Week in Review: Number 24

Contactless Robotic Optical Coherence Tomography
Scientists developed a fully automated ocular coherence tomography (OCT) system that captures a three-dimensional image of the entire eye, including an active-tracking scanning head that aligns itself with the patient's pupil as well as the ability to safely distance operator and patient. The system builds off of earlier work in intraoperative OCT and handheld OCT from colleagues within their institution, now adapted for physical distancing challenges posed by the COVID-19 pandemic. For example, the present iteration of a robotic OCT foregoes the need for chin and forehead rests for stabilization, relying instead on two sets of cameras that track the face and pupil, moving with patient movements to compensate for subtle motions. Additionally, as a comfort feature, patients use a foot pedal to control the robotic arm; when the patient takes his/her foot off the pedal, the robot moves away. A contactless OCT imaging modality also improves accessibility for patients with mobility difficulties, such as children and those who are wheelchair-bound. Advancements in the optical design of this system include working on a wider depth of field and the ability to visualize both the anterior and posterior segments of the eye simultaneously, capturing the entire eye in 3D. The ultimate goal of the project is to provide a one-to-one exact digital replica of the eye, which would enable studies related to the shape of the eye. They are testing applications of the technology in people with elevated intracranial pressure as a potential substitute for more expensive MRIs or more invasive spinal taps. The robotic design of the system should also facilitate telemedicine, since the robot can be sent to rural settings to capture images, the interpretation of which can be done by eye doctors remotely. The investigators report that early results of their system are encouraging both in terms of patient satisfaction and accuracy compared to traditional OCT.

OCT Angiography to Monitor Sickle Cell Retinopathy
Researchers developed a new technique using optical coherence tomography angiography (OCTA) to evaluate progression of sickle cell retinopathy before symptoms manifest as well as assess the efficacy of pharmacological treatment over time. OCT angiography is especially useful in monitoring of sickle cell disease, an inherited red blood cell disorder, in the sense that the visualization of blood flow can detect areas where the sickle-shaped cells clump together and cause local vascular damage, potentially leading to ischemia or hemorrhages. Their study involved 27 participants, 14 of whom were controls and 13 of whom had sickle cell disease with retinopathy of varying severity; some were on standard therapy (hydroxyurea) and others were not on treatment. The series of OCTA scans showed, unsurprisingly, that healthy participants had consistent blood flow with no or very minimal fluctuations. The untreated sickle cell patients, however, had substantially more intermittent vaso-occlusion (more flickering between scan images) than patients on treatment, indicating that treatment was effective. The investigators then used a computer algorithm to assess risk of retinal blood blockages based on flicker frequency and locations of flickering. The lead investigator explains, “We have added a new dimension to ocular imaging technology that no one has thought of before. For the first time, we have shown that by doing rapid, repeated retinal imaging of sickle cell patients, you can see microscopic changes in blood vessels and blood flow. The more the blood flow fluctuates across images, the more at risk patients are for a permanent blockage, which severely damage their eyesight.” The early detection of retinopathy in asymptomatic sickle cell patients could prevent irreversible vision loss when it's too late. Additionally, given a multitude of pharmacological treatments for sickle cell disease, monitoring the effectiveness of a particular drug regimen with OCTA can better inform the choice of drugs.

Case Report: Fundus Albipunctatus Diagnosis Using Adaptive Optics SLO and Genetic Testing
Investigators presented a rare case of a 62-year-old man who was misdiagnosed with Stargardt's disease for ten years before adaptive optics scanning laser ophthalmoscopy (AOSLO) and genetic testing revealed a diagnosis of fundus albipunctatus. Much rarer than Stargardt's macular dystrophy, fundus albipunctatus is an inherited retinal disease categorized under congenital stationary night blindness (CSNB) that is associated with fundus findings. In fundus albipunctatus, an autosomal recessive mutation in the RDH5 gene disrupts proper retinoid recycling. Also unlike the progressive retinal deterioration seen in Stargardt's disease, the clinical course of fundus albipunctatus does not change over time. While a diagnosis of fundus albipunctatus is confirmed with genetic testing, AOSLO was helpful in explaining the patient's clinical presentation and characterizing the photoreceptor status of the disease. The investigators explain, "Previous studies using AOSLO in Stargardt’s disease have revealed increased cone and rod spacing, with reduced foveal cone density and enlarged cone size, and dark cones thought to be associated with foreshortened outer segments. These findings are similar to our patient’s photoreceptor characteristics on AOSLO, except for the profound sparing observed in the central fovea." The use of AOSLO to characterize the cellular structure of photoreceptors in a patient with fundus albipunctatus revealed that in the later stages of the disease, some structures of photoreceptor cells were preserved, advancing understanding of the condition for future gene therapies. While fundus albipunctatus currently has no cure, earlier detection can inform better genetic counseling for family members as well as better counsel patients regarding the prognosis of the condition.

Nano-SOD1 Antioxidant to Treat Ocular Inflammation
A team of Russian scientists, along with American colleagues, developed an anti-inflammatory drug specifically for the eyes, which they tested in vivo in a rabbit model of uveitis. Their project involved creating multilayer polyion nanoparticles of the superoxide dismutase enzyme (Nano-SOD1), with a new formulation specifically manufactured for topical use on eyes. Superoxide dismutase is among antioxidant enzymes that are potent scavengers of reactive oxygen species (ROS), the excessive production of which during cellular processes leads to inflammation and tissue degeneration. When this inflammation occurs in ocular structures that are intricately arranged—such as the cornea, the lens, and the retina—the subsequent tissue damage can lead to blurred vision and even vision loss. While superoxide dismutase is an effective antioxidant, its very limited permeability presents a challenge for drug delivery to the eye, a relatively isolated organ that is often preferentially treated locally rather than systemically. As seen in their rabbit model of immunogenic uveitis, one of the authors states, "It was shown during preclinical studies that topical instillations of Nano-SOD1 were 35% more effective in reducing the manifestations of uveitis compared to the free enzyme SOD1. In particular, we noted statistically significant differences in such inflammatory signs of the eye as corneal and conjunctival edema, iris hyperemia and fibrin clots." The authors also report improved penetration into interior eye structures, longer retention of enzyme activity, and greater antioxidant activity for Nano-SOD1 compared to SOD1 by itself. So far, Nano-SOD1 shows a good drug safety profile at wide concentration ranges and at extremely high doses. Preclinical trials continue to explore its potential as a therapeutic agent for the treatment of ocular inflammation.

Transparent Diffractive Corneal Inlay for Presbyopia
As part of a doctoral thesis, researchers in Spain designed the first transparent diffractive trifocal intracorneal inlay as a potential treatment option for presbyopia. Consisting of an extremely thin (less than 5 microns) lens made of a biocompatible material, the corneal inlay would be placed inside the corneal stroma in a cavity created with a femtosecond laser. According to the research team, the lens is micro-perforated to allow the flow of nutrients within the cornea and to reduce the likelihood of rejection, and its transparent design does not prevent retinal examination. Like existing trifocal refractive technology, this corneal inlay corrects for vision at far, intermediate (computer range), and near vision. However, the authors highlight that the main novelty of this corneal inlay is that it uses a diffractive lens, as compared to existing small-aperature corneal inlays and refractive corneal inlays. One of the researchers explains, “The first type produce an extension of the depth of focus with which intermediate vision is gained but their luminous efficiency, since it is partially opaque, is low. The second type achieve[s] bifocality, so it has a good performance for nearsighted and farsighted individuals, although it loses quality at intermediate distances. Our design, according to the tests we have developed, overcomes all these handicaps.” Additionally, they state that their corneal inlay is fully compatible with other laser refractive surgeries as well as subsequent cataract surgery. The tests the team performed compared their design with another model currently used in clinical practice using commercial optical design software and subsequently on an artificial eye with an optical simulator. Their next step is to test the design in non-invasive clinical trials.

In Other News
(1) Get the best eye care during COVID-19
(2) Healthy vision development begins from birth
(3) Eye conditions on the the rise during the pandemic (Related)

OCT Angiography to Monitor Sickle Cell Retinopathy

Article: Mount Sinai Ophthalmologists Develop New Technique to Assess Progression of Sickle Cell Retinopathy
Source: New York Eye and Ear Infirmary of Mount Sinai, via EurekAlert  and EyeWire News
Published: May 11, 2021

Sickle cell patients on treatment (hydroxyurea)
showed fewer areas of retinal non-perfusion

Researchers developed a new technique using optical coherence tomography angiography (OCTA) to evaluate progression of sickle cell retinopathy before symptoms manifest as well as assess the efficacy of pharmacological treatment over time. OCT angiography is especially useful in monitoring of sickle cell disease, an inherited red blood cell disorder, in the sense that the visualization of blood flow can detect areas where the sickle-shaped cells clump together and cause local vascular damage, potentially leading to ischemia or hemorrhages. Their study involved 27 participants, 14 of whom were controls and 13 of whom had sickle cell disease with retinopathy of varying severity; some were on standard therapy (hydroxyurea) and others were not on treatment. The series of OCTA scans showed, unsurprisingly, that healthy participants had consistent blood flow with no or very minimal fluctuations. The untreated sickle cell patients, however, had substantially more intermittent vaso-occlusion (more flickering between scan images) than patients on treatment, indicating that treatment was effective. The investigators then used a computer algorithm to assess risk of retinal blood blockages based on flicker frequency and locations of flickering. The lead investigator explains, “We have added a new dimension to ocular imaging technology that no one has thought of before. For the first time, we have shown that by doing rapid, repeated retinal imaging of sickle cell patients, you can see microscopic changes in blood vessels and blood flow. The more the blood flow fluctuates across images, the more at risk patients are for a permanent blockage, which severely damage their eyesight.” The early detection of retinopathy in asymptomatic sickle cell patients could prevent irreversible vision loss when it's too late. Additionally, given a multitude of pharmacological treatments for sickle cell disease, monitoring the effectiveness of a particular drug regimen with OCTA can better inform the choice of drugs.

My rating of this study: ⭐⭐⭐

Zhou DB, Castanos MV, Pinhas A, et al. "Quantification of intermittent retinal capillary perfusion in sickle cell disease." Biomedical Optics Express.  12(5):2825-2840. 2021. https://doi.org/10.1364/BOE.418874

Nano-SOD1 Antioxidant to Treat Ocular Inflammation

Article: Scientists Develop a Safe Drug to Treat Eye Inflammation
Source: National University of Science and Technology MISIS (Russia), via EyeWire News
Published: April 26, 2021

Antioxidant activity in the intraocular fluid of rabbits
on the 8th day of experimental immunogenic uveitis

A team of Russian scientists, along with American colleagues, developed an anti-inflammatory drug specifically for the eyes, which they tested in vivo in a rabbit model of uveitis. Their project involved creating multilayer polyion nanoparticles of the superoxide dismutase enzyme (Nano-SOD1), with a new formulation specifically manufactured for topical use on eyes. Superoxide dismutase is among antioxidant enzymes that are potent scavengers of reactive oxygen species (ROS), the excessive production of which during cellular processes leads to inflammation and tissue degeneration. When this inflammation occurs in ocular structures that are intricately arranged—such as the cornea, the lens, and the retina—the subsequent tissue damage can lead to blurred vision and even vision loss. While superoxide dismutase is an effective antioxidant, its very limited permeability presents a challenge for drug delivery to the eye, a relatively isolated organ that is often preferentially treated locally rather than systemically. As seen in their rabbit model of immunogenic uveitis, one of the authors states, "It was shown during preclinical studies that topical instillations of Nano-SOD1 were 35% more effective in reducing the manifestations of uveitis compared to the free enzyme SOD1. In particular, we noted statistically significant differences in such inflammatory signs of the eye as corneal and conjunctival edema, iris hyperemia and fibrin clots." The authors also report improved penetration into interior eye structures, longer retention of enzyme activity, and greater antioxidant activity for Nano-SOD1 compared to SOD1 by itself. So far, Nano-SOD1 shows a good drug safety profile at wide concentration ranges and at extremely high doses. Preclinical trials continue to explore its potential as a therapeutic agent for the treatment of ocular inflammation.

My rating of this study: ⭐⭐

Vaneev AN, Kost OA, Eremeev NL, et al. "Superoxide Dismutase 1 Nanoparticles (Nano-SOD1) as a Potential Drug for the Treatment of Inflammatory Eye Diseases." Biomedicines.  9(4):396. 7 April 2021. https://doi.org/10.3390/biomedicines9040396

Transparent Diffractive Corneal Inlay for Presbyopia

Article: Against presbyopia
Source: Universitat de València (Spain), via Corneal Physician
Published: April 23, 2021

Transparent diffractive trifocal corneal inlay design

As part of a doctoral thesis, researchers in Spain designed the first transparent diffractive trifocal intracorneal inlay as a potential treatment option for presbyopia. Consisting of an extremely thin (less than 5 microns) lens made of a biocompatible material, the corneal inlay would be placed inside the stroma of the cornea in a cavity created with a femtosecond laser. According to the research team, the lens is micro-perforated to allow the flow of nutrients within the cornea and to reduce the likelihood of rejection, and its transparent design does not prevent retinal examination. Like existing trifocal refractive technology, this corneal inlay corrects for vision at far, intermediate (computer range), and near vision. However, the authors highlight that the main novelty of this corneal inlay is that it uses a diffractive lens, as compared to existing small-aperature corneal inlays and refractive corneal inlays. One of the researchers explains, “The first type produce an extension of the depth of focus with which intermediate vision is gained but their luminous efficiency, since it is partially opaque, is low. The second type achieve[s] bifocality, so it has a good performance for nearsighted and farsighted individuals, although it loses quality at intermediate distances. Our design, according to the tests we have developed, overcomes all these handicaps.” Additionally, they state that their corneal inlay is fully compatible with other laser refractive surgeries as well as subsequent cataract surgery. The tests the team performed compared their design with another model currently used in clinical practice using commercial optical design software and subsequently on an artificial eye with an optical simulator. Their next step is to test the design in non-invasive clinical trials.

My rating of this study: ⭐⭐

Furlan WD, Montagud-Martínez D, Ferrando V, et al. "A new trifocal corneal inlay for presbyopia." Scientific Reports.  11:6620. 23 March 2021. https://doi.org/10.1038/s41598-021-86005-8

Contactless Robotic Optical Coherence Tomography

Article: Robotic Optical Coherence Tomography Revolutionizes Imaging
Source: Duke University Health
Published: April 20, 2021
Article: NEI-funded technology promises to broaden access to retinal scanning
Source: National Eye Institute
Published: July 22, 2021
Article: Robotic Scanner Automates Diagnostic Imaging in the Eye
Source: Duke Pratt School of Engineering
Published: August 4, 2021

OCT with a robotic arm and an active-tracking scan head
that automatically aligns itself with the patient’s pupil

Scientists developed a fully automated ocular coherence tomography (OCT) system that captures a three-dimensional image of the entire eye, including an active-tracking scanning head that aligns itself with the patient's pupil as well as the ability to safely distance operator and patient. The system builds off of earlier work in intraoperative OCT and handheld OCT from colleagues within their institution, now adapted for physical distancing challenges posed by the COVID-19 pandemic. For example, the present iteration of a robotic OCT foregoes the need for chin and forehead rests for stabilization, relying instead on two sets of cameras that track the face and pupil, moving with patient movements to compensate for subtle motions. Additionally, as a comfort feature, patients use a foot pedal to control the robotic arm; when the patient takes his/her foot off the pedal, the robot moves away. A contactless OCT imaging modality also improves accessibility for patients with mobility difficulties, such as children and those who are wheelchair-bound. Advancements in the optical design of this system include working on a wider depth of field and the ability to visualize both the anterior and posterior segments of the eye simultaneously, capturing the entire eye in 3D. The ultimate goal of the project is to provide a one-to-one exact digital replica of the eye, which would enable studies related to the shape of the eye. They are testing applications of the technology in people with elevated intracranial pressure as a potential substitute for more expensive MRIs or more invasive spinal taps. The robotic design of the system should also facilitate telemedicine, since the robot can be sent to rural settings to capture images, the interpretation of which can be done by eye doctors remotely. The investigators report that early results of their system are encouraging both in terms of patient satisfaction and accuracy compared to traditional OCT.

My rating of this study: ⭐⭐⭐

Draelos M, Ortiz P, Qian R, et al. "Contactless optical coherence tomography of the eyes of freestanding individuals with a robotic scanner." Nature Biomedical Engineering.  5:726–736. 12 July 2021. https://doi.org/10.1038/s41551-021-00753-6

Case Report: Fundus Albipunctatus Diagnosis Using Adaptive Optics SLO and Genetic Testing

Article: Mount Sinai Ophthalmologists use Breakthrough Imaging to Understand Eye Damage from Rare Congenital Retinal Disease
Source: New York Eye and Ear Infirmary of Mount Sinai, in EyeWire News
Published: April 20, 2021
Article: Adaptive optics SLO sheds light on fundus albipunctatus
Source: Modern Retina
Published: July 5, 2021

AOSLO of the cone photoreceptor mosaic in the central fovea
of the right eye of the patient (A,C) and a healthy control (B,D)

Investigators presented a rare case of a 62-year-old man who was misdiagnosed with Stargardt's disease for ten years before adaptive optics scanning laser ophthalmoscopy (AOSLO) and genetic testing revealed a diagnosis of fundus albipunctatus. Much rarer than Stargardt's macular dystrophy, fundus albipunctatus is an inherited retinal disease categorized under congenital stationary night blindness (CSNB) that is associated with fundus findings. In fundus albipunctatus, an autosomal recessive mutation in the RDH5 gene disrupts proper retinoid recycling. Also unlike the progressive retinal deterioration seen in Stargardt's disease, the clinical course of fundus albipunctatus does not change over time. While a diagnosis of fundus albipunctatus is confirmed with genetic testing, AOSLO was helpful in explaining the patient's clinical presentation and characterizing the photoreceptor status of the disease. The investigators explain, "Previous studies using AOSLO in Stargardt’s disease have revealed increased cone and rod spacing, with reduced foveal cone density and enlarged cone size, and dark cones thought to be associated with foreshortened outer segments. These findings are similar to our patient’s photoreceptor characteristics on AOSLO, except for the profound sparing observed in the central fovea." The use of AOSLO to characterize the cellular structure of photoreceptors in a patient with fundus albipunctatus revealed that in the later stages of the disease, some structures of photoreceptor cells were preserved, advancing understanding of the condition for future gene therapies. While fundus albipunctatus currently has no cure, earlier detection can inform better genetic counseling for family members as well as better counsel patients regarding the prognosis of the condition.

My rating of this study: ⭐🌸

Sobolab EK, Deobhaktaab A, Wilkins CS, et al. "Fundus albipunctatus photoreceptor microstructure revealed using adaptive optics scanning light ophthalmoscopy." American Journal of Ophthalmology Case Reports.  22:101090. June 2021. https://doi.org/10.1016/j.ajoc.2021.101090

Week in Review: Number 23

Advancements in Contact Lenses as Theranostics
The Centre for Ocular Research & Education (CORE) published a comprehensive review of upcoming advancements in contact lenses as part of a special edition Contact Lens Evidence-based Academic Reports (CLEAR) series. The review highlights contact lens applications beyond refractive error correction, using diverse technologies such as "novel biomaterials, nanotechnology progress, unique optical designs, biosensing discoveries, antibacterial agents and even battery miniaturization and power transfer," according to the paper's lead author. Applications range from detecting ocular and systemic disease biomarkers in the tear film to monitoring intraocular pressure with progress in integrated circuitry, to microelectronics that monitor corneal gaze direction and adjust in real-time for presbyopia correction. Advancements in fluid dynamics and materials science could also provide alternative therapies for dry eye disease or mitigate color vision deficiencies (by filtering light). Another exciting potential for contact lenses is in theranostics, "the multi-disciplinary medical field that combines therapeutics and diagnostics." For example, contact lenses could act as a drug delivery alternative to eye drops, with advantages such as increasing the residence time of a drug on the ocular surface for more continuous drug regimens. The review ends with highlights of contact lens case designs that mitigate contamination. While many of these projects are in early stages, a review of developments in the field of contact lenses beyond refractive error correction, where contact lenses continue to make ubiquitous impact, is a worthwhile addition to the literature.

Treatment for Herpetic Stromal Keratitis with CRISPR
Herpes stromal keratitis (HSK) is a leading cause of infectious blindness worldwide. Yet, despite its high prevalence, there is as yet no vaccine for herpes simplex virus type 1 (HSV-1). After primary infection and replication in the cornea, HSV-1 is transported via ophthalmic nerves to the trigeminal ganglia (TG) where it can stay dormant and be reactivated in immunocompromised situations. With frequent recurrence and aggravation, the disease can lead to herpetic stromal keratitis (HSK) and potential blindness due to immune-mediated tissue damage. Researchers in China explored using CRISPR-Cas9 to edit HSV-1 in the treatment of HSK in mice, and blocked HSV-1 replication in human corneas. Previous studies using CRISPR-Cas9 or adeno-associated virus (AAV) vector were conducted in vitro, as compared to the present in vivo studies. The Shanghai researchers developed mRNA-carrying lentiviral particles "that simultaneously deliver SpCas9 [CRISPR-Cas9] mRNA and viral-gene-targeting guide RNAs (designated HSV-1-erasing lentiviral particles, termed HELP)." These particles move by retrograde transport to the trigeminal ganglia to target HSV-1 genomes directly. This approach is interesting in the sense that clearing neuronal reservoirs of latent HSV-1 could prevent the recurrence of HSK, as was shown in mice in this case. The researchers reported no significant side effects in the HELP-treated mice, and whole-genome sequencing of human-derived corneas infected with HSV-1 showed inhibited viral replication without causing off-target effects. Current treatments for HSK, such as acyclovir (ACV) and other anti-virals, inhibit DNA replication but do not prevent disease recurrence. The researchers think their findings "could support the potential clinical utility of HELP for treating refractory HSK."

Topical Drops to Close Macular Holes without Surgery
A retrospective multicenter case series explored the use of medicated eye drops to close small full-thickness macular holes (FTMHs). Depending on its size, a macular hole can cause distorted vision or an area of missing vision (scotoma) at the center of a person's field of view. For macular holes that do not close on their own, a vitrectomy can be performed. However, the gas bubble that serves as a temporary bandage to help the edges of the macular hole join and close requires the patient to remain face down for several days and nights so that the gas bubble can float in the correct position at the back of the eye. As with any intraocular surgery, vitrectomy also carries risks, notably cataract formation and retinal detachment. An alternative medical treatment to close macular holes could be attempted prior to considering surgery. The senior author of the study comments, “For certain patients, medicated drops may heal their macular hole by decreasing inflammation and increasing fluid absorption in the retina.” The study involved 14 patients prescribed a daily three-drug regimen of a steroid (prednisolone or difluprednate), an NSAID (ketorolac or bromfenac), and a carbonic anhydrase inhibitor (brinzolamide or dorzolamide). The study reports that 12 of the 14 patients had holes that closed within 2 to 8 weeks, 2-4 times faster than spontaneous closure rates. The treatment was most effective for patients whose macular holes were less than 200 microns in diameter, with treatment duration (including tapering) ranging from 3.5 to 20 months. The two patients who did not experience hole closure during the study duration had multiple missed follow-up appointments, the article noted. The researchers next plan to conduct a prospective randomized trial to definitively determine the benefits of the eye drops compared to the natural history of macular hole closure.

Joint Research in RGC Stem Cell Transplants in Mice
Researchers presented the first successful attempt to transplant retinal ganglion cells (RGCs) derived from induced pluripotent stem cells, tested in mice. The axons of retinal ganglion cells comprise the optic nerve that transmits visual information from the retina to the brain; it is these axons that are damaged in optic neuropathies, including glaucoma, and could lead to irreversible vision loss. The retinal cells were grown using special organoids and transplanted in mice induced to have glaucoma with microbeads as well as chemically induced neurotoxity. The transplanted donor cells exceeded a 65% success rate and survived in the disease-like microenvironments for up to 12 months (a long duration for mice), independent of the donor's age and location of transplantation. The researchers confirmed that the cells could receive signals from other neurons in the retina, but the ability of the cells to transmit signals to the brain was not definitively assessed "due to the relatively low number of cells surviving the procedure." This proof-of-concept technology is many years from clinical applications. However, as an author of the study explains, "This technique will enable countless future studies into the cross talk between transplanted cells and the host microenvironment. This will allow us to find and employ molecular mechanisms which will help transplanted cells to function properly and, as a result, improve visual function when transplanted in the right quantity." Because the eye is an immune-privileged organ, future possibilities could include growing cells in advance from universal donors (as compared to autologous transplants), and storing them in cell banks for the selection of the most suitable cells for transplantation in different patients. They look forward to the research bringing "hope and excitement for cell therapy development."

Depth Perception in Strabismus with Perspective Cues
Colloquially called an eye turn, strabismus is the misalignment of the eyes from the straight forward (orthotropic) position, with one eye turned, for example, outward (temporally) in exotropia or inward (nasally) in esotropia, which can be constant or intermittent. Because the deviation affects normal binocular vision, it is commonly thought that strabismus limits stereopsis or depth perception. Scientists in the U.K. argue that much of what is known about strabismus (in the U.K.) is based on conjecture or on studies that measured motor tasks at near distances. They instead investigated the perception of relative depth from the cue of linear perspective in strabismus using pictorial images and found that "contrary to popular opinion, there is little impact on a major aspect of 3-D vision for a person with a strabismic vision." The senior author of the study explains, “These results support a theory that the brain constructs at least three distinct types of representation that support different competencies in 3-D vision: the perception of distances of objects near the viewer (in personal space), the perception of object distances at walking distances, and the perception of 3-D object shape/layout (relative depth). Our results, combined with a few other studies, suggest that individuals with strabismus may be impaired only in the first component (perception of near distances) but have largely intact perception in the other two.” In other words, although strabismics do experience deficit in depth perception, that limitation applies mostly to near vision, with depth perception of farther distances still largely functional. The study, however, is limited by its very small sample size of 32 individuals, 24 of whom had normal stereovision. A quantitative measure of heterotropia was also not reported in the 8 participants with "no or limited stereovision."

In Other News
(1) Contact lenses: Moving beyond refractive error correction
(2) The scientifically best way to create a photo gallery wall
(3) Increased screen time during pandemic affects eyesight (Related)

Joint Research in RGC Stem Cell Transplants in Mice

Article: MIPT and Harvard researchers grow stem cells to cure glaucoma
Source: Moscow Institute of Physics and Technology (Russia), via News Medical  and EurekAlert
Published: April 19, 2021

Donor RGCs (green) survive past 12 months
and form axonal projections to the optic nerve

Researchers presented the first successful attempt to transplant retinal ganglion cells derived from induced pluripotent stem cells, tested in mice. The axons of retinal ganglion cells comprise the optic nerve that transmits visual information from the retina to the brain; it is these axons that are damaged in optic neuropathies, including glaucoma, and could lead to irreversible vision loss. The retinal cells were grown using special organoids and transplanted in mice induced to have glaucoma with microbeads as well as chemically induced neurotoxity. The transplanted donor cells exceeded a 65% success rate and survived in the disease-like microenvironments for up to 12 months (a long duration for mice), independent of the donor's age and location of transplantation. The researchers confirmed that the cells could receive signals from other neurons in the retina, but the ability of the cells to transmit signals to the brain was not definitively assessed "due to the relatively low number of cells surviving the procedure." This proof-of-concept technology is many years from clinical applications. However, as an author of the study explains, "This technique will enable countless future studies into the cross talk between transplanted cells and the host microenvironment. This will allow us to find and employ molecular mechanisms which will help transplanted cells to function properly and, as a result, improve visual function when transplanted in the right quantity." Because the eye is an immune-privileged organ, future possibilities could include growing cells in advance from universal donors (as compared to autologous transplants), and storing them in cell banks for the selection of the most suitable cells for transplantation in different patients. They look forward to the research bringing "hope and excitement for cell therapy development."

My rating of this study: ⭐⭐

Oswald J, Kegeles E, Minelli T, et  al. "Transplantation of miPSC/mESC-derived retinal ganglion cells into healthy and glaucomatous retinas." Molecular Therapy - Methods and Clinical Development.  10 March 2021. https://doi.org/10.1016/j.omtm.2021.03.004

Advancements in Contact Lenses as Theranostics

Article: Contact Lenses to Advance Well Beyond Refractive Error Correction
Source: University of Waterloo (Canada), via EyeWire News
Published: March 29, 2021

Microbiosensor in a contact lens case
that can indicate microbial contamination

The Centre for Ocular Research & Education (CORE) published a comprehensive review of upcoming advancements in contact lenses as part of a special edition Contact Lens Evidence-based Academic Reports (CLEAR) series. The review highlights contact lens applications beyond refractive error correction, using diverse technologies such as "novel biomaterials, nanotechnology progress, unique optical designs, biosensing discoveries, antibacterial agents and even battery miniaturization and power transfer," according to the paper's lead author. Applications range from detecting ocular and systemic disease biomarkers in the tear film to monitoring intraocular pressure with progress in integrated circuitry, to microelectronics that monitor corneal gaze direction and adjust in real-time for presbyopia correction. Advancements in fluid dynamics and materials science could also provide alternative therapies for dry eye disease or mitigate color vision deficiencies (by filtering light). Another exciting potential for contact lenses is in theranostics, "the multi-disciplinary medical field that combines therapeutics and diagnostics." For example, contact lenses could act as a drug delivery alternative to eye drops, with advantages such as increasing the residence time of a drug on the ocular surface for more continuous drug regimens. The review ends with highlights of contact lens case designs that mitigate contamination. While many of these projects are in early stages, a review of developments in the field of contact lenses beyond refractive error correction, where contact lenses continue to make ubiquitous impact, is a worthwhile addition to the literature.

My rating of this study: ⭐⭐⭐

Jones L, Hui A, Phan C, et al. "CLEAR - Contact lens technologies of the future." Contact Lens & Anterior Eye.  44(2):398–430. 1 April 2021. https://doi.org/10.1016/j.clae.2021.02.007

Topical Drops to Close Macular Holes without Surgery

Article: Study finds medicated eye drops may help close macular holes without surgery
Source: University of Chicago Medicine, via Retinal Physician
Published: January 29, 2021

A retrospective multicenter case series explored the use of medicated eye drops to close small full-thickness macular holes (FTMHs). Depending on its size, a macular hole can cause distorted vision or an area of missing vision (scotoma) at the center of a person's field of view. For macular holes that do not close on their own, a vitrectomy can be performed. However, the gas bubble that serves as a temporary bandage to help the edges of the macular hole join and close requires the patient to remain face down for several days and nights so that the gas bubble can float in the correct position at the back of the eye. As with any intraocular surgery, vitrectomy also carries risks, notably cataract formation and retinal detachment. An alternative medical treatment to close macular holes could be attempted prior to considering surgery. The senior author of the study comments, “For certain patients, medicated drops may heal their macular hole by decreasing inflammation and increasing fluid absorption in the retina.” The study involved 14 patients prescribed a daily three-drug regimen of a steroid (prednisolone or difluprednate), an NSAID (ketorolac or bromfenac), and a carbonic anhydrase inhibitor (brinzolamide or dorzolamide). The study reports that 12 of the 14 patients had holes that closed within 2 to 8 weeks, 2-4 times faster than spontaneous closure rates. The treatment was most effective for patients whose macular holes were less than 200 microns in diameter, with treatment duration (including tapering) ranging from 3.5 to 20 months. The two patients who did not experience hole closure during the study duration had multiple missed follow-up appointments, the article noted. The researchers next plan to conduct a prospective randomized trial to definitively determine the benefits of the eye drops compared to the natural history of macular hole closure.

My rating of this study: ⭐⭐⭐

Sokol JT, Schechet SA, Komati R, et al. "Macular Hole Closure with Medical Treatment." Ophthalmology Retina.  15 December 2020. https://doi.org/10.1016/j.oret.2020.11.018

Depth Perception in Strabismus with Perspective Cues

Article: Common eyesight myth demystified
Source: University of St. Andrews (U.K.), via Medical Xpress
Published: January 28, 2021

Colloquially called an eye turn, strabismus is the misalignment of the eyes from the straight forward (orthotropic) position, with one eye turned, for example, outward (temporally) in exotropia or inward (nasally) in esotropia, which can be constant or intermittent. Because the deviation affects normal binocular vision, it is commonly thought that strabismus limits stereopsis or depth perception. Scientists in the U.K. argue that much of what is known about strabismus (in the U.K.) is based on conjecture or on studies that measured motor tasks at near distances. They instead investigated the perception of relative depth from the cue of linear perspective in strabismus using pictorial images and found that "contrary to popular opinion, there is little impact on a major aspect of 3-D vision for a person with a strabismic vision." The senior author of the study explains, “These results support a theory that the brain constructs at least three distinct types of representation that support different competencies in 3-D vision: the perception of distances of objects near the viewer (in personal space), the perception of object distances at walking distances, and the perception of 3-D object shape/layout (relative depth). Our results, combined with a few other studies, suggest that individuals with strabismus may be impaired only in the first component (perception of near distances) but have largely intact perception in the other two.” In other words, although strabismics do experience deficit in depth perception, that limitation applies mostly to near vision, with depth perception of farther distances still largely functional. The study, however, is limited by its very small sample size of 32 individuals, 24 of whom had normal stereovision. A quantitative measure of heterotropia was also not reported in the 8 participants with "no or limited stereovision."

My rating of this study: ⭐

Zlatkute G, Sagnay de la Bastida VC and Vishwanath D. "Unimpaired perception of relative depth from perspective cues in strabismus." Royal Society Open Science.  7:200955. 23 December 2020. https://doi.org/10.1098/rsos.200955

Treatment for Herpetic Stromal Keratitis with CRISPR

Article: Local researchers develop cure for keratitis
Source: Shanghai Jiao Tong University (China) in Shanghai Daily, via Corneal Physician
Published: January 12, 2021
Article: HELP is on its way for herpetic stromal keratitis
Source: BioWorld
Published: January 19, 2021

Mouse eyes with different treatments

Herpes stromal keratitis (HSK) is a leading cause of infectious blindness worldwide. Yet, despite its high prevalence, there is as yet no vaccine for herpes simplex virus type 1 (HSV-1). After primary infection and replication in the cornea, HSV-1 is transported via ophthalmic nerves to the trigeminal ganglia (TG) where it can stay dormant and be reactivated in immunocompromised situations. With frequent recurrence and aggravation, the disease can lead to herpetic stromal keratitis (HSK) and potential blindness due to immune-mediated tissue damage. Researchers in China explored using CRISPR-Cas9 to edit HSV-1 in the treatment of HSK in mice, and blocked HSV-1 replication in human corneas. Previous studies using CRISPR-Cas9 or adeno-associated virus (AAV) vector were conducted in vitro, as compared to the present in vivo studies. The Shanghai researchers developed mRNA-carrying lentiviral particles "that simultaneously deliver SpCas9 [CRISPR-Cas9] mRNA and viral-gene-targeting guide RNAs (designated HSV-1-erasing lentiviral particles, termed HELP)." These particles move by retrograde transport to the trigeminal ganglia to target HSV-1 genomes directly. This approach is interesting in the sense that clearing neuronal reservoirs of latent HSV-1 could prevent the recurrence of HSK, as was shown in mice in this case. The researchers reported no significant side effects in the HELP-treated mice, and whole-genome sequencing of human-derived corneas infected with HSV-1 showed inhibited viral replication without causing off-target effects. Current treatments for HSK, such as acyclovir (ACV) and other anti-virals, inhibit DNA replication but do not prevent disease recurrence. The researchers think their findings "could support the potential clinical utility of HELP for treating refractory HSK."

My rating of this study: ⭐⭐

Yin D, Ling S, Wang D, et al. "Targeting herpes simplex virus with CRISPR–Cas9 cures herpetic stromal keratitis in mice." Nature Biotechnology.  39:567–577. 11 January 2021. https://doi.org/10.1038/s41587-020-00781-8

Week in Review: Number 22

CIB2 Regulates mTORC1 in Photoreceptor Autophagy
Researchers studying the pathology of age-related macular degeneration (AMD) are looking into two proteins that play a role in photoreceptor autophagy, a process that is essential for proper light detection. Without this clean up by support cells, the undigested photoreceptor material accumulates over time, contributing to a toxic environment that eventually leads to retinal degeneration and potential vision loss. One protein called calcium and integrin binding protein 2 (CIB2) seems to be responsible for this photoreceptor maintenance; eyes of mice engineered without the CIB2 gene exhibited sub-retinal pigment epithelium (RPE) deposits, marked accumulation of drusen markers, reduced lysosomal capacity and autophagic clearance, and impaired visual function. The researchers also investigated the role of a protein called mTORC1. Involved in other human diseases such as cancer, obesity, and epilepsy, mTORC1 plays a regulatory role in processes such as cleaning up cellular debris. They found that mTORC1 was overactive in mice with CIB2 mutation and also overactive in human eye tissue samples from people with dry AMD. The mTOR protein comes in two varieties: mTORC1 and mTORC2. Because mTOR is involved in many other cellular processes, tinkering with it leads to major side effects. The researchers think regulating mTORC1 via CIB2 can bypass many of these adverse effects. One of the authors says, “Using the evolving understanding of the mechanistic role of mTORC1, this study has provided great insights into new ways that researchers can begin to find ways to preserve, to treat, and/or improve macular degeneration, and thus improve the quality of life and independent living in many older adults.”

Selective Threshold Modulation by Inhibitory Neurons Increases Information Transmission
Researchers at the Salk Institute explored modulation of neuronal thresholds via inhibitory neurons, testing their theory with cells in the retina. Although ubiquitous in the brain, threshold modulation also reduces the strength of the original signal, prompting the question as to why such a dampening effect is so widespread. The researchers found that the decrease in information transmission can be nearly completely eliminated if modulation by inhibitory neurons is applied selectively to the most sparsely responding neurons (i.e., neurons with the lowest spike rate) in a circuit or group of neurons. The authors provide a mathematical (but intuitive) explanation for this outcome relating to the curve of the information function of a single neuron with respect to its threshold. As they explain in the paper, "This function is concave for small thresholds and convex for large thresholds. This is important because concave functions decrease their value upon averaging of their inputs, as occurs as a result of threshold modulation, while convex functions increase their value. This means that neurons with small thresholds, i.e., high spike rates, will suffer a decrease in information transmission upon modulation. In contrast, neurons with large thresholds, i.e., small spike rates, will increase information transmission upon threshold modulation." Therefore, neurons with the smallest spike rates will experience the greatest increase in information transmission upon threshold modulation. The researchers tested their predictions on retinal (sustained Off-type) amacrine cells modulating pairs of ganglion cells, termed "adapting" and "sensitizing," with "the main differences between these cell types are that adapting cells have higher thresholds and larger noise levels than sensitizing cells." They found that the amacrine cells exert a stronger effect on the threshold of adapting ganglion cells than on the threshold of sensitizing ganglion cells. The exploratory techniques involve intracellular current injection of the neurons, which is far from clinical applications. Nonetheless, these findings at the cellular level are relevant in providing knowledge in foundational science that could inform later clinical treatments.

U.K. Research Project Aims to Validate the OHTS-EGPS Glaucoma Risk Calculator
A research team in the U.K. will begin a two-year project launched in June 2021 to improve upon ways to monitor ocular hypertension (elevated intraocular pressure) as a risk factor for developing glaucoma, which could lead to vision loss if not detected and treated early. Eye pressure is considered high if it is above 21 mmHg, although there is debate as to whether and how to treat individuals with eye pressures in the low and mid-20s. Benefit from treatment with pressure-lowering eye drops are weighed against adverse effects, cost, and inconvenience given a lifetime commitment to daily use of medications. An alternative first-line therapy is selective laser trabeculoplasty (SLT), although this option is not effective on all patients. Thus, having a tool to assess risk of progression from ocular hypertension to glaucoma, which is defined by irreversible damage to the optic nerve (and other ocular structures), would support decision-making for both clinicians and patients. A glaucoma risk calculator called OHTS-EGPS is available that makes use of data from the Ocular Hypertension Treatment Study and the European Glaucoma Prevention Study clinical trials. However, the U.K. researchers state that this risk calculator has not been tested on U.K. patients. Their project aims to validate the OHTS-EGPS for use within the U.K. health care system through review of anonymised NHS electronic medical records of over 23,000 people who have ocular hypertension. More specific goals of the project include investigating the value of treatment in people with ocular hypertension of 22 or 23 mmHg and validating the tool for different follow-up intervals of patients according to their glaucoma risk. Ultimately, the project seeks to "both optimise the management of people with ocular hypertension and reduce costs related to the management of the condition."

Visual Backward Masking in Young Infants
Visual perception begins in the retina and is serially processed in increasingly higher levels of the visual cortex in a bottom-up manner. However, top-down feedback is also sent from higher to lower visual areas. Researchers in Japan interested in the interference of this feedback processing in the brain studied visual backward masking in infants ages 3 to 8 months. Visual backward masking occurs when the perception of a second object masks the perception of an immediately preceding object. This phenomenon occurs even if the second object does not spatially overlap the first object, such as a contour or four dots surrounding the object in the present experiments. To test visual perception in infants, the researchers presented them with images of faces on a computer screen and measured the time they spent looking at the images, taking into account that infants look longer at faces (compared to other images). The faces were presented in two conditions: followed by a mask image and followed by nothing. They discovered that infants ages 7 to 8 months, similar to adults, could not see the faces if followed by a mask image, indicating that backward masking had occurred. In contrast, infants ages 3 to 6 months could perceive the faces even if they were followed by the mask. This indicates that visual backward masking did not occur, suggesting that feedback processing is immature in the brains of infants younger than 7 months of age. The study's first author explains, "[Y]ounger infants do not have feedback processing that backward masking should interfere, and thus, masking is ineffective for them." In the latter half of their first year of life, top-down processing begins to take effect to provide that feedback, which becomes important in robustly perceiving ambiguous visual input. As another author proffers, "In return for susceptibility to visual masking, we acquire the ability to robustly perceive ambiguous visual scenes."

Small Study Investigated the Effect of Pure Cocoa Ingestion on Photopic Visual Acuity
Researchers in Spain explored the effect of two dietary polyphenols—cocoa flavanols and red berry anthocyanins—on visual acuity and cone-mediated dark adaptation in healthy eyes. The study involved 37 healthy volunteers who drank a glass of milk with 2.5 grams of pure cocoa, 10 grams of freeze-dried berry powder, or just milk (control) on three separate visits. Interestingly, this study explored the effect of these polyphenols in acute doses, as compared to longer term dietary routines. Levels of polyphenols in their urine were measured after three hours. Visual acuity was tested in both photopic ("daylight") and mesopic (low light) conditions. Tests of dark adaptation were also performed. Results of the study showed improvement in photopic visual acuity in the group that ingested pure cocoa. This effect was not seen in mesopic conditions with either cocoa or red berry ingestion, and the trend toward improvement in photopic conditions with red berry ingestion was not statistically significant. The investigators hypothesized that the flavanols and theobromine, both alkaloids that stimulate the central nervous system and found in cocoa, increase visual acuity by improving attention or processing of visual information. The authors emphasized that questionnaires and eye examinations were conducted to demonstrate the absence of dietary factors or prior pathology that could influence the analysis of results. However, they also acknowledged that the effect of caffeine, which is also present in pure cocoa, is unknown and could also contribute to the results. Overall, studies into diet potentially have many confounding variables, and a small sample size limits how far results can be extrapolated. The authors conclude, "This work highlights the need for new research that delves deeper into the effect of flavanols, anthocyanins and methylxanthines on visual acuity and attention, both in acute and chronic interventions."

In Other News
(1) Children prioritize hearing over vision given emotionally incongruent input
(2) Art: Sculptural installation explores color and light
(3) Time compression in virtual reality (Related)

U.K. Research Project Aims to Validate the OHTS-EGPS Glaucoma Risk Calculator

Article: Teaming up to monitor high eye pressure
Source: City, University of London (U.K.)
Published: June 28, 2021

A research team in the U.K. will begin a two-year project launched in June 2021 to improve upon ways to monitor ocular hypertension (elevated intraocular pressure) as a risk factor for developing glaucoma, which could lead to vision loss if not detected and treated early. Eye pressure is considered high if it is above 21 mmHg, although there is debate as to whether and how to treat individuals with eye pressures in the low and mid-20s. Benefit from treatment with pressure-lowering eye drops are weighed against adverse effects, cost, and inconvenience given a lifetime commitment to daily use of medications. An alternative first-line therapy is selective laser trabeculoplasty (SLT), although this option is not effective on all patients. Thus, having a tool to assess risk of progression from ocular hypertension to glaucoma, which is defined by irreversible damage to the optic nerve (and other ocular structures), would support decision-making for both clinicians and patients. A glaucoma risk calculator called OHTS-EGPS is available that makes use of data from the Ocular Hypertension Treatment Study and the European Glaucoma Prevention Study clinical trials. However, the U.K. researchers state that this risk calculator has not been tested on U.K. patients. Their project aims to validate the OHTS-EGPS for use within the U.K. health care system through review of anonymised NHS electronic medical records of over 23,000 people who have ocular hypertension. More specific goals of the project include investigating the value of treatment in people with ocular hypertension of 22 or 23 mmHg and validating the tool for different follow-up intervals of patients according to their glaucoma risk. Ultimately, the project seeks to "both optimise the management of people with ocular hypertension and reduce costs related to the management of the condition."

Personal commentary: This article is unusual as a feature on this blog in that it presents an upcoming research project rather than published data. However, the topic of validating the current OHTS-EGPS glaucoma risk calculator in an additional population and large sample size, with aims to improve upon the calculator's predictive value, should be an interesting project to see data from.

My rating of this article: ⭐⭐

Visual Backward Masking in Young Infants

Article: Babies Can See Things Adults Can't Due to Visual Masking Phenomenon
Source: Chuo University (Japan), in Technology Networks
Published: June 28, 2021

Adorable illustration explaining that infants younger than
7 months perceive the face even when followed by the contour
mask, while infants older than 7 months and adults do not

Visual perception begins in the retina and is serially processed in increasingly higher levels of the visual cortex in a bottom-up manner. However, top-down feedback is also sent from higher to lower visual areas. Researchers in Japan interested in the interference of this feedback processing in the brain studied visual backward masking in infants ages 3 to 8 months. Visual backward masking occurs when the perception of a second object masks the perception of an immediately preceding object. This phenomenon occurs even if the second object does not spatially overlap the first object, such as a contour or four dots surrounding the object in the present experiments. To test visual perception in infants, the researchers presented them with images of faces on a computer screen and measured the time they spent looking at the images, taking into account that infants look longer at faces (compared to other images). The faces were presented in two conditions: followed by a mask image and followed by nothing. They discovered that infants ages 7 to 8 months, similar to adults, could not see the faces if followed by a mask image, indicating that backward masking had occurred. In contrast, infants ages 3 to 6 months could perceive the faces even if they were followed by the mask. This indicates that visual backward masking did not occur, suggesting that feedback processing is immature in the brains of infants younger than 7 months of age. The study's first author explains, "[Y]ounger infants do not have feedback processing that backward masking should interfere, and thus, masking is ineffective for them." In the latter half of their first year of life, top-down processing begins to take effect to provide that feedback, which becomes important in robustly perceiving ambiguous visual input. As another author proffers, "In return for susceptibility to visual masking, we acquire the ability to robustly perceive ambiguous visual scenes."

My rating of this study: ⭐⭐

Nakashima Y, Kanazawa S and Yamaguchi MK. "Perception of invisible masked objects in early infancy." PNAS.  118(27):e2103040118. 6 July 2021. https://doi.org/10.1073/pnas.2103040118

Small Study Investigated the Effect of Pure Cocoa Ingestion on Photopic Visual Acuity

Article: 2.5 grammes of pure cocoa found to improve visual acuity in daylight
Source: Universidad Complutense de Madrid (Spain)
Published: May13, 2021
Article: Eating Pure Cocoa May Improve Vision in Daylight
Source: Technology Networks
Published: June 25, 2021

Researchers in Spain explored the effect of two dietary polyphenols—cocoa flavanols and red berry anthocyanins—on visual acuity and cone-mediated dark adaptation in healthy eyes. The study involved 37 healthy volunteers who drank a glass of milk with 2.5 grams of pure cocoa, 10 grams of freeze-dried berry powder, or just milk (control) on three separate visits. Interestingly, this study explored the effect of these polyphenols in acute doses, as compared to longer term dietary routines. Levels of polyphenols in their urine were measured after three hours. Visual acuity was tested in both photopic ("daylight") and mesopic (low light) conditions. Tests of dark adaptation were also performed. Results of the study showed improvement in photopic visual acuity in the group that ingested pure cocoa. This effect was not seen in mesopic conditions with either cocoa or red berry ingestion, and the trend toward improvement in photopic conditions with red berry ingestion was not statistically significant. The investigators hypothesized that the flavanols and theobromine, both alkaloids that stimulate the central nervous system and found in cocoa, increase visual acuity by improving attention or processing of visual information. The authors emphasized that questionnaires and eye examinations were conducted to demonstrate the absence of dietary factors or prior pathology that could influence the analysis of results. However, they also acknowledged that the effect of caffeine, which is also present in pure cocoa, is unknown and could also contribute to the results. Overall, studies into diet potentially have many confounding variables, and a small sample size limits how far results can be extrapolated. The authors conclude, "This work highlights the need for new research that delves deeper into the effect of flavanols, anthocyanins and methylxanthines on visual acuity and attention, both in acute and chronic interventions."

My rating of this study: ⭐

Puella MC and Pascual-Teresa S. "The acute effect of cocoa and red-berries on visual acuity and cone-mediated dark adaptation in healthy eyes." Journal of Functional Foods.  81:104435. 10 April 2021. https://doi.org/10.1016/j.jff.2021.104435

Selective Threshold Modulation by Inhibitory Neurons Increases Information Transmission

Article: How Neurons Get Past "No"
Source: Salk Institute, via NEI
Published: June 24, 2021

Information (red line) changes convexity
(dashed vs. solid curves) as a function of threshold

Researchers at the Salk Institute explored modulation of neuronal thresholds via inhibitory neurons, testing their theory with cells in the retina. Although ubiquitous in the brain, threshold modulation also reduces the strength of the original signal, prompting the question as to why such a dampening effect is so widespread. The researchers found that the decrease in information transmission can be nearly completely eliminated if modulation by inhibitory neurons is applied selectively to the most sparsely responding neurons (i.e., neurons with the lowest spike rate) in a circuit or group of neurons. The authors provide a mathematical (but intuitive) explanation for this outcome relating to the curve of the information function of a single neuron with respect to its threshold. As they explain in the paper, "This function is concave for small thresholds and convex for large thresholds. This is important because concave functions decrease their value upon averaging of their inputs, as occurs as a result of threshold modulation, while convex functions increase their value. This means that neurons with small thresholds, i.e., high spike rates, will suffer a decrease in information transmission upon modulation. In contrast, neurons with large thresholds, i.e., small spike rates, will increase information transmission upon threshold modulation." Therefore, neurons with the smallest spike rates will experience the greatest increase in information transmission upon threshold modulation. The researchers tested their predictions on retinal (sustained Off-type) amacrine cells modulating pairs of ganglion cells, termed "adapting" and "sensitizing," with "the main differences between these cell types are that adapting cells have higher thresholds and larger noise levels than sensitizing cells." They found that the amacrine cells exert a stronger effect on the threshold of adapting ganglion cells than on the threshold of sensitizing ganglion cells. The exploratory techniques involve intracellular current injection of the neurons, which is far from clinical applications. Nonetheless, these findings at the cellular level are relevant in providing knowledge in foundational science that could inform later clinical treatments.

My rating of this study: ⭐⭐⭐

Hsu WM, Kastner DB, Baccus SA, et al. "How inhibitory neurons increase information transmission under threshold modulation." Cell Reports.  35(8):109158. 25 May 2021. https://doi.org/10.1016/j.celrep.2021.109158

CIB2 Regulates mTORC1 in Photoreceptor Autophagy

Article: Recycling of the Eye’s Light Sensors Is Faulty in Progressive Blindness of Older Adults
Source: University of Maryland School of Medicine, via ScienceDaily
Published: June 23, 2021
Article: Macular Degeneration Linked to Inability to Remove Damaged Photoreceptors
Source: Genetic Engineering & Biotechnology News
Published: June 24, 2021

Photoreceptors (purple) accumulate in mouse eyes with CIB2
mutation (upper right). Undigested photoreceptor material
(yellow) in mouse eyes with CIB2 mutation (lower right).

Researchers studying the pathology of age-related macular degeneration (AMD) are looking into two proteins that play a role in photoreceptor autophagy, a process that is essential for proper light detection. Without this clean up by support cells, the undigested photoreceptor material accumulates over time, contributing to a toxic environment that eventually leads to retinal degeneration and potential vision loss. One protein called calcium and integrin binding protein 2 (CIB2) seems to be responsible for this photoreceptor maintenance; eyes of mice engineered without the CIB2 gene exhibited sub-retinal pigment epithelium (RPE) deposits, marked accumulation of drusen markers, reduced lysosomal capacity and autophagic clearance, and impaired visual function. The researchers also investigated the role of a protein called mTORC1. Involved in other human diseases such as cancer, obesity, and epilepsy, mTORC1 plays a regulatory role in processes such as cleaning up cellular debris. They found that mTORC1 was overactive in mice with CIB2 mutation and also overactive in human eye tissue samples from people with dry AMD. The mTOR protein comes in two varieties: mTORC1 and mTORC2. Because mTOR is involved in many other cellular processes, tinkering with it leads to major side effects. The researchers think regulating mTORC1 via CIB2 can bypass many of these adverse effects. One of the authors says, “Using the evolving understanding of the mechanistic role of mTORC1, this study has provided great insights into new ways that researchers can begin to find ways to preserve, to treat, and/or improve macular degeneration, and thus improve the quality of life and independent living in many older adults.”

My rating of this study: ⭐⭐⭐

Sethna S, Scott PA, Giese APJ, et al. "CIB2 regulates mTORC1 signaling and is essential for autophagy and visual function." Nature Communications.  12(3906). 23 June 2021.
https://doi.org/10.1038/s41467-021-24056-1

Week in Review: Number 21

Ophthalmic Imaging with Multimodal PARS Microscopy
Engineers in Canada developed a multimodal laser imaging system to measure oxygen saturation and metabolism in living tissue as a potential biomarker for common posterior segment eye diseases. Known as photoacoustic remote sensing (PARS), the technology uses multiwavelength lasers to "almost instantly" image in vivo human tissue noninvasively, with relevant applications for imaging of retinal vasculature and tissue prior to structural changes and functional loss in diseases such as age-related macular degeneration, diabetic retinopathy, and glaucoma. The technical, optical aspects of the technology implement stimulated Raman scattering (SRS), which occurs when the frequency of the emitted light differs from the frequency of the incident light through a material. Multiple wavelengths of light are necessary for oxygen saturation (relative concentration) calculations; however, previous methods using dye lasers or optical parametric oscillators were limited by low pulse repetition rates (i.e., speed) and thus were not suitable for applications of in vivo ocular imaging. Instead, the SRS approach allowed for both high-speed and multiwavelength light sources. The researchers used two excitation wavelengths (532 nm and 558 nm) in this case to estimate the concentration of HbO₂ and Hb. PARS microscopy can be simultaneously combined with swept-source optical coherence tomography (SS-OCT) for enhanced image viewing, and the authors report this being the first instance of combining OCT with optical resolution photoacoustic microscopy (OR-PAM). In contrast to OCT (without the angiography function), photoacoustic microscopy is good at imaging vasculature in greater detail. The project lead comments, “We’re optimistic that our technology, by providing functional details of the eye such as oxygen saturation and oxygen metabolism, may be able to play a critical role in early diagnosis and management of these blinding diseases.” The project is in an early stage and has only been tested in animal models at this point; they are working with several ophthalmologists and hope to start human trials within two years.

Cortical Recycling in High-Level Visual Cortex
A longitudinal study by researchers at Stanford University investigated cortical recycling in the visual cortex during childhood development. The study involved about 30 children ages 5 to 12 at their first MRI and followed with subsequent MRIs over 1 to 5 years. Specifically, the researchers used functional MRI to study areas in the ventral temporal cortex (VTC) that are stimulated by the recognition of images, using a sampling of ten categories of images ranging from faces and body parts to objects, words, and places. They found that areas in childhood (5-9-year-olds) that responded to images of limbs later responded to words and faces during adolescence. The researchers emphasized that increases in face- and word-selectivity in the VTC were directly linked to decreases in limb selectivity, providing surprising evidence of cortical selectivity being repurposed from one category to another, in contrast to prior theories of childhood brain development. The first author of the study comments, “This challenges a theory of cortical development, which states that new representations, like emerging regions involved in word recognition, are sculpted on previously uncommitted cortex. Our study suggests that during childhood, cortical selectivity can change from one category to another.” Word recognition becomes increasingly relevant as children learn to read. Research into vision development in the brain could thus inform strategies related to learning.

Research in Camouflage Breaking
Camouflage is used extensively by the military. From desert sand to dense jungles, visual textures and patterns are used to break up outline and conceal location. At the same time, camouflage breaking or being able to detect and localize another's camouflage is equally important, with real-world combat implications. For example, a sniper's missed shot also reveals his location, a difference between life and death in warfare. Scientists funded by the Army Research Office are studying how to train individuals to break camouflage. In the published study, six adult volunteers with normal or corrected-to-normal vision were trained to break camouflage using a deep-learning method similar to how computer scientists train self-driving cars. Specifically, the participants looked at digitally synthesized camouflage scenes like foliage or fruit, with each scene having a 50-50 chance of containing no target versus a camouflaged target like a human head. The participants could look for as little as 50 milliseconds or for as long as they wanted. After cleansing the visual palate with a random field of pixels, participants were then asked to both acknowledge whether there was a camouflaged target and identify where the target was on the screen based on memory. Interestingly, accuracy did not decrease much in the 50 millisecond viewing scenario as compared to the free viewing scenario. In a second experiment with seven different volunteers, the researchers tested a more abbreviated training protocol with more overt visual images, and found results similar to the more extensive training protocol. The researchers also plan to study the importance of context in camouflage breaking as well as explore using these techniques to identify medical problems. Beyond the military, research into camouflage breaking could also benefit the civilian sector, for instance, in image-intensive professions such as radiology as well as binocular vision in general.

Neurotrophic Effects of PEDF & Peptides in the Retina
Researchers at the National Eye Institute are studying how a growth factor called pigment epithelium-derived factor (PEDF) protects neurons from cellular stresses such as oxidative stress, inflammation, neovascularization, and cell death. To study the mechanisms behind PEDF's beneficial properties, the team used a cell culture model where immature retinal cells are isolated from the eyes of newborn rats and grown in vitro with minimal nutrients, alongside other types of cells in the retina. They discovered that PEDF has functionally distinct domains, which previous research showed can work independently of the full-length protein. One domain called the 34-mer (formed by 34 amino acid building blocks) halts blood vessel growth. Two other domains called the 44-mer and the 17-mer (a shorter version of the 44-mer) provide anti-apoptotic signals to retinal neurons. The researchers found that like full-length PEDF, both the 44-mer and the 17-mer could preserve photoreceptors from cell death, even in the absence of proteins and cells in the usual retinal environment. Additionally, their research showed that PEDF plays a role in photoreceptor development, triggering the movement of light-sensing opsin into the budding outer segment of photoreceptors where light detection takes place. Another finding showed that the 44-mer and the 17-mer could stimulate amacrine cells, another type of cell in the retina that relays visual information, to grow neurites, projections that facilitate neuronal communication; furthermore, these two fragments were at least as effective, or better, at stimulating these connections as the full-length protein. Finally, the researchers discovered PEDF's function in processing omega-3 fatty acids such as DHA, which is important for eye health both during infant development and for eye health over time. The authors conclude, "Our findings support the neurotrophic PEDF peptides as neuronal guardians for the retina, highlighting their potential as promoters of retinal differentiation, and inhibitors of retina cell death and its blinding consequences."

Optimizing Stimulation of Optic Nerve Fibers
Scientists in Europe developed a personalized protocol for optimizing intraneural stimulation of optic nerve fibers for the blind that takes into account feedback from the viewer’s brain. Although current neurotechnology in optic nerve stimulation can only provide simple visual signals, the researchers envision designing these simple visual signals to be meaningful in assisting the blind with daily living. Their idea is to stimulate the optic nerve to induce phosphenes, the sensation of light in one’s visual field. The current technology is limited by image resolution due to contraints of size difference between the optic nerve fibers compared to the electrodes used for intraneural stimulation. A challenge for neuroprosthetics in general, intraneural stimulation of optic nerve fibers is a greater difficulty due to the extreme complexity of visual signals. The model has thus far been tested on convolutional neural networks (CNN), artificial neural networks based on machine learning and used in computer vision for detecting and classifying objects. Psychophysical tests involving ten healthy subjects were also performed to imitate optic nerve stimulation, with results compatible with CNN, according to the authors. The first author of the study states, "Our study shows that it is possible to elicit desired activity patterns in deep layers of a CNN that simulate cortical visual areas." The researchers acknowledge that the project is tremendous, taken one step at a time. They are considering working with collaborators in Rome, Italy for future clinical trials.

In Other News
(1) Discovery of the "focea" in mice
(2) Neurobiology: How mice see the world
(3) Eyeblink test in piglets, for research in human infants