The Structural Layers of the Porcine Iris Exhibit Inherently Different Biomechanical Properties.

Purpose: The purpose of this study was to isolate the structural components of the ex vivo porcine iris tissue and to determine their biomechanical properties. Methods: The porcine stroma and dilator tissues were separated, and their dimensions were assessed using optical coherence tomography (OCT). The stroma underwent flow test (n = 32) to evaluate for permeability using Darcy's Law (ΔP = 2000 Pa, A = 0.0391 mm2), and both tissues underwent stress relaxation experiments (ε = 0.5 with initial ramp of δε = 0.1) to evaluate for their viscoelastic behaviours (n = 28). Viscoelasticity was characterized by the parameters ß (half width of the Gaussian distribution), τm (mean relaxation time constant), E0 (instantaneous modulus), and E∞ (equilibrium modulus). Results: For the stroma, the hydraulic permeability was 9.49 ± 3.05 × 10-6 mm2/Pa · s, and the viscoelastic parameters were ß = 2.50 ± 1.40, and τm = 7.43 ± 4.96 s, with the 2 moduli calculated to be E0 = 14.14 ± 6.44 kPa and E∞ = 6.08 ± 2.74 kPa. For the dilator tissue, the viscoelastic parameters were ß = 2.06 ± 1.33 and τm = 1.28 ± 1.27 seconds, with the 2 moduli calculated to be E0 = 9.16 ± 3.03 kPa and E∞ = 5.54 ± 1.98 kPa. Conclusions: We have established a new protocol to evaluate the biomechanical properties of the structural layers of the iris. Overall, the stroma was permeable and exhibited smaller moduli than those of the dilator muscle. An improved characterization of iris biomechanics may form the basis to further our understanding of angle closure glaucoma.

In vivo biomechanical assessment of iridial deformations and muscle contractions in human eyes.

The iris is a muscular organ whose deformations can cause primary angle-closure glaucoma (PACG), a leading cause of blindness. PACG risk assessment does not consider iridial biomechanical factors, despite their expected influence on iris deformations. Here, we exploited an existing biometric dataset consisting of near-infrared movies acquired during the pupillary light reflex (PLR) as a unique resource to study iris biomechanics. The PLR caused significant (greater than 100%) and essentially spatially uniform radial strains in the iris in vivo, consistent with previous findings. Inverse finite-element modelling showed that sphincter muscle tractions were ca fivefold greater than iridial stroma stiffness (range 4- to 13-fold, depending on sphincter muscle size). This muscle traction is greater than has been previously estimated, which may be due to methodological differences and/or to different patient populations in our study (European descent) versus previous studies (Asian); the latter possibility is of particular interest due to differential incidence rates of PACG in these populations. Our methodology is fast and inexpensive and may be a useful tool in understanding biomechanical factors contributing to PACG.

Irissometry: Effects of Pupil Size on Iris Elasticity Measured With Video-Based Feature Tracking.

Purpose: It is unclear how the iris deforms during changes in pupil size. Here, we report an application of a multi-feature iris tracking method, which we call irissometry, to investigate how the iris deforms and affects the eye position signal as a function of pupil size. Methods: To evoke pupillary responses, we repeatedly presented visual and auditory stimuli to healthy participants while we additionally recorded their right eye with a macro lens-equipped camera. We tracked changes in iris surface structure between the pupil and sclera border (limbus) by calculating local densities (distance between feature points) across evenly spaced annular iris regions. Results: The time analysis of densities showed that the inner regions of the iris stretched more strongly as compared with the outer regions of the iris during pupil constrictions. The pattern of iris densities across eccentricities and pupil size showed highly similar patterns across participants, highlighting the robustness of this elastic property. Importantly, iris-based eye position detection led to more stable signals than pupil-based detection. Conclusions: The iris regions near the pupil appear to be more elastic than the outer regions near the sclera. This elastic property explains the instability of the pupil border and the related position errors induced by eye movement and pupil size in pupil-based eye-tracking. Tracking features in the iris produce more robust eye position signals. We expect that irissometry may pave the way to novel eye trackers and diagnostic tools in ophthalmology.

Congenital Microcoria: Clinical Features and Molecular Genetics.

Iris integrity is required to regulate both the amount of light reaching the retina and intraocular pressure (IOP), with elevated IOP being a major risk factor for glaucoma. Congenital microcoria (MCOR) is an extremely rare, autosomal dominant disease affecting iris development and hindering both of these functions. It is characterized by absent or underdeveloped dilator muscle fibers and immaturity of the iridocorneal angle-where the aqueous humor is drained-which play a central role in IOP regulation. The dilator muscle anomaly is manifested in pinhole pupils (<2 mm) and thin transilluminable irises, causing both hemeralopia and photoaversion. Axial myopia and juvenile open-angle glaucoma are very frequent (80% and 30% of all cases, respectively). It has been suggested that the immaturity of the chamber angle contributes to glaucoma, and myopia has been ascribed to photoaversion and elevated IOP. Though possible, these mechanisms are insufficient. The disease has been tied to chromosome 13q32.1 structural variations. In addition to compromising iris development, modification of the 13q32.1 architecture could alter signaling pathways for axial ocular length and IOP regulation. Here, we summarize the clinical, histological, and molecular features of this disease, and we discuss the possible etiology of associated anomalies.

ScatT-LOOP: scattering tetrolet-LOOP descriptor and optimized NN for iris recognition at-a-distance.

Iris Recognition at-a Distance (IAAD) is a major challenge for researchers due to the defects associated with the visual imaging and poor image quality in dynamic environments, which imposed bad impacts on the accuracy of recognition. Thus, in order to enable the effective IAAD, this paper proposes a new method, named, Chronological Monarch Butterfly Optimization (Chronological MBO)-enabled Neural Network (NN). The recognition of iris using NN is trained with the proposed Chronological MBO, which is developed through the combination of Chronological theory in Monarch Butterfly Optimization (MBO). The recognition becomes effective with the automatic segmentation and the normalization of iris image on the basis of Hough Transform (HT) and Daugman's rubber sheet model followed with the process of feature extraction with the developed ScatT-LOOP descriptor, which is the integration of scattering transform (ST), Local Optimal Oriented Pattern (LOOP) descriptor, and Tetrolet transform (TT). The developed ScatT-LOOP descriptor extracts the texture as well as the orientation details of image for effective recognition. The analysis is evaluated with the CASIA Iris dataset with respect to the evaluation metrics, accuracy, False Acceptance Rate (FAR), and False Rejection Rate (FRR). The proposed method has the accuracy, FRR, and FAR of 0.97, 0.005, and 0.005, respectively. The experimental results proved that the proposed method is effective than the existing methods of iris recognition.

Quantification of iris elasticity using acoustic radiation force optical coherence elastography.

Careful quantification of the changes in biomechanical properties of the iris can offer insight into the pathophysiology of some ocular diseases. However, to date there has not been much information available regarding this subject because clinical detection for iris elasticity remains challenging. To overcome this limitation, we explore, for the first time to our knowledge, the potential of measuring iris elasticity using acoustic radiation force optical coherence elastography (ARF-OCE). The resulting images and shear wave propagation, as well as the corresponding shear modulus and Young's modulus from ex vivo and in vivo rabbit models confirmed the feasibility of this method. With features of noninvasive imaging, micrometer-scale resolution, high acquisition speed and real-time processing, ARF-OCE is a promising method for reconstruction of iris elasticity and may have great potential to be applied in clinical ophthalmology with further refinement.

The distinctive geographic patterns of common pigmentation variants at the OCA2 gene.

Oculocutaneous Albinism type 2 (OCA2) is a gene of great interest because of genetic variation affecting normal pigmentation variation in humans. The diverse geographic patterns for variant frequencies at OCA2 have been evident but have not been systematically investigated, especially outside of Europe. Here we examine population genetic variation in and near the OCA2 gene from a worldwide perspective. The very different patterns of genetic variation found across world regions suggest strong selection effects may have been at work over time. For example, analyses involving the variants that affect pigmentation of the iris argue that the derived allele of the rs1800407 single nucleotide polymorphism, which produces a hypomorphic protein, may have contributed to the previously demonstrated positive selection in Europe for the enhancer variant responsible for light eye color. More study is needed on the relationships of the genetic variation at OCA2 to variation in pigmentation in areas beyond Europe.

Elucidation of Cellular Mechanisms That Regulate the Sustained Contraction and Relaxation of the Mammalian Iris.

Purpose: In mammals, pupil constriction and dilation form the pupillary light reflex (PLR), which is mediated by both brain-regulated (parasympathetic) and local iris-driven reflexes. To better understand the cellular mechanisms that regulate pupil physiological dynamics via central and local photoreception, we have examined the regulation of the PLR via parasympathetic and local activation, respectively. Methods: In this study, the PLR was examined in mouse enucleated eyes ex vivo in real-time under different ionic conditions in response to acetylcholine and/or blue light (480 nm). The use of pupillometry recordings captured the relaxation, contraction, and pupil escape (redilation) processes for 10 minutes up to 1 hour. Results: Among others, our results show that ryanodine receptor channels are the main driver for iridal stimulation-contraction coupling, in which extracellular influx of Ca2+ is required for amplification of pupil constriction. Both local and parasympathetic iridal activations are necessary, but not sufficient for sustained pupil constriction. Moreover, the degree of membrane potential repolarization in the dark is correlated with the latency and velocity of iridal constriction. Furthermore, pupil escape is driven by membrane potential hyperpolarization where voltage-gated potassium channels play a crucial role. Conclusions: Together, this study presents new mechanisms regulating synchronized pupil dilation and contraction, sustained pupil constriction, iridal stimulation-contraction coupling, and pupil escape.

Genome-wide association studies for iris pigmentation and heterochromia patterns in Large White pigs.

Eye colour genetics have been extensively studied in humans since the rediscovery of Mendel's laws. This trait was first interpreted using simplistic genetic models but soon it was realised that it is more complex. In this study, we analysed eye colour variability in a Large White pig population (n = 897) and report the results of GWASs based on several comparisons including pigs having four main eye colour categories (three with both pigmented eyes of different brown grades: pale, 17.9%; medium, 14.8%; and dark, 54.3%; another one with both eyes completely depigmented, 3.8%) and heterochromia patterns (heterochromia iridis - depigmented iris sectors in pigmented irises, 3.2%; heterochromia iridum - one whole eye iris of depigmented phenotype and the other eye with the iris completely pigmented, 5.9%). Pigs were genotyped with the Illumina PorcineSNP60 BeadChip and GEMMA was used for the association analyses. The results indicated that SLC45A2 (on chromosome 16, SSC16), EDNRB (SSC11) and KITLG (SSC5) affect the different grades of brown pigmentation of the eyes, the bilateral eye depigmentation defect and the heterochromia iridis defect recorded in this white pig population respectively. These genes are involved in several mechanisms affecting pigmentation. Significant associations for the eye depigmented patterns were also identified for SNPs on two SSC4 regions (including two candidate genes: NOTCH2 and PREX2) and on SSC6, SSC8 and SSC14 (including COL17A1 as candidate gene). This study provided useful information to understand eye pigmentation mechanisms, further valuing the pig as animal model to study complex phenotypes in humans.

Blind Quality Assessment of Iris Images Acquired in Visible Light for Biometric Recognition.

Image quality is a key issue affecting the performance of biometric systems. Ensuring the quality of iris images acquired in unconstrained imaging conditions in visible light poses many challenges to iris recognition systems. Poor-quality iris images increase the false rejection rate and decrease the performance of the systems by quality filtering. Methods that can accurately predict iris image quality can improve the efficiency of quality-control protocols in iris recognition systems. We propose a fast blind/no-reference metric for predicting iris image quality. The proposed metric is based on statistical features of the sign and the magnitude of local image intensities. The experiments, conducted with a reference iris recognition system and three datasets of iris images acquired in visible light, showed that the quality of iris images strongly affects the recognition performance and is highly correlated with the iris matching scores. Rejecting poor-quality iris images improved the performance of the iris recognition system. In addition, we analyzed the effect of iris image quality on the accuracy of the iris segmentation module in the iris recognition system.

Deep neural network and data augmentation methodology for off-axis iris segmentation in wearable headsets.

A data augmentation methodology is presented and applied to generate a large dataset of off-axis iris regions and train a low-complexity deep neural network. Although of low complexity the resulting network achieves a high level of accuracy in iris region segmentation for challenging off-axis eye-patches. Interestingly, this network is also shown to achieve high levels of performance for regular, frontal, segmentation of iris regions, comparing favourably with state-of-the-art techniques of significantly higher complexity. Due to its lower complexity this network is well suited for deployment in embedded applications such as augmented and mixed reality headsets.

Relationship of postsaccadic oscillation with the state of the pupil inside the iris and with cognitive processing.

Recent studies using video-based eye tracking have presented accumulating evidence that postsaccadic oscillation defined in reference to the pupil center (PSOp) is larger than that to the iris center (PSOi). This indicates that the relative motion of the pupil reflects the viscoelasticity of the tissue of the iris. It is known that the pupil size controlled by the sphincter/dilator pupillae muscles reflects many aspects of cognition. A hypothesis derived from this fact is that cognitive tasks affect the properties of PSOp due to the change in the state of these muscles. To test this hypothesis, we conducted pro- and antisaccade tasks for human participants and adopted the recent physical model of PSO to evaluate the dynamic properties of PSOp/PSOi. The results showed the dependence of the elasticity coefficient of the PSOp on the antisaccade task, but this effect was not significant for the PSOi. This suggests that cognitive tasks such as antisaccade tasks affect elasticity of the muscle of the iris. We found that the trial-by-trial fluctuation in the presaccade absolute pupil size correlated with the elasticity coefficient of PSOp. We also found the task dependence of the viscosity coefficient and overshoot amount of PSOi, which probably reflects the dynamics of the entire eyeball movement. The difference in task dependence between PSOp and PSOi indicates that the separate measures of these two can be means to distinguish factors related to the oculomotor neural system from those related to the physiological states of the iris tissue.NEW & NOTEWORTHY The state of the eyeball varies dynamically moment by moment depending on underlying neural/cognitive processing. Combining simultaneous measurements of pupil-centric and iris-centric movements and a recent physical model of postsaccadic oscillation (PSO), we show that the pupil-centric PSO is sensitive to the type of saccade task, suggesting that the physical state of the iris muscles reflects the underlying cognitive processes.

The Pupil.

PURPOSE OF REVIEW: The goal of this article is to review the anatomy and physiology of pupillary function and then employ that information to develop a comprehensive framework for understanding and diagnosing pupillary disorders. RECENT FINDINGS: The contribution of rods and cones to the pupillary light reflex has long been known. A third photosensitive cell type, the intrinsically photosensitive retinal ganglion cell, has recently been discovered. This cell type employs melanopsin to mediate a portion of the pupillary light reflex independent of rods and cones (the postillumination pupillary response) and photic regulation of circadian rhythm. SUMMARY: The autonomic nervous system regulates pupil size in response to stimuli. The parasympathetic nervous system causes miosis in response to light and near visual stimuli. These stimuli activate supranuclear pathways that project to the Edinger-Westphal nuclei. The sympathetic nervous system causes mydriasis in response to a variety of arousing factors, both physiologic (wakefulness) and pathologic (pain). Abnormalities of physiologic function cause disturbances of pupil size, shape, and response to stimuli. The clinical approach to pupillary abnormalities should focus on the clinical and pharmacologic assessment of the pupil's expected response to diverse stimuli.

Associations between macular pigment, iris color and reflectance, ethnicity, and color vision: An observational study.

PURPOSE: Conflicting findings exist in the literature with regard to the relationship between iris color, ethnicity, macular pigment optical density (MPOD), and hue discrimination. This study re-examined these relationships, accounting for factors that may have confounded prior studies. Clinically, the relationship between MPOD and hue discrimination may impact the utility of macular pigment supplementation as a treatment for conditions such as macular degeneration. METHODS: Subjects (n = 30, mean/SD age = 25.1/2.5 yrs.) with normal color vision completed MPOD testing and Farnsworth-Munsell 100 hue (FM100) testing. MPOD data was derived from the average of three measurements using the QuantifEYE II device and FM100 testing included training runs. The total error score of the FM100 test was used for analysis. Iris color was determined subjectively, while iris reflectance was derived using calibrated iris images. Spearman correlations were used to determine the relationship between MPOD and FM100 test scores. Kruskal-Wallis testing was used to investigate MPOD differences among different ethnicities and iris colors. RESULTS: MPODs were normally distributed with a mean/SD = 0.38/0.13. Total error scores had a mean/SD of 10.7/9.7, but were not normally distributed. Iris reflectances had a mean/SD = 11.0/8.7. MPODs were not correlated to total error scores (p = 0.93). MPODs were also not correlated with iris reflectances (p = 0.28) even though MPODs differed significantly by iris color (brown = 0.44, hazel = 0.31, blue = 0.33, p = 0.04). Iris reflectances were not correlated with total error scores (p = 0.68). MPODs differed significantly (p = 0.003) between Asian and Caucasian subjects, 0.44 and 0.33, respectively. CONCLUSIONS: This study did not find a correlation between MPOD and hue discrimination as in some previous studies. While MPOD was associated with iris color and ethnicity as found in prior studies, it was not associated with iris reflectance, which may be a better indicator of ocular pigmentation compared to either iris color or ethnicity.

Visual modelling supports the potential for prey detection by means of diurnal active photolocation in a small cryptobenthic fish.

Active sensing has been well documented in animals that use echolocation and electrolocation. Active photolocation, or active sensing using light, has received much less attention, and only in bioluminescent nocturnal species. However, evidence has suggested the diurnal triplefin Tripterygion delaisi uses controlled iris radiance, termed ocular sparks, for prey detection. While this form of diurnal active photolocation was behaviourally described, a study exploring the physical process would provide compelling support for this mechanism. In this paper, we investigate the conditions under which diurnal active photolocation could assist T. delaisi in detecting potential prey. In the field, we sampled gammarids (genus Cheirocratus) and characterized the spectral properties of their eyes, which possess strong directional reflectors. In the laboratory, we quantified ocular sparks size and their angle-dependent radiance. Combined with environmental light measurements and known properties of the visual system of T. delaisi, we modeled diurnal active photolocation under various scenarios. Our results corroborate that diurnal active photolocation should help T. delaisi detect gammarids at distances relevant to foraging, 4.5 cm under favourable conditions and up to 2.5 cm under average conditions. To determine the prevalence of diurnal active photolocation for micro-prey, we encourage further theoretical and empirical work.

Young's Modulus Determination of Normal and Glaucomatous Human Iris.

Purpose: To evaluate the biomechanical properties (Young's modulus) of normal (control) and glaucomatous human iris using atomic force microscopy (AFM). Methods: Iris tissue obtained from eighteen glaucomatous subjects (equal number of eyes with primary angle closure glaucoma (PACG) and primary open angle glaucoma (POAG) and five normal subjects who underwent elective eye surgery were subjected to the estimation of Young's modulus by AFM. Force measurements were done at room temperature using Nanowizard II BioAFM. The iris samples were immersed in the liquid media (PBS with 0.1% BSA) during force measurements. Young's modulus values were calculated for each recorded curve using JPK Data Processing Software, which uses a Hertz's contact model for spherical indenters fitted to the extend curves. Results: The iris from the normal controls had the least Young's modulus (0.85 ± 0.31 kPa) while those from PACG patients had the highest Young's modulus (2.40 ± 0.82 kPa). The Young's modulus of PACG iris was significantly higher compared to that of the normal controls (P = 0.005) and POAG iris (P = 0.001). However, there was no significant difference in the Young's modulus of POAG iris (1.13 ± 0.36 kPa) compared to that of the normal controls (P = 0.511). Conclusions: Variations in biomechanical properties of iris tissue may have a significant role in the pathogenesis of angle closure glaucoma. This study suggests the existence of fundamental biomechanical differences in eyes with angle closure versus open angle glaucoma. An understanding of this basis creates a new platform to understand disease pathology better and work on therapeutic strategies that will address the same.

Photoacoustic properties of anterior ocular tissues.

Clinical imaging techniques for the anterior segment of the eye provide excellent anatomical information, but molecular imaging techniques are lacking. Molecular photoacoustic imaging is one option to address this need, but implementation requires use of contrast agents to distinguish molecular targets from background photoacoustic signals. Contrast agents are typically selected based on a priori knowledge of photoacoustic properties of tissues. However, photoacoustic properties of anterior ocular tissues have not been studied yet. Herein, anterior segment anatomy and corresponding photoacoustic signals were analyzed in brown and blue porcine eyes ex vivo. Measured photoacoustic spectra were compared to known optical absorption spectra of endogenous chromophores. In general, experimentally measured photoacoustic spectra matched expectations based on absorption spectra of endogenous chromophores reported in the literature, and similar photoacoustic spectra were observed in blue and brown porcine eyes. However, unique light-tissue interactions at the iris modified photoacoustic signals from melanin. Finally, we demonstrated how the measured PA spectra established herein can be used for one application of molecular PA imaging, detecting photoacoustically labeled stem cells in the anterior segment for glaucoma treatment.

Visualization of the Superior Ocular Sulcus during Danio rerio Embryogenesis.

Congenital ocular coloboma is a genetic disorder that is typically observed as a cleft in the inferior aspect of the eye resulting from incomplete choroid fissure closure. Recently, the identification of individuals with coloboma in the superior aspect of the iris, retina, and lens led to the discovery of a novel structure, referred to as the superior fissure or superior ocular sulcus (SOS), that is transiently present on the dorsal aspect of the optic cup during vertebrate eye development. Although this structure is conserved across mice, chick, fish, and newt, our current understanding of the SOS is limited. In order to elucidate factors that contribute to its formation and closure, it is imperative to be able to observe it and identify abnormalities, such as delay in the closure of the SOS. Here, we set out to create a standardized series of protocols that can be used to efficiently visualize the SOS by combining widely available microscopy techniques with common molecular biology techniques such as immunofluorescent staining and mRNA overexpression. While this set of protocols focuses on the ability to observe SOS closure delay, it is adaptable to the experimenter's needs and can be easily modified. Overall, we hope to create an approachable method through which our understanding of the SOS can be advanced to expand the current knowledge of vertebrate eye development.