Closed-loop optogenetic control of the dynamics of neural activity in non-human primates.

Electrical neurostimulation is effective in the treatment of neurological disorders, but associated recording artefacts generally limit its applications to open-loop stimuli. Real-time and continuous closed-loop control of brain activity can, however, be achieved by pairing concurrent electrical recordings and optogenetics. Here we show that closed-loop optogenetic stimulation with excitatory opsins enables the precise manipulation of neural dynamics in brain slices from transgenic mice and in anaesthetized non-human primates. The approach generates oscillations in quiescent tissue, enhances or suppresses endogenous patterns in active tissue and modulates seizure-like bursts elicited by the convulsant 4-aminopyridine. A nonlinear model of the phase-dependent effects of optical stimulation reproduced the modulation of cycles of local-field potentials associated with seizure oscillations, as evidenced by the systematic changes in the variability and entropy of the phase-space trajectories of seizures, which correlated with changes in their duration and intensity. We also show that closed-loop optogenetic neurostimulation could be delivered using intracortical optrodes incorporating light-emitting diodes. Closed-loop optogenetic approaches may be translatable to therapeutic applications in humans.

High-frequency limit of neural stimulation with ChR2.

Optogenetic technology based on light activation of genetically targeted single component opsins such as Channelrhodopsin-2 (ChR2) has been changing the way neuroscience research is conducted. This technology is becoming increasingly important for neural engineering as well. The efficiency of neural stimulation with ChR2 drops at high frequencies, often before the natural limit of the neuron is reached. This study aims to investigate the underlying mechanisms that limit the efficiency of the stimulation at high frequencies. The study analyzes the dynamics of the spikes induced by ChR2 in comparison to control stimulations using patch clamp current injection. It shows that the stimulation dynamics is limited by two mechanisms: 1) a frequency independent reduction in the conductance-to-irradiance yield due to the ChR2 light adaptation process and 2) a frequency dependent reduction in the conductance-to-current yield due to a decrease in membrane re-polarization level between spikes that weakens the ionic driving force. The effect of the first mechanism can be minimized by using ChR2 mutants with lower irradiance threshold. In contrast the effect of the second mechanism is fundamentally limited by the rate the native ion channels re-polarize the membrane potential.

Insertion experiments of a biologically inspired microtextured and multi-part probe based on reciprocal motion.

While there have been significant advances in minimally invasive surgical instrumentation, the majority of tools still rely on a push from the back to aid insertion into the tissue, whether the process is manual or servo assisted. In this work, a novel approach to tool insertion is proposed which is based on the concept of a multi-part probe with at least three interlocking segments. By means of a sequential insertion process, where each segment is pushed further into the tissue while stabilized by the remaining stationary parts, the multi-part probe concept is shown to successfully "insinuate itself" within a synthetic soft tissue specimen without the need for an overall forward push. The presence of an anisotropic microtextured outer probe surface is also shown to affect the overall speed of insertion and can thus be used to optimize the interaction forces at the probe-tissue interface. A measured reduction in the force transferred to the back of the specimen also suggests that this approach to tool insertion may result in reduced tissue disruption, a result which could lead to less tissue damage and a reduction in target displacement.

A New Individually Addressable Micro-LED Array for Photogenetic Neural Stimulation.

Here, we demonstrate the use of a micro light emitting diode (LED) array as a powerful tool for complex spatiotemporal control of photosensitized neurons. The array can generate arbitrary, 2-D, excitation patterns with millisecond and micrometer resolution. In particular, we describe an active matrix control address system to allow simultaneous control of 256 individual micro LEDs. We present the system optically integrated into a microscope environment and patch clamp electrophysiology. The results show that the emitters have sufficient radiance at the required wavelength to stimulate neurons expressing channelrhodopsin-2 (ChR2).

Biologically inspired microtexturing: investigation into the surface topography of next-generation neurosurgical probes.

Minimally Invasive (MI) surgery represents the future of many types of medical intervention (keyhole neurosurgery, natural orifice trans-luminal endoscopic surgery, etc.). However, the shortcomings of today's surgical tools fuel the need for the development of next-generation 'smart instrumentation', which will be more accurate and safer for the patient. This paper presents the preliminary results of a biologically inspired microtexturing method, based on UV-lithography, and its application to MI neurosurgery. These results suggest that the size and geometry of the texture 'printed' on the outer surface of a neurosurgical probe clearly affect the insertion and extraction forces generated at the brain-probe interface. Thus, by carefully choosing an appropriate microtexture, unique insertion characteristics can be obtained, which can improve the performance of existing instruments (e.g. reducing slippage in permanent electrodes such as those used in deep brain stimulation) or enable the development of novel designs altogether.

A non-invasive retinal prosthesis - testing the concept.

We have developed a testing platform for a novel type of retinal prosthesis. Our system uses an array of light sources as non-contact stimulators. The platform consists of an imaging system based on a CMOS camera, PC based image processing, and a stimulation address system carried out on a Field Programmable Gated Array which addresses a matrix array of LEDs. Special optics are used to focus the light from the LED array onto light sensitized cells.

A method for micrometer resolution patterning of primary culture neurons for SPM analysis.

In this work we present a method for ultra-fine patterning of primary culture neuron cell growth, which is compatible for scanning near-field optical atomic force microscopy (SNOAM) analysis. SNOAM uses near-field optics to break the fundamental diffraction limit imposed on normal microscopy. SNOAM can achieve sub-100 nm optical resolutions, but requires transparent, open substrates. The ability to do physiological measurements on patterns of neurons, combined with ultra high resolution optical and fluorescent analysis, is useful in the study of long-term potentiation. The patterning method consists of chemical guidance with an element of physical confinement and allows for ultra-fine patterning of neural growth on transparent glass substrates. Substrates consist of microfabricated perfluoropolymer barrier structures on glass. Poly-L-lysine was selectively deposited using a silicone-based microfluidic stencil aligned to the perfluoropolymer/glass substrate. Primary culture neurons were extracted from 8-day-old chicks and grown for 3 days to form good networks. This patterning system shows very specific growth with patterning separations down to the level of individual neurites. Fluorescent imaging was carried out on both cell viability during growth and immuno-tagged microtubule-associated proteins on the neurites. Neurons inside the patterned structures were imaged and analyzed with a tapping mode SNOAM.