimode optical fibers (P600-2-VIS-NIR, Ocean Optics, Dunedin, Florida) were placed on either side of the same nerve cross section to produce more uniform IR exposure. They were coupled to 2 Capella lasers (Lockheed Martin Aculight, Bothell, Washington) centered at 1860 and 1863 nm due to different wavelength tuning ranges, but with similar water absorption coefficients. The optical fibers gently touched the nerve sheath, producing a 600-µm spot at the nerve surface. The fibers had a numerical aperture of 0.22 (i.e., a beam divergence of 25.4 deg in air), so the spot was slightly larger at the axons due to divergence and scattering through the typically 100-µm thick sheath. Since the onset response travels both anterogradely and retrogradely from the KHFAC block electrodes, applying the lasers to block the onset response near the proximal electrodes allowed the distal electrodes to serve as a control for the same KHFAC block. Varying placement of the optical fibers between the KHFAC and the distal or proximal recording electrodes had no effect on the results.