The application of photonics to manipulate and stimulate neurons and to study neural networks has gained momentum over the last decade. Two general methods have been used: the genetic expression of light or temperature sensitive ion channels in the plasma membrane of neurons (Optogenetics and Thermogenetics) and the direct stimulation of neurons using infrared radiation (Infrared Neural Stimulation, INS). Both approaches have their strengths and challenges, which are well understood with a profound understanding of the light tissue interaction(s). This paper compares the opportunities of the methods for the use in cochlear prostheses. Ample data are already available on the stimulation of the cochlea with INS. The data show that the stimulation is selective, feasible at rates that would be sufficient to encode acoustic information and may be beneficial over conventional pulsed electrical stimulation. A third approach, using lasers in stress confinement to generate pressure waves and to stimulate the functional cochlea mechanically will also be discussed.
Neural stimulation using infrared optical pulses has numerous potential advantages over traditional
electrical stimulation, including improved spatial precision and no stimulation artifact. However,
realization of optical stimulation in neural prostheses will require a compact and efficient optical
source. One attractive candidate is the vertical cavity surface emitting laser. This paper presents
the first report of VCSELs developed specifically for neurostimulation applications. The target
emission wavelength is 1860 nm, a favorable wavelength for stimulating neural tissues. Continuous
wave operation is achieved at room temperature, with maximum output power of 2.9 mW. The
maximum lasing temperature observed is 60° C. Further development is underway to achieve
power levels necessary to trigger activation thresholds.
In the 1990's a new laser technology, Vertical Cavity Surface Emitting Lasers, or VCSELs, emerged and transformed
the data communication industry. The combination of performance characteristics, reliability and performance/cost ratio
allowed high data rate communication to occur over short distances at a commercially viable price. VCSELs have not
been widely used outside of this application space, but with the development of new attributes, such as a wider range of
available wavelengths, the demonstration of arrays of VCSELs on a single chip, and a variety of package form factors,
VCSELs can have a significant impact on medical diagnostic and therapeutic applications.
One area of potential application is neurostimulation. Researchers have previously demonstrated the feasibility of using
1850nm light for nerve stimulation. The ability to create an array of VCSELs emitting at this wavelength would allow
significantly improved spatial resolution, and multiple parallel channels of stimulation. For instance, 2D arrays of 100
lasers or more can be integrated on a single chip less than 2mm on a side. A second area of interest is non-invasive
sensing. Performance attributes such as the narrow spectral width, low power consumption, and packaging flexibility
open up new possibilities in non-invasive and/or continuous sensing. This paper will suggest ways in which VCSELs
can be implemented within these application areas, and the advantages provided by the unique performance
characteristics of the VCSEL. The status of VCSEL technology as a function of available wavelength and array size and
form factors will be summarized.
Light can artificially stimulate nerve activity in vivo. A significant advantage of optical neural
stimulation is the potential for higher spatial selectivity when compared with electrical stimulation.
An increased spatial selectivity of stimulation could improve significantly the function of
neuroprosthetics, such as cochlear implants. Cochlear implants restore a sense of hearing and
communication to deaf individuals by directly electrically stimulating the remaining neural cells in
the cochlea. However, performance is limited by overlapping electric fields from neighboring
electrodes.
Here, we report on experiments with a new laser, offering a previously unavailable
wavelength, 1.94μm, and pulse durations down to 5μs, to stimulate cochlear neurons. Compound
action potentials (CAP) were evoked from the gerbil cochlea with pulse durations as short as 1μs.
Data show that water absorption of light is a significant factor in optical stimulation, as evidenced by
the required distance between the optical fiber and the neurons during stimulation. CAP threshold
measurements indicate that there is an optimal range of pulse durations over which to deposit the
laser energy, less than ~100μs. The implications of these data could direct further research and
design of an optical cochlear implant.
Since lasers were first used in medicine and biomedical related research there have been a variety of
documented effects following the irradiation of neural tissues. The first systematic studies to report
the direct stimulatory effect of infrared light on neural tissues were performed by researchers at
Vanderbilt University in the rat sciatic nerve. These initial studies demonstrated a set of associated
advantages of standard stimulation methods, which lead to much excitement and anticipation from
the neuroscience community and industry. The inception of this new field included a partnership
between industry and academia to foster the development, not only of the applications but also a
series of devices to support the research and ultimate commercialization of technology.
Currently several institutions are actively utilizing this technique in various applications including in
the cochlear and vestibular systems. As more researchers enter the field and new devices are
developed we anticipate the number of applications will continue to grow. Some of the next steps
will include the establishment of the safety and efficacy data to move this technique to clinical trials
and human use.
One drawback with traditional cochlear implants, which use electrical currents to stimulate spiral ganglion cells, is the ability to stimulate spatially discrete cells without overlap and electric current spread. We have recently demonstrated that spatially selective stimulation of the cochlea is possible with optical stimulation. However, for light to be a useful stimulation paradigm for stimulation of neurons, including cochlear implants, the neurons must be stimulated at high stimulus repetition rates. In this paper we utilize single fiber recordings from the auditory nerve to demonstrate that stimulation is possible at high repetition rates of the light pulses. Results showed that action potentials occurred 2.5-4. ms after the laser pulse. Maximum rates of discharge were up to 300 Hz. The action potentials did not respond strictly after the light pulse with high stimulation rates, i.e. >300 pulses per second. The correlation between the action potentials and the laser pulses decreased drastically for laser pulse repetition rate larger than 300 pulses per second.
Pulsed, mid-infrared 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 neural stimulation and very little of the available optical parameter
space has been studied. We found that pulse durations as short as 20 ?s elicit a compound action potential from the gerbil cochlea. Moreover, stimulation thresholds are not a function of absolute energy or absolute power deposited. Compound action potential peak-to-peak amplitude remained constant over extended periods
of stimulation. Stimulation occurred up six hours continuously and up to 50 Hz in repetition rate. Single fiber experiments were made using repetition rates of up to 1 kHz. Action potentials occurred 2.5-4 ms after the laser pulse. Maximum rates of discharge were up to 250 action potentials per second. With increasing stimulation rate
(300 Hz), the action potentials did not respond strictly after the light pulse. The results from these experiments
are important for designing the next generation of neuroprostheses, specifically cochlear implants.
This paper introduces our recent research on vehicle lateral control, a component of an automated highway system (AHS). We have developed a laser radar based automatic steering control system to study the performance and feasibility of such a system in an actual highway environment. Our approach does not require significant infrastructure investment because it is based on the retroreflective road markers currently installed on many roads. The system consists of a laser radar sensor, steering actuator, steering controller, and data processing system. The newly developed laser radar system measures not only the range to a target, but also the azimuth angle of the target in the sensor field of view. The azimuth angle measurement, which is essential for the lateral control system, has been implemented without any moving parts in the sensor system. A line scan CCD is used as the azimuth detector while an APD is used for the range measurement. The vehicle control algorithm was studied using computer simulation and a full-scale test vehicle was developed. Some previous related projects are reviewed and our unique sensor concept is described. The total system performance was demonstrated and evaluated on a California highway. Several issues remaining for further improvement of the system are also discussed.
Approaches for future 'mixed application' monolithic integrated circuits (ICs) employing optical receive/transmit, RF amplification and modulation and digital control functions are discussed. We focus on compatibility of the photonic component fabrication with conventional RF and digital IC technologies. Recent progress at Honeywell in integrating several parts of the desired RF/digital/photonic circuit integration suite required for construction of a future millimeter-wave optically-controlled phased-array element are illustrated.
Optical interconnects are being considered for the high speed distribution of multiplexed control signals in GaAs MMIC-based phased array antennas. This paper describes the performance of a hybrid GaAs optoelectronic integrated circuit (OEIC), along with a description of its design and fabrication. The OEIC converts a 16-bit serial optical input to a 16 parallel line electrical output using an on-board 1:16 demultiplexer and operates at data rates as high as 305 Mbps. The performance characteristics as well as potential applications of the device are presented.
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