The importance of being protected: How to choose your sunglasses

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Posted on 2nd September 2016 by Pacific ClearVision Institute in General |Retina

Experts from the University of Alicante (UA) have found that prolonged exposure to the sun increases the chance of developing alterations in the lens by 4%. Choosing the right pair of sunglasses can reduce such complications.

Despite the fact that the vast majority of the harmful radiation from the sun is absorbed by the atmosphere, enough ultraviolet rays do reach Earth’s surface to cause skin burns and eye complications in structures like the retina and cornea. In this sense, studies highlight that each hour spent exposed to the sun in the summer increases the chance of developing alterations in the lens by 4%.

The researchers, from the UA’s Department of Optics, Pharmacology and Anatomy, stress that using approved sunglasses from 10 to 4pm can reduce this chance by around 2%.

But how do we choose the right pair of sunglasses?

One of the biggest ‘musts’ is that the glasses bear the European ‘CE’ seal, since this means they adhere to European safety standards.

Further advice from UA professor David Piñero urges us to remember “that sunglasses are very important for visual health and, therefore, their purchase should be supervised by an optician-optometrist.”

Filter category

Another important thing to know when you’re buying a part of sunglasses is what filter category you need. This is determined by your location and activity. In accordance with European regulations on sunglasses, UV filters are classified into five categories, from 0 to 4.

When driving, for instance, you need use a category 1, 2 or 3 filter lens, depending on the conditions; never a 4, since this can interfere with your perception of traffic signs. Piñero tells us that “in the summer in Alicante a category 2 or 3 filter is more than enough, though if we’re going out onto the water or into the mountains, where light is reflected much more strongly, protection level 4 would be advisable.”

Special care should be paid to the glasses worn by children and old people. Children are especially sensitive to ultraviolet radiation, since the lens is still very transparent until adolescence. Category 2 or 3 is recommended, and very sturdy lenses and frames. Old people should wear the same category lens.

Debunking myths

Just as price is not an indicator of quality, neither is the colour of the lense an indicator of the level of protection offered. “There are very dark lenses that do not correctly filter ultraviolet light, leading to greater pupil dilation and an increase in the radiation that enters the eye,” says Piñero.

Ultraviolet radiation is a risk factor for our eyes and “has an accumulative effect that can, in some cases, set off problems in the photoreceptors in the retina (the rods and cones), progressively bad vision, macular degeneration, or the onset of pterygion, where tissues invade the cornea, known colloquially as Surfer’s Eye.”

New test needed to assess the quality, safety of sunglasses

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Posted on 2nd September 2016 by Pacific ClearVision Institute in General |Retina

Revision of standards is needed to test sunglasses quality and establish safe limits for the lenses’ UV filters, according to research published in the open access journal Biomedical Engineering OnLine.

Exposure to the sun may deteriorate your sunglasses over time and the lenses may become lighter and so alter the category under which they are classified. It may also diminish the impact resistance of lenses (how ‘shatterproof’ the lens is). Current national and regional standards require that sunglasses provide levels of UV protection linked to the luminous transmittance, which decides the category of the lenses.

The aging test, used in Europe, Brazil, New Zealand and Australia, calculates the extent to which the lenses’ category deteriorates as a result of exposure to the sun. The test exposes sun glasses to a sun simulator for 50 hours at 30cm from a 450 W lamp. The lamp exposure is equivalent to two days in a natural environment on a summer’s day, or four days in winter.

Liliane Ventura, the corresponding author, from São Carlos School of Engineering, University of São Paulo, Brazil, said: “50 hours of exposure to the sun simulator equates to 23.5 hours of exposure to natural sun in Sao Paulo in Brazil. Most Brazilians replace their sunglasses every two years. To test the sunglasses are safe to wear for these two years, with the assumption they are worn for a period of two hours a day, they should be tested for 134.6 hours at a distance of 5cm. Although our calculations are mainly based on Brazilian cities, other countries may also benefit, especially those located at similar latitudes.”

Exposure will vary among world latitudes, with tropical countries being of most concern, as UV indexes are extremely high in summer and remain high in the winter. Therefore, sunglasses worn in the southern hemisphere may need replacing more often than in those worn in the northern hemisphere.

Liliane Ventura adds: “We need adequate lamp power, exposure time, distance from the bulb and controlled temperature. To overcome the current limitations one may either increase the time the lenses are exposed to the lamp or decrease the distance of the lenses from the lamp. We could also consider using a higher power lamp, switching from a 450W to 1600 W lamp.”

A Brazilian national survey indicated that most Brazilians wear the same pair of sunglasses for a minimum of two years for a period of two hours a day. Therefore, the standard must guarantee that the sunglasses are safe over this period.

The study calculations were carried out in 27 Brazilian state capitals and data for 110 national capitals in the northern hemisphere were also included. Calculating the equivalence of the simulator to natural light is an estimate because when an individual wears sunglasses, the lenses are not directly exposed to the sun, as they are usually worn in the vertical position.

Lenses should provide adequate UV filters, because insufficient protection could lead to pathological modifications to the cornea and to the internal structure of the eye. This could cause edema (swelling of the eye which can distort vision), pterygium (growth of pink, fleshy tissue on the white of the eye that can interfere with vision), cataract (clouding of the lens of the eye) and retina damage.

Artificial retinas: Promising leads towards clearer vision

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Posted on 2nd September 2016 by Pacific ClearVision Institute in General |Retina

A major therapeutic challenge, the retinal prostheses that have been under development during the past ten years can enable some blind subjects to perceive light signals, but the image thus restored is still far from being clear. By comparing in rodents the activity of the visual cortex generated artificially by implants against that produced by “natural sight,” scientists from CNRS, CEA, INSERM, AP-HM and Aix-Marseille Université identified two factors that limit the resolution of prostheses. Based on these findings, they were able to improve the precision of prosthetic activation. These multidisciplinary efforts, published on 23 August 2016 in eLife, thus open the way towards further advances in retinal prostheses that will enhance the quality of life of implanted patients.

A retinal prosthesis comprises three elements: a camera (inserted in the patient’s spectacles), an electronic microcircuit (which transforms data from the camera into an electrical signal) and a matrix of microscopic electrodes (implanted in the eye in contact with the retina). This prosthesis replaces the photoreceptor cells of the retina: like them, it converts visual information into electrical signals which are then transmitted to the brain via the optic nerve. It can treat blindness caused by a degeneration of retinal photoreceptors, on condition that the optical nerve has remained functional[1]. Equipped with these implants, patients who were totally blind can recover visual perceptions in the form of light spots, or phosphenes. Unfortunately, at present, the light signals perceived are not clear enough to recognize faces, read or move about independently.

To understand the resolution limits of the image generated by the prosthesis, and to find ways of optimizing the system, the scientists carried out a large-scale experiment on rodents. By combining their skills in ophthalmology and the physiology of vision, they compared the response of the visual system of rodents to both natural visual stimuli and those generated by the prosthesis.

Their work showed that the prosthesis activated the visual cortex of the rodent in the correct position and at ranges comparable to those obtained under natural conditions. However, the extent of the activation was much too great, and its shape was much too elongated. This deformation was due to two separate phenomena observed at the level of the electrode matrix. Firstly, the scientists observed excessive electrical diffusion: the thin layer of liquid situated between the electrode and the retina passively diffused the electrical stimulus to neighboring nerve cells. And secondly, they detected the unwanted activation of retinal fibers situated close to the cells targeted for stimulation.

Armed with these findings, the scientists were able to improve the properties of the interface between the prosthesis and retina, with the help of specialists in interface physics. Together, they were able to generate less diffuse currents and significantly improve artificial activation, and hence the performance of the prosthesis.

This lengthy study, because of the range of parameters covered (to study the different positions, types and intensities of signals) and the surgical problems encountered (in inserting the implant and recording the images generated in the animal’s brain) has nevertheless opened the way towards making promising improvements to retinal prostheses for humans.

This work was carried out by scientists from the Institut de Neurosciences de la Timone (CNRS/AMU) and AP-HM, in collaboration with CEA-Leti and the Institut de la Vision (CNRS/INSERM/UPMC).

Surprise discovery in the blink of an eye

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Posted on 2nd September 2016 by Pacific ClearVision Institute in General |Retina

We probably do it every day, but scientists have only just discovered a distinct new way in which we move our eyes.

The team from the University of Tübingen in Germany assessed the eye movements of 11 subjects using tiny wires attached to the cornea and with infrared video tracking. In results published in eLife, they discovered a new type of eye movement that is synchronised with blinking.

The movement they discovered helps to reset the eye after it twists when viewing a rotating object. It is like avoiding tiny rotations of a camera to stabilise the image we perceive. We don’t notice the eye resetting in this way because it happens automatically when we blink.

“We were really surprised to discover this new type of eye movement and it was not what we had anticipated from the experiment,” says lead author Mohammad Khazali.

“We had expected to find that another, already well-known type of eye movement is synchronized to blinking.”

Although it is brief, blinking creates an interruption in our visual perception. We spend up to a tenth of our waking hours blinking but hardly notice it. It serves an essential role in lubricating the eye and may even provide the brain with small, frequent mental breaks.

The scientists sought to investigate whether a reflexive, involuntary eye movement called torsional optokinetic nystagmus (tOKN) occurs at the same time as blinking. The theory was that this reflex also creates a break in the visual system so synchronising them minimises downtime.

The subjects’ eye movements were tracked as they viewed a rotating pattern of dots. As their eyes twisted to follow the dots, they frequently reset, via tOKN, to avoid moving beyond the mechanical limits of the eye muscles. However, this resetting was imperfect and the eyes gradually twisted until the muscles couldn’t twist any more. This varied between subjects from three to eight degrees of rotation.

Once they reached their maximum, the eyes reset so they were no longer twisted at all. This happened at the same time as blinking. The scientists have called this newly-discovered movement blink-associated resetting movement (BARM).

“The eye’s sharpest vision is enabled by a spot on the light-sensitive sheet of the retina called the fovea and this needs to stay balanced to ensure objects of interest can be scrutinised in an optimum way,” says Khazali.

The frequency and size of the movement is determined by how far the eyes have deviated from a neutral position. It helps to reduce strain in the eyes as they move to assess the world around us. In further experiments, the scientists discovered that it even occurs when the eye is not tracking a rotating object.

“To discover such a ubiquitous phenomenon in such a well-studied part of the human body was astonishing to us and we’re very grateful to the volunteers who took part in the study,” says Khazali.

June 2016 is Fireworks Eye Safety Month

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Posted on 2nd June 2016 by Pacific ClearVision Institute in General |Retina

PCVI and many other eye health providers support the sound warnings regarding firework eye safety. With the Fourth of July only a matter of weeks away, it’s important to review reminders that can serve as a real protection.

According to the U.S. Consumer Product Safety Commission, fireworks are involved in thousands of injuries treated in U.S. hospital emergency rooms each year.

Most fireworks injuries occur during the one month period surrounding the Fourth of July.

- Fireworks devices were involved in an estimated 10,500 injuries treated in U.S. hospital emergency rooms in 2014 (the latest year for which data is available).
- An estimated 7,000 injuries were treated in hospital emergency rooms during the one-month period (June 20–July 20) surrounding the Fourth of July.
- 19 percent, or 1,200, of those injuries were to the eyes. Sparklers accounted for 1,400 injuries, firecrackers (1,400) and bottle rockets (100).
- Males accounted for 74% of fireworks injuries.
- 40% of fireworks injuries were to children under age 15.
- For children under 5 years old, sparklers accounted for the most estimated injuries for that specific age group.
- Data from the U.S. Eye Injury Registry shows that bystanders are more often injured by fireworks than operators themselves.
- Contusions, lacerations and foreign bodies were the most common injuries to eyes.
- There were 11 fireworks-related deaths in 2014.

Do Not Let Children Play With Fireworks

Fireworks and celebrations go together, especially during the Fourth of July, but there are precautions parents can take to prevent these injuries. The best defense against kids suffering severe eye injuries and burns is to not let kids play with any fireworks.

These Six Steps Can Help Save Your Child’s Sight

If an accident does occur, minimize the damage to the eye. In the event of an eye emergency:

- Do not rub the eye. Rubbing the eye may increase bleeding or make the injury worse.

- Do not attempt to rinse out the eye. This can be even more damaging than rubbing.

- Do not apply pressure to the eye itself. Holding or taping a foam cup or the bottom of a juice carton to the eye are just two tips. Protecting the eye from further contact with any item, including the child’s hand, is the goal.

- Do not stop for medicine! Over-the-counter pain relievers will not do much to relieve pain. Aspirin (should never be given to children) and ibuprofen can thin the blood, increasing bleeding. Take the child to the emergency room at once – this is more important than stopping for a pain reliever.

- Do not apply ointment. Ointment, which may not be sterile, makes the area around the eye slippery and harder for the doctor to examine.

- Do not let your child play with fireworks, even if his/her friends are setting them off. Sparklers burn at 1800 degrees Farenheit, and bottle rockets can stray off course or throw shrapnel when they explode.

If possible leave fireworks to the professional displays that are held at various venues on the Fourth of July.

(Thanks to PreventBlindness.Org)

Visual acuity

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Posted on 2nd June 2016 by Pacific ClearVision Institute in General |Retina

Visual acuity (VA) is acuteness or clearness of vision, especially form vision, which is dependent on the sharpness of the retinal focus within the eye, the sensitivity of the nervous elements, and the interpretative faculty of the brain.

VA is a quantitative measure of the ability to identify black symbols on a white background at a standardized distance as the size of the symbols is varied.

The VA represents the smallest size that can be reliably identified.

VA is the most common clinical measurement of visual function.

A visual acuity of 20/20 is frequently described as meaning that a person can see detail from 20 feet away the same as a person with normal eyesight would see from 20 feet.

If a person has a visual acuity of 20/40, he is said to see detail from 20 feet away the same as a person with normal eyesight would see it from 40 feet away.

It is possible to have vision superior to 20/20: the maximum acuity of the human eye without visual aids (such as binoculars) is generally thought to be around 20/10 (6/3).

Recent developments in optometry have resulted in corrective lenses conferring upon the wearer a vision of up to 20/10.

Some birds, such as hawks, are believed to have an acuity of around 20/2, which is much better than human eyesight.

Many humans have one eye that has superior visual acuity over the other.

If a person cannot achieve a visual acuity of 20/200 (6/60) or above in the better eye, even with the best possible glasses, then that person is considered legally blind in the United States.

A person with a visual field narrower than 20 degrees in diameter also meets the definition of legally blind.

Our brain suppresses perception related to heartbeat, for our own good

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Posted on 2nd June 2016 by Pacific ClearVision Institute in General |Retina

Our heart is constantly beating yet we normally do not feel it. It turns out that our brain is capable of filtering out the cardiac sensation so that it doesn’t interfere with the brain’s ability to perceive external sensations. For the first time, researchers from the Center for Neuroprosthetics at EPFL have identified this mechanism. They discovered that a certain region in the brain determines where internal and external sensations interact. Their work appears in the Journal of Neuroscience.

EPFL’s neuroscientists noted that the brain perceives visual stimuli less effectively if they occur in time with the heartbeat. It seems as if the brain wants to avoid processing information that is synchronized with the body’s heartbeat.

“We don’t see the same way as a video camera does”

“We are not objective, and we don’t see everything that hits our retina like a video camera does,” said Roy Salomon from the Laboratory of Cognitive Neuroscience, one of the study’s co-authors. “The brain itself decides which information to bring to awareness. But what’s surprising is that our heart also affects what we see!”

The researchers carried out an initial series of experiments with more than 150 volunteers. The volunteers were subjected to a visual stimulus — an octagonal shape flashing on a screen. When this geometric shape flashed in sync with the subject’s heartbeat, the subject had more difficulties perceiving it.

What’s happening in the brain — a first insight

The researchers just needed to figure out what was happening in the brain. They were able to show that a specific region, the insular cortex, acts as a filter and intercepts the sensations coming from the body’s beating heart.

They did this by running the experiment again in an MRI scanner. When the visual stimuli were not in sync with the subject’s heartbeat, the insular cortex functioned normally and the subject perceived the flashing octagon easily. But when the stimuli occurred in time with the heart rate, the level of activity in the insular cortex dropped noticeably: the subject was less aware — or totally unaware — of the flashing shape being shown.

It did not take long for Roy to get over his initial surprise at his discovery. “You don’t want your internal sensations to interfere with your external ones. It’s in your interest to be aware of what’s outside you. Since our heart was already beating while our brain was still forming, we’ve been exposed to it since the very start of our existence. So it’s not surprising that the brain acts to suppress it and make it less apparent.”

Is feeling one’s heartbeat related to anxiety?

Awareness of one’s heartbeat is known to be correlated with a number of psychological problems, including anxiety disorders. Patients typically perceive their heart rate more clearly than most people. “But someone who does not suffer from this type of disorder can also be aware of their heartbeat,” said Roy. “This can happen at times of intense excitement or fear, for example.”

Could anxiety disorders be, at least in part, the cause or effect of someone’s inability to silence their heartbeat? “We don’t know that yet. What we do know now is that, under most conditions, we are not aware of our own heartbeat and that there is a specific region of the brain whose task is to suppress it.”

Vessel damage may precede diabetic retinopathy, researchers find

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Posted on 1st May 2016 by Pacific ClearVision Institute in General |Retina

A University of Iowa-led study of diabetes-related vision impairment holds good news — and some bad news — for patients with signs of these disorders.

Scientists have long known that patients with diabetes mellitus — both Type 1 and Type 2 — are at high risk for developing diabetic retinopathy, the most common cause of irreversible blindness in adults. Vision loss occurs due to microvascular damage to the retina. People with diabetes are typically not aware that they are also at risk for developing retinal diabetic neuropathy, a loss of the nerve cells in the retina.

For many years, scientists believed patients developed retinopathy and, as a result of the damage to the blood vessels, later developed neuropathy. Doctors were focusing on early detection and treatment of retinopathy to prevent blindness, which they thought would then prevent the damage caused by neuropathy.

In this new study, however, researchers discovered that the sequence of events occurring in the retina due to diabetes is just the opposite of these long-held beliefs.

“What we’re finding here, unfortunately, is that the nerve damage actually does come first, before the vessel damage,” says Michael Abramoff, MD, PhD, UI professor in the Department of Ophthalmology and Visual Sciences and a member of the Stephen A. Wynn Institute for Vision Research, and senior author on the study. “Even people with diabetes who never get retinopathy can still develop this damage, and after many years, damage may be severe, similar to glaucoma.”

The study appears online the week of April 25, 20916 in the journal PNAS.

“Essentially, the order of damage in the retina from diabetes is different from what we originally thought, and preventing the effects of retinopathy by itself would not protect the nerves in the retina,” says Elliott Sohn, MD, UI associate professor in the Department of Ophthalmology and Visual Sciences and a member of the Stephen A. Wynn Institute for Vision Research, and first author on the study.

In the study, Sohn and Abramoff with their colleagues from the UI and the University of Amsterdam studied 45 people with diabetes and little to no diabetic retinopathy over a four-year span. They found “significant, progressive loss of the nerve fiber and ganglion cell layer,” proof of damage to the nerves before vascular changes typically found in the retina from diabetes.

At the same time, researchers found corresponding thinning of the nerve fiber layer in six donor eyes from patients with diabetes and little to no diabetic retinopathy, and the layer was considerably thinner than the layer in six donor eyes from patients who did not have diabetes. Similar results were found in diabetic mouse models in this study.

The good news, Abramoff says, is that better understanding of the sequence of damage may lead to new treatments that focus on preventing the nerve damage, and thereby hopefully also preventing retinopathy.

Gene therapy shows long-term benefit for treating rare blindness

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Posted on 1st May 2016 by Pacific ClearVision Institute in General |Retina

Pioneering gene therapy has restored some vision to patients with a rare form of genetic blindness for as long as four years, raising hopes it could be used to cure common causes of vision loss, new University of Oxford research published today shows.

A technique which involves injecting a virus into the eye to deliver billions of healthy genes to replace a key missing gene for choroideremia sufferers has provided sustained improvement in vision for up four years for some patients.

This provides the strongest evidence so far in humans that the effects of gene therapy are potentially permanent and could therefore provide a single treatment cure for many types of inherited blindness. These include retinitis pigmentosa, which affects young people, and age-related macular degeneration, which affects the older age group.

Reporting the results this week in the New England Journal of Medicine, doctors from the University of Oxford examined the vision of six patients up to four years after receiving gene therapy at Oxford’s John Radcliffe Hospital. These six were the first in the world to have the procedure for choroideremia in a trial funded by the Department of Health and the Wellcome Trust.

The gene therapy treatment was designed to slow or stop sight loss, however, two of the patients experienced a significant improvement in vision that was sustained for at least four years, despite a decline in their untreated eyes over this period. A further three maintained their vision in their treated eyes throughout this period. The sixth patient who had a lower dose had a slow decline in vision in both eyes.

It is hoped that gene therapy would ideally be applied to patients early in the disease process to prevent sight loss because the treatment is expected to be long lasting. Patients with choroideremia are missing a key gene in their retina and the technique involves injecting a virus to deliver billions of healthy genes to replace the missing gene.

Professor Robert MacLaren, the lead investigator of the study, said: “There have recently been questions about the long term efficacy of gene therapy, but now we have unequivocal proof that the effects following a single injection of viral vector are sustained. Even sharpening up the little bit of central vision that these patients have can give them considerable independence.

“Gene therapy is a new technique in medicine that has great potential. As we learn more about genetics we realise that correcting faulty genes even before a disease starts may be the most effective treatment. Gene therapy uses the infectious properties of a virus to insert DNA into a cell, but the virial DNA is removed and replaced with DNA that is reprogrammed in the lab to correct whichever gene is faulty in the patient.

“In this case, success in getting a treatment effect that lasts at least several years was achieved because the viral DNA had an optimal design and the viral vector was delivered into the correct place, using advanced surgical techniques. In brief, this is the breakthrough we have all been waiting for.”

Dr Stephen Caddick, Director of Innovation at the Wellcome Trust, added: “To permanently restore sight to people with inherited blindness would be a remarkable medical achievement.

“This is the first time we’ve seen what appears to be a permanent change in vision after just one round of treatment. It’s a real step forwards towards an era where gene therapy is part of routine care for these patients.”

Jonathan Wyatt, the first patient in the world to be treated with this gene therapy is still sight impaired, but he was able to double the level of vision in his treated left eye, which has been maintained for four years so far.

The retired barrister, 68, of Bristol, suffered vision problems since the age of 20. The right eye has continued to degenerate and the left eye is now dominant.

Mr Wyatt, married to Diana, for nearly 30 years, could read 23 letters in eye chart tests prior to the operation but by three-and-a-half years could read 44.

Mr Wyatt said: “I feel very lucky, privileged and honoured to be part of the fantastic John Radcliffe research group. I feel that even though I am the meat in a sandwich, my life will be making a contribution to help others.”

“The left eye is much improved to such an extent that I use it mostly to get about these days. It has substantially improved, it is fantastic.

“It has made me more independent, I think I would be more dependent. I think I would feel more cautious about train journeys on my own. Without it I think I would be tapping with a white stick, I think I would have remained cheerful but I would be at home more.”

Joe Pepper, a 24-year-old teacher from Croydon, who was the last patient to receive the gene therapy treatment (not in the original cohort of six), said: “I sat down and began the reading chart test on my treated right eye and I read the first two lines and for the first time in my memory I read on and on.

“I will remember that day for the rest of my life. I could see more than before the operation. I could read four lines beyond where I was earlier. I laughed and shed a tear. It was special.

“Six months on from the surgery the results have been more than I ever imagined. My vision now has a new found clarity and I am no longer putting stress on my vision when reading or looking into the distance. Instead of looking into the distance and seeing outlines of people or trees I am seeing their defined features. At night I now have a new found confidence in dimly lit areas that means I can feel independent and safe after dark.

“Without Professor MacLaren and his team, and their pioneering work I could have lost my sight and for the last 14 years I have feared I could. The work they do is special and I have nothing but thanks to them.”

Surprising central role of darks in brain visual maps

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Posted on 1st May 2016 by Pacific ClearVision Institute in General |Retina

Scientists have been studying how visual space is mapped in the cerebral cortex for many decades under the assumption that the map is equal for lights and darks. Surprisingly, recent work demonstrates that visual brain maps are dark-centric and that, just as stars rotate around black holes in the Universe, lights rotate around darks in the brain representation of visual space. The work was done by Jens Kremkow and collaborators in the laboratories of Jose Manuel Alonso at the State University of New York College of Optometry and will be published in the May 5, 2016 issue of Nature. A similar result will be reported in the same issue of Nature by Kuo-Sheng Lee et al. in the laboratories of David Fitzpatrick at the Max Planck Florida Institute for Neuroscience.

The primary visual cortex is the area of the cerebral cortex with the most detailed representation of visual space and the main recipient of eye inputs coming through the visual thalamus. In the first description of its functional architecture, Hubel and Wiesel demonstrated that the cortical map of visual space coexisted with other maps for eye input and stimulus orientation. Neurons responding to inputs from the left and right eyes segregated in visual cortex forming a map of alternating stripes that resembled a zebra pattern. Neurons responding to similar stimulus orientations also clustered forming a map with a pinwheel pattern that was discovered later by other scientists. More recent work also found additional maps for stimulus features related with motion and depth. However, while the maps for visual space and eye input were thought to originate from the segregation of thalamic afferents in visual cortex, the origin of the other maps remained unclear.

The work of Kremkow and colleagues indicates that the organization of all maps originates from the same principle: an arrangement of thalamic afferents that minimizes differences in spatial position, eye input and light/dark polarity among neighboring cortical neurons. Moreover, they show that the organization of visual space, for both monocular and binocular vision, is more precise for darks than lights. As a consequence of the greater spatial mapping of darks, a 0.5 x 0.5 mm cube of visual cortex can represent the same position of a dark spot but different positions of light spots that appear to rotate around a dark anchor in visual space. This surprising dark-centric organization could be a consequence of a size distortion for lights that originates at the photoreceptor (Kremkow et al., PNAS, 2014), the very first neuron in the visual pathway. Taken together, these findings explain why visual acuity is commonly measured with dark characters on light backgrounds and why visual resolution is lower for lights, as already noted by Leonardo da Vinci and Galileo Galilei when judging the size of light objects in paintings and the dark sky.