Laser stimulation of auditory neurons: effect of shorter pulse duration and penetration depth.

We have pioneered what we believe is a novel method of stimulating cochlear neurons, using pulsed infrared radiation, based on the hypothesis that optical radiation can provide more spatially selective stimulation of the cochlea than electric current. Very little of the available optical parameter space has been used for optical stimulation of neurons. Here, we use a pulsed diode laser (1.94 microm) to stimulate auditory neurons of the gerbil. Radiant exposures measured at CAP threshold are similar for pulse durations of 5, 10, 30, and 100 micros, but greater for 300-micros-long pulses. There is evidence that water absorption of optical radiation is a significant factor in optical stimulation. Heat-transfer-based analysis of the data indicates that potential structures involved in optical stimulation of cochlear neurons have a dimension on the order of approximately 10 microm. The implications of these data could direct further research and design of an optical cochlear implant.

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

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.

Slow-stirring method for determining the n-octanol/water partition coefficient (Pow) for highly hydrophobic chemicals: performance evaluation in a ring test.

The n-octanol/water partition coefficient (Pow) is one of the most important parameters employed for estimating a chemical's environmental fate and toxicity. The currently adopted test guidelines for its determination do not allow for reliable determination of log Pow greater than 5. The slow-stirring experiment, in contrast, has been demonstrated to provide reliable log Pow data up to log Pow of 8.3. To validate this method and to obtain its approval as an official Organization for Economic Cooperation and Development (Paris, France) test guideline, a ring test was performed to evaluate the accuracy and precision of the slow-stirring experiment for determination of log Pow, particularly for highly hydrophobic compounds. Up to 15 volunteer laboratories tested 1,2,3,4-tetrachlorobenzene, hexachlorobenzene. 2,2',3,3',5,5',6,6'-octachlorobiphenyl, and decachlorobiphenyl (4.5 < log Pow < 8.2). The ring-test results for the respective chemicals were 4.62, 5.50, 7.39, and 8.18. The results deviated by less than 0.1 log Pow units from the reference log Pow. In addition, the relative standard deviations of log Pow were less than 2%. Using the protocol of the ring test, log Pow of p,p'-DDT was determined to be 6.24 +/- 0.05 (mean +/- standard deviation). In combination, these results indicate that the slow-stirring method is precise and accurate and, thus, allows for reliable determination of log Pow of highly hydrophobic chemicals.