Hearing ability of prairie voles (Microtus ochrogaster).

The hearing abilities of mammals are impacted by factors such as social cues, habitat, and physical characteristics. Despite being used commonly to study social behaviors, hearing of the monogamous prairie vole (Microtus ochrogaster) has never been characterized. In this study, anatomical features are measured and auditory brainstem responses (ABRs) are used to measure auditory capabilities of prairie voles, characterizing monaural and binaural hearing and hearing range. Sexually naive male and female voles were measured to characterize differences due to sex. It was found that prairie voles show a hearing range with greatest sensitivity between 8 and 32 kHz, binaural hearing across interaural time difference ranges appropriate for their head sizes. No differences are shown between the sexes in binaural hearing or hearing range (except at 1 kHz), however, female voles have increased amplitude of peripheral ABR waves I and II and longer latency of waves III and IV compared to males. The results confirm that prairie voles have a broad hearing range, binaural hearing consistent with rodents of similar size, and differences in amplitudes and thresholds of monaural physiological measures between the sexes. These data further highlight the necessity to understand sex-specific differences in neural processing that may underly variability in responses between sexes.

A recording chamber for small volume slice electrophysiology.

Electrophysiological recordings from brain slices are typically performed in small recording chambers that allow for the superfusion of the tissue with artificial extracellular solution (ECS), while the chamber holding the tissue is mounted in the optical path of a microscope to image neurons in the tissue. ECS itself is inexpensive, and thus superfusion rates and volumes of ECS consumed during an experiment using standard ECS are not critical. However, some experiments require the addition of expensive pharmacological agents or other chemical compounds to the ECS, creating a need to build superfusion systems that operate on small volumes while still delivering appropriate amounts of oxygen and other nutrients to the tissue. We developed a closed circulation tissue chamber for slice recordings that operates with small volumes of bath solution in the range of 1.0 to 2.6 ml and a constant oxygen/carbon dioxide delivery to the solution in the bath. In our chamber, the ECS is oxygenated and recirculated directly in the recording chamber, eliminating the need for tubes and external bottles/containers to recirculate and bubble ECS and greatly reducing the total ECS volume required for superfusion. At the same time, the efficiency of tissue oxygenation and health of the section are comparable to standard superfusion methods. We also determined that the small volume of ECS contains a sufficient amount of nutrients to support the health of a standard brain slice for several hours without concern for either depletion of nutrients or accumulation of waste products.

Third harmonic generation microscopy of a mouse retina.

PURPOSE: To demonstrate lipid-specific imaging of the retina through the use of third harmonic generation (THG), a multiphoton microscopic technique in which tissue contrast is generated from optical inhomogeneities. METHODS: A custom fiber laser and multiphoton microscope was constructed and optimized for simultaneous two-photon autofluorescence (TPAF) and THG retinal imaging. Imaging was performed using fixed-frozen sections of mouse eyes without the use of exogenous fluorescent dyes. In parallel experiments, a fluorescent nuclear stain was used to verify the location of the retinal cell nuclei. RESULTS: Simultaneous THG and TPAF images revealed all retinal layers with subcellular resolution. In BALB/c strains, the THG signal stems from the lipidic organelles of the cellular and nuclear membranes. In the C57BL/6 strain, the THG signal from the RPE cells originates from the pigmented granules. CONCLUSIONS: THG microscopy can be used to image structures of the mouse retina using contrast inherent to the tissue and without the use of a fluorescent dye or exogenously expressed recombinant protein.

A fully automatic multichannel neural spike sorting algorithm with spike reduction and positional feature.

Objective: The sorting of neural spike data recorded by multichannel and high channel neural probes such as Neuropixels, especially in real-time, remains a significant technical challenge. Most neural spike sorting algorithms focus on sorting neural spikes post-hoc for high sorting accuracy-but reducing the processing delay for fast sorting, potentially even live sorting, is generally not possible with these algorithms.Approach: Here we report our Graph nEtwork Multichannel sorting (GEMsort) algorithm, which is largely based on graph network, to allow rapid neural spike sorting for multiple neural recording channels. This was accomplished by two innovations: In GEMsort, duplicated neural spikes recorded from multiple channels were eliminated from duplicate channels by only selecting the highest amplitude neural spike in any channel for subsequent processing. In addition, the channel from which the representative neural spike was recorded was used as an additional feature to differentiate between neural spikes recorded from different neurons having similar temporal features.Main results: Synthetic and experimentally recorded multichannel neural recordings were used to evaluate the sorting performance of GEMsort. The sorting results of GEMsort were also compared with two other state-of-the-art sorting algorithms (Kilosort and Mountainsort) in sorting time and sorting agreements.Significance: GEMsort allows rapidly sort neural spikes and is highly suitable to be implemented with digital circuitry for high processing speed and channel scalability.

Optical profiles of cathode ray tube and liquid crystal display monitors: implication in cutaneous phototoxicity in photodynamic therapy.

Recent clinical reports suggest that overexposure to light emissions generated from cathode ray tube (CRT) and liquid crystal display (LCD) color monitors after topical or systemic administration of a photosensitizer could cause noticeable skin phototoxicity. In this study, we examined the light emission profiles (optical irradiance, spectral irradiance) of CRT and LCD monitors under simulated movie and video game modes. Results suggest that peak emissions and integrated fluence generated from monitors are clinically relevant and therefore prolonged exposure to these light sources at a close distance should be avoided after the administration of a photosensitizer or phototoxic drug.

A multiphoton microscope platform for imaging the mouse eye.

PURPOSE: To demonstrate the ability of multiphoton microscopy to obtain full three-dimensional high-resolution images of the intact mouse eye anterior chamber without need for enucleation. METHODS: A custom multiphoton microscope was constructed and optimized for deep tissue imaging. Simultaneous two-photon autofluorescence (2PAF) and second harmonic generation (SHG) imaging were performed. A mouse holder and stereotaxic platform were designed to access different parts of the eye for imaging. A reservoir for keeping the eye moist was used during imaging sessions. RESULTS: Non-invasive multiphoton images deep inside the anterior chamber of the mouse eye were obtained without the need for enucleation. The iris, corneal epithelium and endothelium, trabecular meshwork region and conjunctiva were visualized by the 2PAF and SHG signals. Identification of the anatomy was achieved by the intrinsic properties of the native tissue without any exogenous labeling. Images as deep as 600 microns into the eye were clearly demonstrated. Full three-dimensional image reconstructions of the entire anterior chamber were performed and analyzed using custom software. CONCLUSIONS: Multiphoton imaging is a highly promising tool for ophthalmic research. We have demonstrated the ability to image the entire anterior chamber of the mouse eye in its native state. These results provide a foundation for future in vivo studies of the eye.

Predicting the Influence of Axon Myelination on Sound Localization Precision Using a Spiking Neural Network Model of Auditory Brainstem.

Spatial hearing allows animals to rapidly detect and localize auditory events in the surrounding environment. The auditory brainstem plays a central role in processing and extracting binaural spatial cues through microsecond-precise binaural integration, especially for detecting interaural time differences (ITDs) of low-frequency sounds at the medial superior olive (MSO). A series of mechanisms exist in the underlying neural circuits for preserving accurate action potential timing across multiple fibers, synapses and nuclei along this pathway. One of these is the myelination of afferent fibers that ensures reliable and temporally precise action potential propagation in the axon. There are several reports of fine-tuned myelination patterns in the MSO circuit, but how specifically myelination influences the precision of sound localization remains incompletely understood. Here we present a spiking neural network (SNN) model of the Mongolian gerbil auditory brainstem with myelinated axons to investigate whether different axon myelination thicknesses alter the sound localization process. Our model demonstrates that axon myelin thickness along the contralateral pathways can substantially modulate ITD detection. Furthermore, optimal ITD sensitivity is reached when the MSO receives contralateral inhibition via thicker myelinated axons compared to contralateral excitation, a result that is consistent with previously reported experimental observations. Our results suggest specific roles of axon myelination for extracting temporal dynamics in ITD decoding, especially in the pathway of the contralateral inhibition.

Double-Sided Sapphire Optrodes with Conductive Shielding Layers to Reduce Optogenetic Stimulation Artifacts.

Optrodes, which are single shaft neural probes integrated with microelectrodes and optical light sources, offer a remarkable opportunity to simultaneously record and modulate neural activities using light within an animal's brain; however, a common problem with optrodes is that stimulation artifacts can be observed in the neural recordings of microelectrodes when the light source on the optrode is activated. These stimulation artifacts are undesirable contaminants, and they cause interpretation complexity when analyzing the recorded neural activities. In this paper, we tried to mitigate the effects of the stimulation artifacts by developing a low-noise, double-sided optrode integrated with multiple Electromagnetic Shielding (EMS) layers. The LED and microelectrodes were constructed separately on the top epitaxial and bottom substrate layers, and EMS layers were used to separate the microelectrodes and LED to reduce signal cross-talks. Compared with conventional single-sided designs, in which the LED and microelectrodes are constructed on the same side, our results indicate that double-sided optrodes can significantly reduce the presence of stimulation artifacts. In addition, the presence of stimulation artifacts can further be reduced by decreasing the voltage difference and increasing the rise/fall time of the driving LED pulsed voltage. With all these strategies, the presence of stimulation artifacts was significantly reduced by ~76%. As well as stimulation suppression, the sapphire substrate also provided strong mechanical stiffness and support to the optrodes, as well as improved electronic stability, thus making the double-sided sapphire optrodes highly suitable for optogenetic neuroscience research on animal models.

Label-free imaging of trabecular meshwork cells using Coherent Anti-Stokes Raman Scattering (CARS) microscopy.

PURPOSE: To image the human trabecular meshwork (TM) using a non-invasive, non-destructive technique without the application of exogenous label. METHODS: Flat-mounted TM samples from a human cadaver eye were imaged using two nonlinear optical techniques: coherent anti-Stokes Raman scattering (CARS) and two-photon autofluorescence (TPAF). In TPAF, two optical photons are simultaneously absorbed and excite molecules in the sample that then emit a higher energy photon. The signal is predominately from collagen and elastin. The CARS technique uses two laser frequencies to specifically excite carbon-hydrogen bonds, allowing the visualization of lipid-rich cell membranes. Multiple images were taken along an axis perpendicular to the surface of the TM for subsequent analysis. RESULTS: Analysis of multiple TPAF images of the TM reveals the characteristic overlapping bundles of collagen of various sizes. Simultaneous CARS imaging revealed elliptical structures of ~7×10 µm in diameter populating the meshwork which were consistent with TM cells. Irregularly shaped objects of ~4 µm diameter appeared in both the TPAF and CARS channels, and are consistent with melanin granules. CONCLUSIONS: CARS techniques were successful in imaging live TM cells in freshly isolated human TM samples. Similar images have been obtained with standard histological techniques, however the method described here has the advantage of being performed on unprocessed, unfixed tissue free from the potential distortions of the fine tissue morphology that can occur due to infusion of fixatives and treatment with alcohols. CARS imaging of the TM represents a new avenue for exploring details of aqueous outflow and TM cell physiology.

Trans-scleral imaging of the human trabecular meshwork by two-photon microscopy.

PURPOSE: To image the native (unfixed) human trabecular meshwork (TM) through the overlying sclera using a non-invasive, non-destructive technique. METHODS: Two-photon microscopic (2PM) methods, including two-photon autofluorescence (2PAF) and second harmonic generation (SHG), were used to image through the sclera of a human cadaver eye into the TM region. Multiple images were analyzed along the tissue axis (z-axis) to generate a three-dimensional (3D) model of the region. The tissue was subsequently fixed, paraffin embedded, and histological sections were photographed for comparison to the 2PM images. RESULTS: 3D analysis of multiple 2PM SHG images revealed an open region deep within the TM consistent with the location of Schlemm's canal (SC). Images of the scleral spur and surrounding tissues were also obtained. The SC, TM, scleral spur, and surrounding tissue images obtained with 2PM matched with histologically stained sections of the same tissue. CONCLUSIONS: 2PM imaging of the outflow system of the human eye documented collagenous structures solely from inherent optical properties. 2PM successfully imaged through the sclera into the SC/TM without the need for fixation, embedding, or histological processing. This work reveals that 2PM imaging has potential as a new metric for evaluating the aqueous outflow region of the human eye and is worthy of further exploration.

Real-time measurements of nicotinamide adenine dinucleotide in live human trabecular meshwork cells: effects of acute oxidative stress.

The trabecular meshwork (TM) region of the eye is exposed to a constant low-level of oxidative insult. The cumulative damage may be the reason behind age-dependent risk for developing primary open angle glaucoma. Chronic and acute effects of hydrogen peroxide (H(2)O(2)) on TM endothelial cells include changes in viability, protein synthesis, and cellular adhesion. However, little if anything is known about the immediate effect of H(2)O(2) on the biochemistry of the TM cells and the initial response to oxidative stress. In this report, we have used two-photon excitation autofluorescence (2PAF) to monitor changes to TM cell nicotinamide adenine dinucleotide (NADPH). 2PAF allows non-destructive, real-time analysis of concentration of intracellular NADPH. Coupled to reduced glutathione, NADPH, is a major component in the anti-oxidant defense of TM cells. Cultured human TM cells were monitored for over 30 min in control and H(2)O(2)-containing solutions. Peroxide caused both a dose- and time-dependent decrease in NADPH signal. NADPH fluorescence in control and in 4 mM H(2)O(2) solutions showed little attenuation of NADPH signal (4% and 9% respectively). TM cell NADPH fluorescence showed a linear decrease with exposure to 20 mM H(2)O(2) (-29%) and 100 mM H(2)O(2) (37%) after a 30 min exposure. Exposure of TM cells to 500 mM H(2)O(2) caused an exponential decrease in NADPH fluorescence to a final attenuation of 46% of starting intensity. Analysis of individual TM cells indicates that cells with higher initial NADPH fluorescence are more refractive to the apparent loss of viability caused by H(2)O(2) than weakly fluorescing TM cells. We conclude that 2PAF of intracellular NADPH is a valuable tool for studying TM cell metabolism in response to oxidative insult.

Axonal Conduction Delay Shapes the Precision of the Spatial Hearing in A Spiking Neural Network Model of Auditory Brainstem.

One method by which the mammalian sound localization pathway localizes sound sources is by analyzing the microsecond-level difference between the arrival times of a sound at the two ears. However, how the neural circuits in the auditory brainstem precisely integrate signals from the two ears, and what the underlying mechanisms are, remains to be understood. Recent studies have reported that variations of axon myelination in the auditory brainstem produces various axonal conduction velocities and sophisticated temporal dynamics, which have not been well characterized in most existing models of sound localization circuits. Here, we present a spiking neural network model of the auditory brainstem to investigate how axon myelinations affect the precision of sound localization. Sound waves with different interaural time differences (ITDs) are encoded and used as stimuli, and the axon properties in the network are adjusted, and the corresponding axonal conduction delays are computed with a multi-compartment axon model. Through the simulation, the sensitivity of ITD perception varies with the myelin thickness of axons in the contralateral input pathways to the medial superior olive (MSO). The ITD perception becomes more precise when the contralateral inhibitory input propagates faster than the contralateral excitatory input. These results indicate that axon myelination and contralateral spike timing influence spatial hearing perception.

Oxytocin modulates neural processing of mitral/tufted cells in the olfactory bulb.

AIM: Oxytocin plays an important role in social recognition in rodents, which is mediated predominantly by the olfactory system. Although oxytocin modulates neural activity in the olfactory bulb, the underlying mechanism is largely unknown. Here, we studied how direct infusion of oxytocin into the olfactory bulb affect social interactions in mice and modulate the neural activity of mitral/tufted cells in the olfactory bulb. METHODS: A three-chamber social interaction test was used in the behavioural test. For in vivo studies, single unit recordings, local field potential recordings and fibre photometry recordings were used to record the neural activity of olfactory bulb. For in vitro studies, we performed patch clamp recordings in the slice of the olfactory bulb. RESULTS: Behaviourally, direct oxytocin infusion in olfactory bulb increased performance in a social interaction task. Moreover, odour-evoked responses of mitral/tufted cells and neural discrimination of odours were both enhanced by oxytocin, whereas the spontaneous firing rate of mitral/tufted cells was reduced. At the neural network level, oxytocin decreased the amplitude of odour-evoked high gamma responses. At the cell population level, oxytocin decreased odour-evoked calcium responses (reflecting neural activity) specifically in granule cells. Moreover, in vitro slice recordings revealed that the inhibitory effect of oxytocin on mitral cell activity is mediated mainly by modulation of ATP-sensitive potassium channels and involves the oxytocin receptor-Gq-PLC-IP3 signalling pathway. CONCLUSION: Oxytocin modulates social interaction, likely by increasing the signal-to-noise ratio of odour responses in mitral cells which is partly through ATP-sensitive potassium channel.

Robotic stereotaxic system based on 3D skull reconstruction to improve surgical accuracy and speed.

BACKGROUND: Some experimental approaches in neuroscience research require the precise placement of a recording electrode, pipette or other tool into a specific brain area that can be quite small and/or located deep beneath the surface. This process is typically aided with stereotaxic methods but remains challenging due to a lack of advanced technology to aid the experimenter. Currently, procedures require a significant amount of skill, have a high failure rate, and take up a significant amount of time. NEW METHOD: We developed a next generation robotic stereotaxic platform for small rodents by combining a three-dimensional (3D) skull profiler sub-system and a full six degree-of-freedom (6DOF) robotic platform. The 3D skull profiler is based on structured illumination in which a series of horizontal and vertical line patterns are projected onto an animal skull. These patterns are captured by two two-dimensional (2D) CCD cameras which reconstruct an accurate 3D skull surface profile based on structured illumination and geometrical triangulation. Using the reconstructed 3D profile, the skull can be repositioned using a 6DOF robotic platform to accurately align a surgical tool. RESULTS: The system was evaluated using mechanical measurement techniques, and the accuracy of the platform was demonstrated using agar brain phantoms and animal skulls. Additionally, a small and deep brain nucleus (the medial nucleus of the trapezoid body) were targeted in rodents to confirm the targeting accuracy. CONCLUSIONS: The new stereotaxic system can accomplish "skull-flat" rapidly and precisely and with minimal user intervention, and thus reduces the failure rate of such experiments.

Influence of the Surface Material and Illumination upon the Performance of a Microelectrode/Electrolyte Interface in Optogenetics.

Integrated optrodes for optogenetics have been becoming a significant tool in neuroscience through the combination of offering accurate stimulation to target cells and recording biological signals simultaneously. This makes it not just be widely used in neuroscience researches, but also have a great potential to be employed in future treatments in clinical neurological diseases. To optimize the integrated optrodes, this paper aimed to investigate the influence of surface material and illumination upon the performance of the microelectrode/electrolyte interface and build a corresponding evaluation system. In this work, an integrated planar optrode with a blue LED and microelectrodes was designed and fabricated. The charge transfer mechanism on the interface was theoretically modeled and experimentally verified. An evaluation system for assessing microelectrodes was also built up. Using this system, the proposed model of various biocompatible surface materials on microelectrodes was further investigated under different illumination conditions. The influence of illumination on the microelectrode/electrolyte interface was the cause of optical artifacts, which interfere the biological signal recording. It was found that surface materials had a great effect on the charge transfer capacity, electrical stability and recoverability, photostability, and especially optical artifacts. The metal with better charge transfer capacity and electrical stability is highly possible to have a better performance on the optical artifacts, regardless of its electrical recoverability and photostability under the illumination conditions of optogenetics. Among the five metals used in our investigation, iridium served as the best surface material for the proposed integrated optrodes. Thus, optimizing the surface material for optrodes could reduce optical interference, enhance the quality of the neural signal recording for optogenetics, and thus help to advance the research in neuroscience.

Two-photon imaging of the trabecular meshwork.

PURPOSE: To image the trabecular meshwork (TM) in its native unfixed state using a non-invasive, non-destructive technique. METHODS: Two-photon microscopy (2PM), including two-photon excitation fluorescence (2PEF) and second harmonic generation (SHG), was used to image flat-mounted trabecular meshwork samples from human cadaver eyes. Multiple images were analyzed along the tissue axis (z-axis) to generate a three-dimensional (3D) model of the region. RESULTS: A lattice of large collagen fibers (approximately 10 microm in diameter) were detected by inherent fluorescence (2PEF) and SHG. There are regions of both tightly overlapping bundles as well as fluid-filled regions visible from the surface of the TM. 3D analysis of multiple images reveals that the open regions deep in the TM penetrate the juxtacanalicular TM (JTM) and connect to the inner wall of Schlemm's canal (IWSC). These open regions may represent low-resistance fluid pathways between the anterior chamber and Schlemm's canal (SC). CONCLUSIONS: 2PM imaging of the outflow system of the human eye documented collagenous structures solely from inherent optical properties, without addition of an exogenous fluorescent label. 2PM successfully imaged into the TM without the need for fixation, embedding, or histological processing. Deep penetration using advanced optical techniques revealed regions likely representing pores in the IWSC that have been documented by multiple electron microscope studies. Our work reveals that 2PM imaging has potential as a new metric for evaluating the aqueous outflow region of the human eye and is worthy of further exploration.

Evaluation of lensed fibers used in photodynamic therapy (PDT).

BACKGROUND: The combination of diode laser and lensed optical fiber is a commonly used technique in photodynamic therapy (PDT). The design of the lensed fiber can affect the beam quality, therefore, directly or indirectly effect treatment outcomes. In this study, several commercially available microlens and convex lens fibers used in PDT were evaluated. MATERIALS AND METHODS: A total of five types of lensed fibers were evaluated. The beam uniformity, expansion, and transmittance properties of lensed fibers to laser beams of 532 nm and 630 nm were analyzed. In addition, the spectral transmissions of lensed fibers were also examined using a broad band light source (400-1000 nm). RESULTS: Using microlens alone could significantly improve the light beam uniformity. The combination of lens and macrobendings could further improve the light beam uniformity. The beam expansion effect was determined by the numerical aperture (NA) of lens. The microlens fibers of higher NA showed better beam expansion effect than the convex lens of lower NA. The overall transmittance of tested lensed fibers ranged between 70 % and 77 %. CONCLUSIONS: Tested fibers showed good transmission property to the tested PDT wavelengths. Generally speaking, utilizing macrobending could improve the transmission quality of lens based frontal fiber-optic light distributors in PDT applications.

Computationally inexpensive enhanced growing neural gas algorithm for real-time adaptive neural spike clustering.

OBJECTIVE: Real-time closed-loop neural feedback control requires the analysis of action potential traces within several milliseconds after they have been recorded from the brain. The current generation of spike clustering algorithms were mostly designed for off-line use and also require a significant amount of computational resources. A new spike clustering algorithm, termed 'enhanced growing neural gas (EGNG)', was therefore developed that is computationally lightweight and memory conserving. The EGNG algorithm can adapt to changes of the electrophysiological recording environment and can classify both pre-recorded and streaming action potentials. APPROACH: The algorithm only uses a small number of EGNG nodes and edges to learn the neural spike distributions which eliminates the need of retaining the neural data in the system memory to conserve computational resources. Most of the computations revolve around calculating Euclidian distances, which is computationally inexpensive and can be implemented in parallel using digital circuit technology. MAIN RESULTS: EGNG was evaluated off-line using both synthetic and pre-recorded neural spikes. Streaming synthetic neural spikes were also used to evaluate the ability of EGNG to classify action potentials in real-time. The algorithm was also implemented in hardware with a Field Programming Gate Array (FPGA) chip, and the worst-case clustering latency was 3.10 µs, allowing a minimum of 322 580 neural spikes to be clustered per second. SIGNIFICANCE: The EGNG algorithm provides a viable solution to classification of neural spikes in real-time and can be implemented with limited computational resources as a front-end spike clustering unit for future tethered-free and miniaturized closed-loop neural feedback systems.

Quantitative assessment of skin swelling using optical coherence tomography.

BACKGROUND: Preclinical and clinical studies suggest that optical coherence tomography (OCT) is a useful tool to visualize inflammatory conditions. OBJECTIVE: The objective of this study was to evaluate the usefulness of combining OCT and various image processing techniques for quantitative assessment of histamine-induced tissue swelling. METHODS: Both time-domain and frequency-domain OCT were used on a mouse ear model. The ear thickness and volume before and after histamine challenge were determined from pixel locations in 2D scans and voxel number counting in 3D scans. Swelling kinetics was analyzed on 3D contour mapping. Microvessel network was visualized using speckle decorrelation analysis. RESULTS: OCT images showed that the thickness and volume changes were histamine dose and contact time dependent. The 3D mapping showed that the histamine-induced swelling spread slowly and directionally. OCT data indicated that microvessel opening and vessel dilation occurred prior to tissue swelling. CONCLUSION: OCT is a robust and quantitative non-invasive imaging tool for assessing skin swelling.

Low-latency single channel real-time neural spike sorting system based on template matching.

Recent technical advancements in neural engineering allow for precise recording and control of neural circuits simultaneously, opening up new opportunities for closed-loop neural control. In this work, a rapid spike sorting system was developed based on template matching to rapidly calculate instantaneous firing rates for each neuron in a multi-unit extracellular recording setting. Cluster templates were first generated by a desktop computer using a non-parameter spike sorting algorithm (Super-paramagnetic clustering) and then transferred to a field-programmable gate array digital circuit for rapid sorting through template matching. Two different matching techniques-Euclidean distance (ED) and correlational matching (CM)-were compared for the accuracy of sorting and the performance of calculating firing rates. The performance of the system was first verified using publicly available artificial data and was further confirmed with pre-recorded neural spikes from an anesthetized Mongolian gerbil. Real-time recording and sorting from an awake mouse were also conducted to confirm the system performance in a typical behavioral neuroscience experimental setting. Experimental results indicated that high sorting accuracies were achieved for both template-matching methods, but CM can better handle spikes with non-Gaussian spike distributions, making it more robust for in vivo recording. The technique was also compared to several other off-line spike sorting algorithms and the results indicated that the sorting accuracy is comparable but sorting time is significantly shorter than these other techniques. A low sorting latency of under 2 ms and a maximum spike sorting rate of 941 spikes/second have been achieved with our hybrid hardware/software system. The low sorting latency and fast sorting rate allow future system developments of neural circuit modulation through analyzing neural activities in real-time.