Light-evoked changes in near-infrared transmission by the ON and OFF channels of the anuran retina

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.

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.

Functional imaging of retinal photoreceptors and inner neurons using stimulus-evoked intrinsic optical signals

Retinal development is a dynamic process both anatomically and functionally. High-resolution imaging and dynamic monitoring of photoreceptors and inner neurons can provide important information regarding the structure and function of the developing retina. In this chapter, we describe intrinsic optical signal (IOS) imaging as a high spatiotemporal resolution method for functional study of living retinal tissues. IOS imaging is based on near infrared (NIR) light detection of stimulus-evoked transient change of inherent optical characteristics of the cells. With no requirement for exogenous biomarkers, IOS imaging is totally noninvasive for functional mapping of stimulus-evoked spatiotemporal dynamics of the photoreceptors and inner retinal neurons.

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.

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.

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.

Optophysiology: depth-resolved probing of retinal physiology with functional ultrahigh-resolution optical coherence tomography

Noncontact, depth-resolved, optical probing of retinal response to visual stimulation with a <10-microm spatial resolution, achieved by using functional ultrahigh-resolution optical coherence tomography (fUHROCT), is demonstrated in isolated rabbit retinas. The method takes advantage of the fact that physiological changes in dark-adapted retinas caused by light stimulation can result in local variation of the tissue reflectivity. fUHROCT scans were acquired from isolated retinas synchronously with electrical recordings before, during, and after light stimulation. Pronounced stimulus-related changes in the retinal reflectivity profile were observed in the inner/outer segments of the photoreceptor layer and the plexiform layers. Control experiments (e.g., dark adaptation vs. light stimulation), pharmacological inhibition of photoreceptor function, and synaptic transmission to the inner retina confirmed that the origin of the observed optical changes is the altered physiological state of the retina evoked by the light stimulus. We have demonstrated that fUHROCT allows for simultaneous, noninvasive probing of both retinal morphology and function, which could significantly improve the early diagnosis of various ophthalmic pathologies and could lead to better understanding of pathogenesis.

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.