Fast Intrinsic Optical Signals Research Articles

Functional intrinsic optical signal imaging for objective optoretinography of human photoreceptors.

Functional mapping of photoreceptor physiology is important for better disease diagnosis and treatment assessment. Fast intrinsic optical signal (IOS), which arises before light-evoked pupillary response, promises a unique biomarker of photoreceptor physiology for objective optoretinography with high resolution. This study is to test the feasibility of non-mydriatic IOS mapping of retinal photoreceptors in awake human. Depth-resolved optical coherence tomography verified outer segment (OS) as the anatomic origin of fast photoreceptor-IOS. Dynamic IOS changes are primarily confined at OS boundaries connected with inner segment and retinal pigment epithelium, supporting transient OS shrinkage due to phototransduction process as the mechanism of the fast photoreceptor-IOS response.

Fast intrinsic optical signal correlates with activation phase of phototransduction in retinal photoreceptors.

As the center of phototransduction, retinal photoreceptors are responsible for capturing and converting photon energy to bioelectric signals for following visual information processing in the retina. This article summarizes experimental observation and discusses biophysical mechanism of fast photoreceptor-intrinsic optical signal (IOS) correlated with early phase of phototransduction. Quantitative imaging of fast photoreceptor-IOS may provide objective optoretinography to advance the study and diagnosis of age-related macular degeneration, retinitis pigmentosa, diabetic retinopathy, and other eye diseases that can cause photoreceptor dysfunctions.

Impact of motion-associated noise on intrinsic optical signal imaging in humans with optical coherence tomography.

A growing body of evidence suggests that phototransduction can be studied in the human eye in vivo by imaging of fast intrinsic optical signals (IOS). There is consensus concerning the limiting influence of motion-associated imaging noise on the reproducibility of IOS-measurements, especially in those employing spectral-domain optical coherence tomography (SD-OCT). However, no study to date has conducted a comprehensive analysis of this noise in the context of IOS-imaging. In this study, we discuss biophysical correlates of IOS, and we address motion-associated imaging noise by providing correctional post-processing methods. In order to avoid cross-talk of adjacent IOS of opposite signal polarity, cellular resolution and stability of imaging to the level of individual cones is likely needed. The optical Stiles-Crawford effect can be a source of significant IOS-imaging noise if alignment with the peak of the Stiles-Crawford function cannot be maintained. Therefore, complete head stabilization by implementation of a bite-bar may be critical to maintain a constant pupil entry position of the OCT beam. Due to depth-dependent sensitivity fall-off, heartbeat and breathing associated axial movements can cause tissue reflectivity to vary by 29% over time, although known methods can be implemented to null these effects. Substantial variations in reflectivity can be caused by variable illumination due to changes in the beam pupil entry position and angle, which can be reduced by an adaptive algorithm based on slope-fitting of optical attenuation in the choriocapillary lamina.

In vivo confocal intrinsic optical signal identification of localized retinal dysfunction.

The purposes of this study were to investigate the physiological mechanism of stimulus-evoked fast intrinsic optical signals (IOSs) recorded in dynamic confocal imaging of the retina, and to demonstrate the feasibility of in vivo confocal IOS mapping of localized retinal dysfunctions. A rapid line-scan confocal ophthalmoscope was constructed to achieve in vivo confocal IOS imaging of frog (Rana pipiens) retinas at cellular resolution. In order to investigate the physiological mechanism of confocal IOS, comparative IOS and electroretinography (ERG) measurements were made using normal frog eyes activated by variable-intensity stimuli. A dynamic spatiotemporal filtering algorithm was developed to reject the contamination of hemodynamic changes on fast IOS recording. Laser-injured frog eyes were employed to test the potential of confocal IOS mapping of localized retinal dysfunctions. Comparative IOS and ERG experiments revealed a close correlation between the confocal IOS and retinal ERG, particularly the ERG a-wave, which has been widely used to evaluate photoreceptor function. IOS imaging of laser-injured frog eyes indicated that the confocal IOS could unambiguously detect localized (30 μm) functional lesions in the retina before a morphological abnormality is detectable. The confocal IOS predominantly results from retinal photoreceptors, and can be used to map localized photoreceptor lesion in laser-injured frog eyes. We anticipate that confocal IOS imaging can provide applications in early detection of age-related macular degeneration, retinitis pigmentosa, and other retinal diseases that can cause pathological changes in the photoreceptors.

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.

Correlation of visually evoked intrinsic optical signals and electroretinograms recorded from chicken retina with a combined functional optical coherence tomography and electroretinography system

Visually evoked fast intrinsic optical signals (IOSs) were recorded for the first time in vivo from all layers of healthy chicken retina by using a combined functional optical coherence tomography (fOCT) and electroretinography (ERG) system. The fast IOSs were observed to develop within ∼5 ms from the on-set of the visual stimulus, whereas slow IOSs were measured up to 1 s later. The visually evoked IOSs and ERG traces were recorded simultaneously, and a clear correlation was observed between them. The ability to measure visually evoked fast IOSs non-invasively and in vivo from individual retinal layers could significantly improve the understanding of the complex communication between different retinal cell types in healthy and diseased retinas.

In vivo confocal imaging of fast intrinsic optical signals correlated with frog retinal activation.

Using freshly isolated animal retinas, we have conducted a series of experiments to test fast intrinsic optical signals (IOSs) that have time courses comparable to electrophysiological kinetics. In this Letter, we demonstrate the feasibility of in vivo imaging of fast IOSs in intact frogs. A rapid line-scan confocal ophthalmoscope was constructed to achieve high-speed IOS recording. By rejecting out-of-focus background light, the line-scan confocal imager provided the resolution to differentiate individual photoreceptors in vivo. Rapid confocal imaging disclosed robust IOSs with time courses comparable to retinal electroretinogram kinetics. High-resolution IOS images revealed both positive (increasing) and negative (decreasing) light responses, with subcellular complexity.

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.

Imaging fast intrinsic optical signals for studies of retinal function

Imaging fast intrinsic optical signals for studies of retinal function

High-speed line-scan confocal imaging of stimulus-evoked intrinsic optical signals in the retina

A rapid line-scan confocal imager was developed for functional imaging of the retina. In this imager, an acousto-optic deflector was employed to produce mechanical vibration- and inertia-free light scanning, and a high-speed (68,000 Hz) linear CCD camera was used to achieve subcellular and submillisecond spatiotemporal resolution imaging. Two imaging modalities, i.e., frame-by-frame and line-by-line recording, were validated for the reflected light detection of intrinsic optical signals (IOSs) in visible light stimulus activated frog retinas. Experimental results indicated that fast IOSs were tightly correlated with retinal stimuli and could track visible light flicker stimulus frequency up to at least 2 Hz.

Intrinsic optical signal imaging of retinal activation

Fast intrinsic optical signals (IOSs) correlated with stimulus-activated retinal responses are reviewed. Fast IOSs have a time course comparable to the stimulus-evoked electrophysiological kinetics of the retina, and thus promise a new methodology for high-resolution evaluation of the physiological health of the retina. However, practical application of fast IOSs for retinal study and diagnosis is challenging because of their low sensitivity and limited specificity. Using isolated amphibian retinas, a series of experiments to optimize and characterize fast IOSs has been conducted. Fast, high-resolution nearinfrared light imaging disclosed both positive (increasing) and negative (decreasing) optical responses in adjacent retinal areas, which satisfied spatial resolution essential to the differentiation of IOSs from opposite polarities. At the subcellular (approximately microm) level, fast IOSs often exceeded 5% DeltaI/I, where I is the dynamic optical change, and I is the background light intensity. Experiments with isolated frog retinas suggest that negative IOSs stem primarily from the photoreceptor layer, while positive IOSs come from inner retinal layers.

INTRINSIC OPTICAL SIGNAL IMAGING OF RETINAL ACTIVITY IN FROG EYE

Using a near-infrared (NIR) light flood-illumination imager equipped with a high-speed (120 Hz) CCD camera, we demonstrated optical imaging of stimulus-evoked retinal activity in isolated, but intact, frog eye. Both fast and slow transient intrinsic optical signals (IOSs) were observed. Fast optical response occurred immediately after the stimulus onset, could reach peak magnitude within 100 ms, and correlated tightly with ON and OFF edges of the visible light stimulus; while slow optical response lasted a relatively long time (many seconds). High-resolution images revealed both positive (increasing) and negative (decreasing) IOSs, and dynamic optical change at individual CCD pixels could often exceed 10% of the background light intensity. Our experiment on isolated eye suggests that further development of fast, high (sub-cellular) resolution fundus imager will allow robust detection of fast IOSs in vivo, and thus allow noninvasive, three-dimensional evaluation of retinal neural function.

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.

Imaging cortical electrical stimulation in vivo: fast intrinsic optical signal versus voltage-sensitive dyes

We applied high-temporal-resolution optical imaging utilizing both the fast intrinsic optical signal (fIOS) and voltage-sensitive dyes (VSDs) to observe the spatiotemporal characteristics of rat somatosensory cortex during electrical stimulation. We find that changes in both the fIOS and VSD signals occur rapidly (<30 ms) after the stimulus is applied, suggesting that both membrane depolarization and transmembrane ion movement occur shortly after the stimulus, preceding the more gradual physiological changes in oxygen consumption revealed by the slower component of the intrinsic optical signal. We find that the VSD signal spreads through a much larger area of cortex than the fIOS.

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.