Optical lever recording of displacements from activated lobster nerve bundles and Nitella internodes

Toward a clinical optoretinogram: a review of noninvasive, optical tests of retinal neural function

The past few years have witnessed rapid development of the optoretinogram-a noninvasive, optical measurement of neural function in the retina, and especially the photoreceptors (Ph). While its recent development has been rapid, it represents the culmination of hundreds of experiments spanning decades. Early work showed measurable and reproducible changes in the optical properties of retinal explants and suspensions of Ph, and uncovered some of the biophysical and biochemical mechanisms underlying them. That work thus provided critical motivation for more recent work based on clinical imaging platforms, whose eventual goal is the improvement of ophthalmic care and streamlining the discovery of novel therapeutics. The first part of this review consists of a selective summary of the early work, and identifies four kinds of stimulus-evoked optical signals that have emerged from it: changes in light scattered from the membranous discs of the Ph's outer segment (OS), changes in light scattered by the front and back boundaries of the OS, rearrangement of scattering material in and near the OS, and changes in the OS length. In the past decade, all four of these signals have continued to be investigated using imaging systems already used in the clinic or intended for clinical and translational use. The second part of this review discusses these imaging modalities, their potential to detect and quantify the signals of interest, and other factors influencing their translational promise. Particular attention is paid to phase-sensitive optical coherence tomography (OCT) with adaptive optics (AO), a method in which both the amplitude and the phase of light reflected from individual Ph is monitored as visible stimuli are delivered to them. The record of the light's phase is decoded to reveal a reproducible pattern of deformation in the OS, while the amplitude reveals changes in scattering and structural rearrangements. The method has been demonstrated in a few labs and has been used to measure responses from both rods and cones. With the ability to detect responses to stimuli isomerizing less than 0.01% of photopigment, this technique may prove to be a quick, noninvasive, and objective way to measure subtle disease-related dysfunction at the cellular level, and thus to provide an entirely new and complementary biomarker for retinal disease and recovery.

Optical Electrophysiology: Toward the Goal of Label-Free Voltage Imaging

Measuring and monitoring the electrical signals transmitted between neurons is key to understanding the communication between neurons that underlies human perception, information processing, and decision-making. While electrode-based electrophysiology has been the gold standard, optical electrophysiology has opened up a new area in the past decade. Voltage-dependent fluorescent reporters enable voltage imaging with high spatial resolution and flexibility to choose recording locations. However, they exhibit photobleaching as well as phototoxicity and may perturb the physiology of the cell. Label-free optical electrophysiology seeks to overcome these hurdles by detecting electrical activities optically, without the incorporation of exogenous fluorophores in cells. For example, electrochromic optical recording detects neuroelectrical signals via a voltage-dependent color change of extracellular materials, and interferometric optical recording monitors membrane deformations that accompany electrical activities. Label-free optical electrophysiology, however, is in an early stage, and often has limited sensitivity and temporal resolution. In this Perspective, we review the recent progress to overcome these hurdles. We hope this Perspective will inspire developments of label-free optical electrophysiology techniques with high recording sensitivity and temporal resolution in the near future.

What Triggers the Interictal Epileptic Spike? A Multimodal Multiscale Analysis of the Dynamic of Synaptic and Non-synaptic Neuronal and Vascular Compartments Using Electrical and Optical Measurements

Interictal spikes (IISs) may result from a disturbance of the intimate functional balance between various neuronal (synaptic and non-synaptic), vascular, and metabolic compartments. To better characterize the complex interactions within these compartments at different scales we developed a simultaneous multimodal-multiscale approach and measure their activity around the time of the IIS. We performed such measurements in an epileptic rat model (n = 43). We thus evaluated (1) synaptic dynamics by combining electrocorticography and multiunit activity recording in the time and time-frequency domain, (2) non-synaptic dynamics by recording modifications in light scattering induced by changes in the membrane configuration related to cell activity using the fast optical signal, and (3) vascular dynamics using functional near-infrared spectroscopy and, independently but simultaneously to the electrocorticography, the changes in cerebral blood flow using diffuse correlation spectroscopy. The first observed alterations in the measured signals occurred in the hemodynamic compartments a few seconds before the peak of the IIS. These hemodynamic changes were followed by changes in coherence and then synchronization between the deep and superficial neural networks in the 1 s preceding the IIS peaks. Finally, changes in light scattering before the epileptic spikes suggest a change in membrane configuration before the IIS. Our multimodal, multiscale approach highlights the complexity of (1) interactions between the various neuronal, vascular, and extracellular compartments, (2) neural interactions between various layers, (3) the synaptic mechanisms (coherence and synchronization), and (4) non-synaptic mechanisms that take place in the neuronal network around the time of the IISs in a very specific cerebral hemodynamic environment.

Cell Surface Deformation during an Action Potential

The excitation of many cells and tissues is associated with cell mechanical changes. The evidence presented herein corroborates that single cells deform during an action potential. It is demonstrated that excitation of plant cells (Chara braunii internodes) is accompanied by out-of-plane displacements of the cell surface in the micrometer range (∼1-10 μm). The onset of cellular deformation coincides with the depolarization phase of the action potential. The mechanical pulse: 1) propagates with the same velocity as the electrical pulse (within experimental accuracy, ∼10 mm s^-1), 2) is reversible, 3) in most cases is of biphasic nature (109 out of 152 experiments), and 4) is presumably independent of actin-myosin-motility. The existence of transient mechanical changes in the cell cortex is confirmed by micropipette aspiration experiments. A theoretical analysis demonstrates that this observation can be explained by a reversible change in the mechanical properties of the cell surface (transmembrane pressure, surface tension, and bending rigidity). Taken together, these findings contribute to the ongoing debate about the physical nature of cellular excitability.

Functional optical coherence tomography of retinal photoreceptors

IMPACT STATEMENT

Retinal photoreceptors are the primary target of age-related macular degeneration (AMD) which is the leading cause of severe vision loss and legal blindness. An objective method for functional assessment of photoreceptor physiology can benefit early detection and better treatment evaluation of AMD and other eye diseases that are known to cause photoreceptor dysfunctions. This article summarizes in vitro study of IOS mechanisms and in vivo demonstration of IOS imaging of intact animals. Further development of the functional IOS imaging may provide a revolutionary solution to achieve objective assessment of human photoreceptors.

Imaging Action Potential in Single Mammalian Neurons by Tracking the Accompanying Sub-Nanometer Mechanical Motion

Action potentials in neurons have been studied traditionally by intracellular electrophysiological recordings and more recently by the fluorescence detection methods. Here we describe a label-free optical imaging method that can measure mechanical motion in single cells with a sub-nanometer detection limit. Using the method, we have observed sub-nanometer mechanical motion accompanying the action potential in single mammalian neurons by averaging the repeated action potential spikes. The shape and width of the transient displacement are similar to those of the electrically recorded action potential, but the amplitude varies from neuron to neuron, and from one region of a neuron to another, ranging from 0.2-0.4 nm. The work indicates that action potentials may be studied noninvasively in single mammalian neurons by label-free imaging of the accompanying sub-nanometer mechanical motion.

Surface deformation during an action potential in pearled cells

Electric pulses in biological cells (action potentials) have been reported to be accompanied by a propagating cell-surface deformation with a nanoscale amplitude. Typically, this cell surface is covered by external layers of polymer material (extracellular matrix, cell wall material, etc.). It was recently demonstrated in excitable plant cells (Chara braunii) that the rigid external layer (cell wall) hinders the underlying deformation. When the cell membrane was separated from the cell wall by osmosis, a mechanical deformation, in the micrometer range, was observed upon excitation of the cell. The underlying mechanism of this mechanical pulse has, to date, remained elusive. Herein we report that Chara cells can undergo a pearling instability, and when the pearled fragments were excited even larger and more regular cell shape changes were observed (∼10-100μm in amplitude). These transient cellular deformations were captured by a curvature model that is based on three parameters: surface tension, bending rigidity, and pressure difference across the surface. In this paper these parameters are extracted by curve-fitting to the experimental cellular shapes at rest and during excitation. This is a necessary step to identify the mechanical parameters that change during an action potential.

Shedding light on interictal epileptic spikes: An in vivo study using fast optical signal and electrocorticography

OBJECTIVE

Interictal epileptic spikes (IESs), apart from being a key marker of epileptic neuronal networks, constitute a nice model of the widespread endogenous phenomenon of neuronal hypersynchronization. Many questions concerning the mechanisms that drive neurons to hypersynchronize remain unresolved, but synaptic as well as nonsynaptic events are likely to be involved. In this study, changes in optical properties of neural tissues were observed in rats with penicillin-induced IES using fast optical signal (FOS) concomitantly with electrocorticography (ECoG).

METHODS

In this study, near-infrared optical imaging was used with ECoG to investigate variations in the optical properties of cortical tissue directly associated with neuronal activity in 15 rats. FOS changes correspond to variations of scattered light from neuronal tissue when neurons are activated. To independently evaluate our method, a control experiment on somatosensory was designed and applied to seven different rats. Time-frequency analysis was also used to track variations of (de)synchronization concomitantly with changes in optical signals during IES.

RESULTS

FOS responses revealed that changes in optical signals occurred 320 msec before to 370 msec after the IES peak. These changes started before any changes in ECoG signal. In addition, time-frequency domain electrocorticography revealed an alternating decrease-increase-decrease in the ECoG spectral power (pointing to desynchronization-synchronization-desynchronization), which occurred concomitantly with an increase-decrease-increase in relative optical signal during the IES. These results suggest a relationship between (de)synchronization and optical changes.

SIGNIFICANCE

These changes in the neuronal environment around IESs raise new questions about the mechanisms that induce changes in optical properties of neural tissues before the IES, which may provide suitable conditions for neuronal synchronization during IESs. FOS-ECoG constitutes a multimodal approach and opens new avenues to study the mechanisms of neuronal synchronization in the pathologic brain, which has clinical implications, at least in epilepsy.

Stimulus-evoked outer segment changes in rod photoreceptors

Rod-dominated transient retinal phototropism (TRP) has been recently observed in freshly isolated mouse and frog retinas. Comparative confocal microscopy and optical coherence tomography revealed that the TRP was predominantly elicited from the rod outer segment (OS). However, the biophysical mechanism of rod OS dynamics is still unknown. Mouse and frog retinal slices, which displayed a cross-section of retinal photoreceptors and other functional layers, were used to test the effect of light stimulation on rod OSs. Time-lapse microscopy revealed stimulus-evoked conformational changes of rod OSs. In the center of the stimulated region, the length of the rod OS shrunk, while in the peripheral region, the rod OS swung toward the center region. Our experimental observation and theoretical analysis suggest that the TRP may reflect unbalanced rod disc-shape changes due to localized visible light stimulation.

Elucidating the temporal dynamics of optical birefringence changes in crustacean nerves

Intrinsic optical properties, such as optical birefringence, may serve as a tool for minimally invasive neuroimaging methods with high spatiotemporal resolution to aid in the study of neuronal activation patterns. To facilitate imaging neuronal activity by sensing dynamic birefringence, temporal characteristics behind the signal must be better understood. We have developed a novel nerve chamber to investigate changes in birefringence at the stimulation site, and at distances ~4-28 mm from that site. Using crustacean nerves with either heterogeneous or homogeneous size distributions of axon diameters, we found that the gradual (slow) recovery of the crossed-polarized signal is not explained by the arrival times of action potentials in smaller axons. Through studying the effects of stimulating current and voltage pulses, we hypothesize that the recovery may be caused by a capacitive-like coupling between firing axons and adjacent tissue structures, and we report data consistent with this hypothesis. This study will aid in the utilization of action-potential-related changes in birefringence to study fast changes in neuronal network activity.

Intrinsic optical signal imaging of retinal physiology: a review

Intrinsic optical signal (IOS) imaging promises to be a noninvasive method for high-resolution examination of retinal physiology, which can advance the study and diagnosis of eye diseases. While specialized optical instruments are desirable for functional IOS imaging of retinal physiology, in depth understanding of multiple IOS sources in the complex retinal neural network is essential for optimizing instrument designs. We provide a brief overview of IOS studies and relationships in rod outer segment suspensions, isolated retinas, and intact eyes. Recent developments of line-scan confocal and functional optical coherence tomography (OCT) instruments have allowed in vivo IOS mapping of photoreceptor physiology. Further improvements of the line-scan confocal and functional OCT systems may provide a feasible solution to pursue functional IOS mapping of human photoreceptors. Some interesting IOSs have already been detected in inner retinal layers, but better development of the IOS instruments and software algorithms is required to achieve optimal physiological assessment of inner retinal neurons.

Optical coherence tomography for cross-sectional imaging of neural activity

We report a functional optical coherence tomography cross-sectional scanner to detect neural activity using unmyelinated nerves dissected from squid. The nerves, unstained or stained with a voltage-sensitive dye, were imaged in a nerve chamber. Transient phase changes from backscattered light were detected during action potential propagation. The results show that the scanner can provide high spatiotemporal resolution cross-sectional images of neural activity ([Formula: see text]; [Formula: see text]; [Formula: see text] in [Formula: see text]). The advantage of this method compared to monitoring a single depth profile [Formula: see text] is a dramatic increase in the number of available sites that can be measured in two spatial dimensions [Formula: see text] with lateral scanning; therefore, the study demonstrates that two-dimensional monitoring of small-scale functional activity would also be feasible.

En face optical coherence tomography of transient light response at photoreceptor outer segments in living frog eyecup

This study was designed to test the feasibility of en face mapping of the transient intrinsic optical signal (IOS) response at photoreceptor outer segments and to assess the effect of spatial resolution on functional IOS imaging of retinal photoreceptors. A line-scan optical coherence tomography (LS-OCT) was constructed to achieve depth-resolved functional IOS imaging of living frog eyecups. Rapid en face OCT revealed transient IOS almost immediately (<3 ms) after the onset of visible light flashes at photoreceptor outer segments. Quantitative analysis indicated that the IOS kinetics may reflect dynamics of G-protein binding and releasing in early phases of visual transduction, and high resolution is essential to differentiate positive and negative IOS changes in adjacent locations.

Label-free imaging of membrane potential using membrane electromotility

Electrical activity may cause observable changes in a cell's structure in the absence of exogenous reporter molecules. In this work, we report a low-coherence interferometric microscopy technique that can detect an optical signal correlated with the membrane potential changes in individual mammalian cells without exogenous labels. By measuring milliradian-scale phase shifts in the transmitted light, we can detect changes in the cells' membrane potential. We find that the observed optical signals are due to membrane electromotility, which causes the cells to deform in response to the membrane potential changes. We demonstrate wide-field imaging of the propagation of electrical stimuli in gap-junction-coupled cell networks. Membrane electromotility-induced cell deformation may be useful as a reporter of electrical activity.

Dynamic intrinsic optical signal monitoring of electrically stimulated inner retinal neural response

Dynamic monitoring of stimulus-evoked inner neural response is important for functional validation of stimulation protocols of retinal prosthetic devices. In this paper, we demonstrate label-free intrinsic optical signal (IOS) imaging of electrically stimulated inner neural response in freshly isolated mouse retinas. While single-pulse stimulation evoked rapid IOS within 20 ms, pulse-train stimulation indicated that the fast IOS response can follow frequency stimulation up to at least 8 Hz. Fast IOS imaging promises a noninvasive method for high resolution examination of electrically evoked retinal response, without artifact contamination of electrical stimulus.

Comparative intrinsic optical signal imaging of wild-type and mutant mouse retinas

Functional measurement is important for retinal study and disease diagnosis. Transient intrinsic optical signal (IOS) response, tightly correlated with functional stimulation, has been previously detected in normal retinas. In this paper, comparative IOS imaging of wild-type (WT) and rod-degenerated mutant mouse retinas is reported. Both 2-month and 1-year-old mice were measured. In 2-month-old mutant mice, time course and peak value of the stimulus-evoked IOS were significantly delayed (relative to stimulus onset) and reduced, respectively, compared to age matched WT mice. In 1-year-old mutant mice, stimulus-evoked IOS was totally absent. However, enhanced spontaneous IOS responses, which might reflect inner neural remodeling in diseased retina, were observed in both 2-month and 1-year-old mutant retinas. Our experiments demonstrate the potential of using IOS imaging for noninvasive and high resolution identification of disease-associated retinal dysfunctions. Moreover, high spatiotemporal resolution IOS imaging may also lead to advanced understanding of disease-associated neural remodeling in the retina.

Parallel optical monitoring of visual signal propagation from the photoreceptors to the inner retina layers

Understanding of visual signal processing can benefit from simultaneous measurement of different types of retinal neurons working together. In this Letter, we demonstrate that intrinsic optical signal (IOS) imaging of frog retina slices allows simultaneous observation of stimulus-evoked responses propagating from the photoreceptors to the inner neurons. High-resolution imaging revealed robust IOSs at the photoreceptor, the inner plexiform, and the ganglion cell layers. While IOSs of the photoreceptor layer were mainly confined to the area directly stimulated by the visible light, IOSs of the inner retinal layers spread from the stimulus site into relatively large areas with a characteristic near-to-far time course.

High spatiotemporal resolution imaging of fast intrinsic optical signals activated by retinal flicker stimulation

High resolution monitoring of stimulus-evoked retinal neural activities is important for understanding retinal neural mechanisms, and can be a powerful tool for retinal disease diagnosis and treatment outcome evaluation. Fast intrinsic optical signals (IOSs), which have the time courses comparable to that of electrophysiological activities in the retina, hold the promise for high resolution imaging of retinal neural activities. However, application of fast IOS imaging has been hindered by the contamination of slow, high magnitude optical responses associated with transient hemodynamic and metabolic changes. In this paper we demonstrate the feasibility of separating fast retinal IOSs from slow optical responses by combining flicker stimulation and dynamic (temporal) differential image processing. A near infrared flood-illumination microscope equipped with a high-speed (1000 Hz) digital camera was used to conduct concurrent optical imaging and ERG measurement of isolated frog retinas. High spatiotemporal resolution imaging revealed that fast IOSs could follow flicker frequency up to at least 6 Hz. Comparable time courses of fast IOSs and ERG kinetics provide evidence that fast IOSs are originated from stimulus activated retinal neurons.

Measurement of intrinsic optical backscattering characteristics of cells using fiber-guided near infrared light

BACKGROUND

Intrinsic optical signals (IOS), which reflect changes in transmittance and scattering light, have been applied to characterize the physiological conditions of target biological tissues. Backscattering approaches allow mounting of the source and detector on the same side of a sample which creates a more compact physical layout of device. This study presents a compact backscattering design using fiber-optic guided near-infrared (NIR) light to measure the amplitude and phase changes of IOS under different osmotic challenges.

METHODS

High-frequency intensity-modulated light was guided via optic fiber, which was controlled by micromanipulator to closely aim at a minimum cluster of cortical neurons. Several factors including the probe design, wavelength selection, optimal measuring distance between the fiber-optical probe and cells were considered. Our experimental setup was tested in cultured cells to observe the relationship between the changes in backscattered NIR light and cellular IOS, which is believed mainly caused by cell volume changes in hypo/hyperosmotic solutions (+/- 20, +/- 40 and +/- 60 mOsm).

RESULTS

The critical parameters of the current setup including the optimal measuring distance from fiber-optical probe to target tissue and the linear relationship between backscattering intensity and cell volume were determined. The backscattering intensity was found to be inversely proportional to osmotic changes. However, the phase shift exhibited a nonlinear feature and reached a plateau at hyperosmotic solution.

CONCLUSIONS

Our study indicated that the backscattering NIR light guided by fiber-optical probe makes it a potential alternative for continuous observation of intrinsic optical properties of cell culture under varied physical or chemical challenges.

Optical coherence tomography phase measurement of transient changes in squid giant axons during activity

Noncontact optical measurements reveal that transient changes in squid giant axons are associated with action potential propagation and altered under different environmental (i.e., temperature) and physiological (i.e., ionic concentrations) conditions. Using a spectral-domain optical coherence tomography system, which produces real-time cross-sectional images of the axon in a nerve chamber, axonal surfaces along a depth profile are monitored. Differential phase analyses show transient changes around the membrane on a millisecond timescale, and the response is coincident with the arrival of the action potential at the optical measurement area. Cooling the axon slows the electrical and optical responses and increases the magnitude of the transient signals. Increasing the NaCl concentration bathing the axon, whose diameter is decreased in the hypertonic solution, results in significantly larger transient signals during action potential propagation. While monophasic and biphasic behaviors are observed, biphasic behavior dominates the results. The initial phase detected was constant for a single location but alternated for different locations; therefore, these transient signals acquired around the membrane appear to have local characteristics.

Optical dissection of stimulus-evoked retinal activation

Better understanding of stimulus-evoked intrinsic optical signals (IOSs) in the retina promises new methodology for study and diagnosis of retinal function. Using a flood-illumination near infrared (NIR) light microscope equipped with high-speed CCD (80 Hz) and CMOS (1000 Hz) cameras, we validated depth-resolved enface imaging of fast IOSs in isolated retina of leopard frog. Both positive (increasing) and negative (decreasing) IOSs were observed at the photoreceptor and inner layers of the retina. The distribution of IOSs with opposite polarities showed a center-surround pattern. At the photoreceptor layer, negative IOSs dominated the center area illuminated by the stimulus light spot, while positive signals dominated the surrounding area. In contrast, at inner retinal layers, positive IOSs dominated the center area covered by the stimulus light spot, and negative IOSs were mainly observed in the surrounding area. Fast CMOS imaging disclosed rapid IOSs within 5 ms after the stimulus onset, and both ON and OFF optical responses were observed associated with a step light stimulus.

In vitro and in vivo noise analysis for optical neural recording

Laser diodes (LD) are commonly used for optical neural recordings in chronically recorded animals and humans, primarily due to their brightness and small size. However, noise introduced by LDs may counteract the benefits of brightness when compared to low-noise light-emitting diodes (LEDs). To understand noise sources in optical recordings, we systematically compared instrument and physiological noise profiles in two recording paradigms. A better understanding of noise sources can help improve optical recordings and make them more practical with fewer averages. We stimulated lobster nerves and a rat cortex, then compared the root mean square (RMS) noise and signal-to-noise ratios (SNRs) of data obtained with LED, superluminescent diode (SLD), and LD illumination for different numbers of averages. The LED data exhibited significantly higher SNRs in fewer averages than LD data in all recordings. In the absence of tissue, LED noise increased linearly with intensity, while LD noise increased sharply in the transition to lasing and settled to noise levels significantly higher than the LED's, suggesting that speckle noise contributed to the LD's higher noise and lower SNRs. Our data recommend low coherence and portable light sources for in vivo chronic neural recording applications.

Depth-resolved measurement of transient structural changes during action potential propagation

We report noncontact optical measurement of fast transient structural changes in the crustacean nerve during action potential propagation without the need for exogenous chemicals or reflection coatings. The technique, spectral domain optical coherence tomography, provides real-time cross-sectional images of the nerve with micron-scale resolution to select a specific region for functional assessment and interferometric phase sensitivity for subnanometer-scale motion detection. Noncontact optical measurements demonstrate nanometer-scale transient movement on a 1-ms timescale associated with action potential propagation in crayfish and lobster nerves.

Design and performance of a sub-nanoradian resolution autocollimating optical lever

Precision goniometry using optics has the advantage that it does not impose much stress on the object of investigation and, as such, is adopted extensively in gravitational wave detection, in torsion balances investigating fundamental forces, in specialized studies of biological samples, and it has potential applications in condensed matter physics. In this article we present the considerations that go into designing optical levers and discuss the performance of the instrument we have constructed. We motivate the design by considering an idealized setup and the limitations to the angular resolution induced by statistical fluctuations of the photon count rate and diffraction at the apertures. The effects of digitization of the count rate and of the spatial location of the photons on the image plane motivating the actual design are discussed next. Based on these considerations, we have developed an autocollimating optical lever which has a very high resolution and dynamic range. An array of 110 slits, of 90 microm width and a pitch of 182 microm, is located in the focal plane of a field lens, of focal length 1000 mm, and is illuminated by a CCFL tube. This array is imaged back onto the focal plane after retroreflection from a mirror placed just beyond the lens. The image is recorded on a linear charge-coupled device array at the rate of 1000 images/s and is processed through a special algorithm to obtain the centroid. The instrument has a centroid stability of approximately 3 x 10(-10) rad Hz(-1/2) and a dynamic range of approximately 10(7).

Optically teasing apart neural swelling and depolarization

We measured birefringence, 90 degree scattered light, and voltage sensitive dye changes from lobster walking leg nerves. Systematic application of key chemical agents revealed separate cellular mechanisms underlying fast optical signals. Each agent exhibited mixed effects, some having a greater effect on cellular swelling and refractive index, and some altering membrane potential. Birefringence changes were tightly correlated with voltage sensitive dye signals and were perturbed by those agents that altered membrane potential. Signals from light scattered at 90 degrees corroborated the hypothesis that large angle scattering signals arise from changes in the interstitial spaces and were perturbed by those agents that altered cellular swelling and refractive index. We conclude that multiple cellular mechanisms can be exploited to measure rapid optical signals. Since birefringence produces much larger changes than scattering, the use of polarized light might lead to improvements in imaging neural activity with high temporal resolution, especially since birefringence changes corresponded closely to membrane potential.

Near-infrared imaging of fast intrinsic optical responses in visible light-activated amphibian retina

High performance functional imaging is needed for dynamic measurements of neural processing in retina. Emerging techniques for visual prosthesis also require advanced methodology for reliable validation of electromagnetic stimulation of the retina. Imaging of fast intrinsic optical responses associated with neural activation promises a variety of technical advantages over traditional single and multichannel electrophysiological techniques for these purposes, but the application of fast optical signals for neural imaging has been limited by low signal-to-noise ratio and high background light intensity. However, by using an optimized near-infrared probe light and improved optical system, we improve the optical signals substantially, allowing single pass measurements with approximately micron resolution. We image fast intrinsic optical responses with different optical modalities, i.e., bright field, dark field, and cross-polarization, from isolated retina activated by visible light stimulation. At single cell resolution, bright-field imaging discloses the maxima of optical responses approximately 5% dI/I, where dI is the dynamic optical change and I is the baseline light intensity. Dark-field imaging techniques further enhance the sensitivity of optical measurements, and show the maxima of optical responses exceeding 10% dI/I. Cross-polarized imaging provides optical sensitivity similar to dark-field imaging, but different patterns of neural activation are observed.

Dynamic neuroimaging of retinal light responses using fast intrinsic optical signals

Transient intrinsic optical responses associated with neural activation offer an attractive strategy for dynamic imaging of neural activity, and may provide a noninvasive methodology for imaging of retinal function. Here we demonstrate the feasibility of near infrared imaging of fast intrinsic optical changes in isolated frog retina activated by visible light. Using a photodiode detector in a transmitted light geometry, we routinely measured dynamic transmitted optical responses in single passes, at the level of one part in 10(4) of background light. Rapid CCD image sequences acquired with transmitted light (bright field) illumination disclosed larger fractional responses and showed evidence of multiple response components with both negative- and positive-going signals with different timecourses. Dark field imaging further enhanced the contrast and sensitivity of optical measures of neural activation. High-resolution imaging disclosed optical responses in single pixels often exceeding 5%, of background light, allowing dynamic imaging at the resolution of single cells, in single passes. Fast optical signals are closely related to identified response components of the electroretinogram. Optical responses showed complex but consistent spatial organization from frame to frame. Our experimental results and theoretical analysis suggest that the optical responses may result from dynamic volume changes corresponding to ion and water flow across the cell membrane, directly associated with the electrophysiological response.

Rapid optical coherence tomography and recording functional scattering changes from activated frog retina

Optical coherence tomography (OCT) has important potential advantages for fast functional neuroimaging. However, dynamic neuroimaging poses demanding requirements for fast and stable acquisition of optical scans. Optical phase modulators based on the electro-optic effect allow rapid phase modulation; however, applications to low-coherence tomography are limited by the optical dispersion of a broadband light source by the electro-optic crystal. We show that the optical dispersion can be theoretically estimated and experimentally compensated. With an electro-optic phase modulator-based, no-moving-parts OCT system, near-infrared scattering changes associated with neural activation were recorded from isolated frog retinas activated by visible light.

Cross-polarized reflected light measurement of fast optical responses associated with neural activation

We developed an optical probe for cross-polarized reflected light measurements and investigated optical signals associated with electrophysiological activation in isolated lobster nerves. The cross-polarized baseline light intensity (structural signal) and the amplitude of the transient response to stimulation (functional signal) measured in reflected mode were dependent on the orientation of the nerve axis relative to the polarization plane of incident light. The maximum structural signal and functional response amplitude were observed at 45 degrees , and the ratio of functional to structural signals was approximately constant across orientations. Functional responses were measured in single trials in both transmitted and reflected geometries and responses had similar waveforms. Both structural and functional signals were an order of magnitude smaller in reflected than in transmitted light measurements, but functional responses had similar signal/noise ratios. We propose a theoretical model based on geometrical optics that is consistent with experimental results. In the model, the cross-polarized structural signal results from light reflection from axonal fibers and the transient functional response arises from axonal swelling associated with neural activation. Polarization-sensitive reflected light measurements could greatly enhance in vivo imaging of neural activation since cross-polarized responses are much larger than scattering signals now employed for dynamic functional neuroimaging.