Nairouz Farah

Patterned Optical Activation of Retinal Ganglion Cells

Neuroprosthetic retinal interfaces depend upon the ability to bypass the damaged photoreceptor layer and directly activate populations of retinal ganglion cells (RGCs). To date, the preferred approach to this task largely relies on electrode array implants. We are currently pursuing two alternative methods for light-based direct activation of the RGCs. The first method is based on applying caged glutamate over the retina and uncaging it locally to obtain RGC excitation. The second method is to artificially cause RGCs to express Channelrhodopsin II (ChR2), a light-gated cation channel. In addition to being non-contact, optical techniques lend themselves relatively easily to a variety of technologies for achieving patterned stimulation with high temporal and spatial resolution. Using the Texas Instruments Digital Light Processing (DLP - DMD) technology, we have developed an optical stimulation system capable of controlled, large-scale, flexible stimulation of the retinal tissue with high temporal accuracy. In preliminary studies, we are performing patterned photo-stimulation experiments using samples of caged fluorescent probes and in rat retinas that were virally transfected with ChR2.

Holographic optogenetic stimulation of patterned neuronal activity for vision restoration

When natural photoreception is disrupted, as in outer-retinal degenerative diseases, artificial stimulation of surviving nerve cells offers a potential strategy for bypassing compromised neural circuits. Recently, light-sensitive proteins that photosensitize quiescent neurons have generated unprecedented opportunities for optogenetic neuronal control, inspiring early development of optical retinal prostheses. Selectively exciting large neural populations are essential for eliciting meaningful perceptions in the brain. Here we provide the first demonstration of holographic photo-stimulation strategies for bionic vision restoration. In blind retinas, we demonstrate reliable holographically patterned optogenetic stimulation of retinal ganglion cells with millisecond temporal precision and cellular resolution. Holographic excitation strategies could enable flexible control over distributed neuronal circuits, potentially paving the way towards high-acuity vision restoration devices and additional medical and scientific neuro-photonics applications.

Design and characteristics of holographic neural photo-stimulation systems.

Computer-generated holography is an emerging technology for stimulation of neuronal populations with light patterns. A holographic photo-stimulation system may be designed as a powerful research tool or a compact neural interface medical device, such as an optical retinal prosthesis. We present here an overview of the main design issues including the choice of holographic device, field-of-view, resolution, physical size, generation of two- and three-dimensional patterns and their diffraction efficiency, choice of algorithms and computational effort. The performance and characteristics of a holographic pattern stimulation system with kHz frame rates are demonstrated using experimental recordings from isolated retinas.

Holographically patterned activation using photo-absorber induced neural–thermal stimulation

OBJECTIVE Patterned photo-stimulation offers a promising path towards the effective control of distributed neuronal circuits. Here, we demonstrate the feasibility and governing principles of spatiotemporally patterned microscopic photo-absorber induced neural-thermal stimulation (PAINTS) based on light absorption by exogenous extracellular photo-absorbers. APPROACH We projected holographic light patterns from a green continuous-wave (CW) or an IR femtosecond laser onto exogenous photo-absorbing particles dispersed in the vicinity of cultured rat cortical cells. Experimental results are compared to predictions of a temperature-rate model (where membrane currents follow I ∝ dT/dt). MAIN RESULTS The induced microscopic photo-thermal transients have sub-millisecond thermal relaxation times and stimulate adjacent cells. PAINTS activation thresholds for different laser pulse durations (0.02 to 1 ms) follow the Lapicque strength-duration formula, but with different chronaxies and minimal threshold energy levels for the two excitation lasers (an order of magnitude lower for the IR system <50 nJ). Moreover, the empirical thresholds for the CW system are found to be in good agreement with detailed simulations of the temperature-rate model, but are generally lower for the IR system, suggesting an auxiliary excitation mechanism. SIGNIFICANCE Holographically patterned PAINTS could potentially provide a means for minimally intrusive control over neuronal dynamics with a high level of spatial and temporal selectivity.

Correction-free remotely scanned two-photon in vivo mouse retinal imaging

Non-invasive fluorescence retinal imaging in small animals is an important requirement for an array of translational vision applications. The in vivo two-photon imaging of the mouse retina may enable the long-term investigation of the structure and function of healthy and diseased retinal tissue. However, to date, this has only been possible using relatively complex adaptive-optics systems. Here, the optical modeling of the murine eye and of the imaging system is used to achieve correction-free two-photon microscopy through the pupil of a mouse eye to yield high-quality, optically sectioned fundus images. By remotely scanning the focus using an electronically tunable lens, high-resolution three-dimensional fluorescein angiograms and cellular-scale images are acquired, thus introducing a correction-free baseline performance level for two-photon in vivo retinal imaging. Moreover, the system enables functional calcium imaging of repeated retinal responses to light stimulation using the genetically encoded indicator, GCaMP6s. These results and the simplicity of the new add-on optics are an important step toward several structural, functional, and multimodal imaging applications that will benefit from the tight optical sectioning and the use of near-infrared light.

Patterned optical activation of Channelrhodopsin II expressing retinal ganglion cells

Neuroprosthetic retinal interfaces depend upon the ability to bypass the damaged photoreceptor layer and directly activate populations of retinal ganglion cells (RGCs). Current approaches to this task largely rely on electrode array implants. We are pursuing an alternative, light-based approach towards direct activation of the RGCs, by artificially causing them to express Channelrhodopsin II (ChR2), a light-gated cation channel. In addition to being non-contact, optical techniques lend themselves relatively easily to a variety of technologies for achieving patterned stimulation with high temporal and spatial resolution. In early studies, we are using viral vectors to obtain wide spread expression of ChR2 in rat retinas, and have developed a system capable of controlled large-scale, flexible stimulation of the retinal tissue with high temporal accuracy through adaptations of video projection technology. Finally, we demonstrate a PC-based wearable system that can perform the image processing transformations required for optical retinal neuroprosthetic interfaces in real time.

Two-photon in vivo imaging of retinal microstructures

Non-invasive fluorescence retinal imaging in small animals is an important requirement in an array of translational vision applications. Two-photon imaging has the potential for long-term investigation of healthy and diseased retinal function and structure in vivo. Here, we demonstrate that two-photon microscopy through a mouse’s pupil can yield high-quality optically sectioned fundus images. By remotely scanning using an electronically tunable lens we acquire highly-resolved 3D fluorescein angiograms. These results provide an important step towards various applications that will benefit from the use of infrared light, including functional imaging of retinal responses to light stimulation.

Holographic fiber bundle system for patterned optogenetic activation of large-scale neuronal networks

Abstract. Optogenetic perturbation has become a fundamental tool in controlling activity in neurons. Used to control activity in cell cultures, slice preparations, anesthetized and awake behaving animals, optical control of cell-type specific activity enables the interrogation of complex systems. A remaining challenge in developing optical control tools is the ability to produce defined light patterns such that power-efficient, precise control of neuronal populations is obtained. Here, we describe a system for patterned stimulation that enables the generation of structured activity in neurons by transmitting optical patterns from computer-generated holograms through an optical fiber bundle. The system couples the optical system to versatile fiber bundle configurations, including coherent or incoherent bundles composed of hundreds of up to several meters long fibers. We describe the components of the system, a method for calibration, and a detailed power efficiency and spatial specificity quantification. Next, we use the system to precisely control single-cell activity as measured by extracellular electrophysiological recordings in ChR2-expressing cortical cell cultures. The described system complements recent descriptions of optical control systems, presenting a system suitable for high-resolution spatiotemporal optical control of wide-area neural networks in vitro and in vivo, yielding a tool for precise neural system interrogation.

Holographic photo-stimulation for dynamic control of neuronal population activity

Spatiotemporal patterns of activity carried across large populations of neurons are the fundamental representation of information within the nervous system. Patterned optical photo-stimulation of neural populations provides a general strategy for controlling such spatial-temporal patterns, but previous realizations of this technology did not allow high-rate, parallel, light-efficient control of large neural populations. To address this challenge, we have been developing a new generation of holographic pattern photo-stimulation systems that use phase-only spatial light modulators (SLM) to create computer-controlled light patterns in two and three dimensions. SLMs use phase modulation and light diffraction to provide a light-efficient method for flexibly creating desired light patterns which can be switched in our systems in millisecond timescales. Holographic photo-stimulation provides a powerful strategy for dynamic patterned photo-stimulation of neural populations and could be used in research and neuroprosthetic interfaces.

Simulation of morphologically structured photo-thermal neural stimulation

OBJECTIVE Rational design of next-generation techniques for photo-thermal excitation requires the development of tools capable of modeling the effects of spatially- and temporally-dependent temperature distribution on cellular neuronal structures. APPROACH We present a new computer simulation tool for predicting the effects of arbitrary spatiotemporally-structured photo-thermal stimulation on 3D, morphologically realistic neurons. The new simulation tool is based on interfacing two generic platforms, NEURON and MATLAB and is therefore suited for capturing different kinds of stimuli and neural models. MAIN RESULTS Simulation results are validated using photo-absorber induced neuro-thermal stimulation (PAINTS) empirical results, and advanced features are explored. SIGNIFICANCE The new simulation tool could have an important role in understanding and investigating complex optical stimulation at the single-cell and network levels.