Optical parameter variability in laser nerve stimulation: a study of pulse duration, repetition rate, and wavelength

Abstract

Pulsed lasers can evoke neural activity from motor as well as sensory neurons in vivo. Lasers allow more selective spatial resolution of stimulation than the conventional electrical stimulation. To date, few studies have examined pulsed, mid-infrared laser stimulation of nerves and very little of the available optical parameter space has been studied. In this study, a pulsed diode laser, with wavelength between 1.844-1.873 microm, was used to elicit compound action potentials (CAPs) from the auditory system of the gerbil. We found that pulse durations as short as 35 micros elicit a CAP from the cochlea. In addition, repetition rates up to 13 Hz can continually stimulate cochlear spiral ganglion cells for extended periods of time. Varying the wavelength and, therefore, the optical penetration depth, allowed different populations of neurons to be stimulated. The technology of optical stimulation could significantly improve cochlear implants, which are hampered by a lack of spatial selectivity.

Fig. 1

Orientation of the optical fiber in the gerbil cochlea. This is a mid-modiolar cross-section of the gerbil cochlea. The arrow indicates the orientation and placement of the optical fiber with respect to the cochlea. The optical path length through the stimulated spiral ganglion cell population (circled) is approximately 250 μm. The calibration bar at the top left of the figure equals 500 μm.

Fig. 2

Compound action potentials elicited by varying pulse durations. The CAPs evoked by the laser change with different pulse durations. At the shortest pulse durations, the CAP is primarily composed of one negative peak (N1) and one positive peak (P1). As the pulse durations increase, there is a secondary peak that increases in amplitude. These CAPs were measured from the same animal. The amount of time between the onset of data acquisition and onset of CAP decreases as the pulse duration increases due to the data acquisition trigger from the laser.

Fig. 3

Stimulation thresholds for various pulse durations. (a) The radiant exposure required to elicit a CAP is smallest at 35-μs pulse duration, 5.29 ± 0.6 mJ/cm² (x̄ ± s.e., n =8) The stimulation threshold increases with increasing pulse duration. On the right-hand axis, the calculated temperature rise for the corresponding radiant exposures is provided for reference. The average data with standard error bars are shown in black squares. The individual data sets, each measured from a different animal, are shown by the gray squares. (b) I/O functions relating radiant exposure to CAP amplitude for 35-μs pulse duration. There is a steady increase in CAP amplitude with increasing radiant exposure, followed by a plateau of CAP amplitude. Each type of data marker represents a single data point measured on a different animal (n = 8).

Fig. 4

Wavelength variability of laser evoked neural response. At longer wavelengths (shorter penetration depths), the CAP amplitude is at a minimum. When the wavelength is decreased (penetration depth increases), the CAP amplitude grows, until it reaches a plateau. Wavelengths between 1.844–1.873 μm were tested. Each type of data marker represents a single data point measured on a different animal. (n = 5).

Fig. 5

Extended stimulation of the gerbil cochlea reveals constant evoked response. The CAP amplitude remains relatively constant over a period of 6 h of continual stimulation (x̄± s.e., n = 6). The laser operated at 1.873 μm, 35 μs, 13 Hz, at a radiant exposure of ~ 10 mJ/cm².

Fig. 6

Peak power at stimulation level as a function pulse duration. The peak power measured for a 50-μ CAP is constant for pulse durations 100 μs−1 ms, but increases at a pulse duration of 35 μs. These are the same experiments as shown in Fig. 3(a) calculated as a function of the pulse duration. The data are averages with standard error bars.