retina

Energy for Visual Processing Provided by Microtubules in Retinal Neurons

Researchers have discovered a thick band of microtubules in certain neurons in the retina that they believe acts as a transport road for mitochondria that help provide energy required for visual processing.

The research is in Journal of General Physiology. (full access paywall)

Research: “A marginal band of microtubules transports and organizes mitochondria in retinal bipolar synaptic terminals” by Malkolm Graffe, David Zenisek, and Justin W. Taraska in Journal of General Physiology doi:10.1085/jgp.201511396

Image: Fluorescently labeled microtubules extend from the tips of the dendrites (top) into the axon and down into the giant synaptic terminal (bottom) of a single isolated goldfish retinal bipolar cell. A loop of microtubules encircles the inner plasma membrane of the terminal and anchors mitochondria. Image credit: Graffe et al.

2

It really gets me how your brain completes what you see. The first picture shows what your brain tells you that you see. But in reality, your retina is covered by blood vessels and you see them all the time. Also, there’s a blind spot on your eye that your brain erases and completes for you by averaging the light conditios around it. And your cornea and lens twist the picture so it is both horizontally and vertically flipped. 

So the second picture shows what is really projected on your retina.

sixpenceee, you might be slightly interested.

The red eye shine seen in alligators arises when light enters the eye and hits a layer of cells called the tapetum lucidum. This membrane is located beneath the photoreceptor cells (rods and cones) in the retina and reflects light back into these cells to increase the amount of light detected, which improves an alligator’s vision in low light conditions.

Several species exhibit this phenomenon, with different colour ‘shines’ observed. Most species with eyeshine are night hunters who must make use of limited light.

-Jean

Photos by Larry Lynch (http://www.lynchphotos.com/) and David Moynahan (http://www.davidmoynahan.com/)

The brain works as a ‘cyclops,’ compensating the optical differences between the eyes

The eyes differ in their optical properties what results in a blur projected in each retina, despite we see sharp images because the visual system calibrates itself. An international research performed by the Consejo Superior de Investigaciones Científicas has discovered that when each eye separately has a different level of blur, our brain uses as sharp reference the image projected through the less aberrated eye. The research has been published in Current Biology.

“Our impression about what is sharp is colossal and it is determined by the sharper image among those which are projected through both eyes”, explain the CSIC researcher Susana Marcos of the Instituto de Óptica Daza de Váldes. The research reveals that, despite these blur differences, the perception of each eye separately about the sharper image is the same, regardless of the eye we use to make the test and coincides with the blur image projected through the less aberrated eye.

The nature of these visual calibrations is important in order to understand the different consequences referred to the refractive errors between both eyes. “For instance, an available solution to correct the presbyopia is monovision, in which different refractive corrections are provided for both eyes. One eye, the dominant eye, is corrected for distance viewing and the other one is corrected for vision viewing. It is essential to understand the visual calibration with different levels of blur to understand the visual processing of the patients, the main objective is to provide the best possible correction”, conclude the researcher.

Modeling cell connections in the retina

With 576-megapixel resolution, our eyes are incredible cameras, capturing 72-times more high-definition detail than the iPhone 6. To do this, our retinas are packed with many different cell types that help transmit light information to the brain. We know very little, however, about how these cells interconnect, so researchers have turned to mapping and tracing how one cell connects with another…and you can help. A team of scientists at MIT has developed an online game called EyeWire that allows anyone to figure out how cells connect in the retina with real science implications. This image was generated from players correctly tracing connections from one cell to the next, generating a complete connectivity map for these seven cells.

Image by Amy Robinson, Alex Norton, Sebastian Seung, William Silversmith, Jinseop Kim, Kisuk Lee, Aleks Zlasteski, Matt Green, Matthew Balkam, Rachel Prentki, Marissa Sorek, Celia David, Devon Jones, and Doug Bland.

The human eye can see ‘invisible’ infrared light

Any science textbook will tell you we can’t see infrared light. Like X-rays and radio waves, infrared light waves are outside the visual spectrum. But an international team of researchers co-led by scientists at Washington University School of Medicine in St. Louis has found that under certain conditions, the retina can sense infrared light after all.

Using cells from the retinas of mice and people, and powerful lasers that emit pulses of infrared light, the researchers found that when laser light pulses rapidly, light-sensing cells in the retina sometimes get a double hit of infrared energy. When that happens, the eye is able to detect light that falls outside the visible spectrum.

“We’re using what we learned in these experiments to try to develop a new tool that would allow physicians to not only examine the eye but also to stimulate specific parts of the retina to determine whether it’s functioning properly,” said senior investigator Vladimir J. Kefalov, PhD, associate professor of ophthalmology and visual sciences at Washington University. “We hope that ultimately this discovery will have some very practical applications.”

The eye can detect light at wavelengths in the visual spectrum. Other wavelengths, such as infrared and ultraviolet, are supposed to be invisible to the human eye, but Washington University scientists have found that under certain conditions, it’s possible for us to see otherwise invisible infrared light. Credit: Sara Dickherber

Read more at: http://phys.org/news/2014-12-human-eye-invisible-infrared.html#jCp

Study sheds new light on low-light vision, could aid people with retinal deficits

Driving down a dimly lit road at midnight can tax even those with 20/20 vision, but according to a recent UC Irvine study, the brain processes the experience no differently than if it were noon. The same study also reveals how quickly the brain adapts to vision loss, contradicting earlier research and opening the door to novel treatments.

The findings, which appear in the April 21 edition of Proceedings of the National Academy of Sciences, are significant for those who have suffered retinal damage or disease, said cognitive scientist Alyssa Brewer, the lead author.

“Previous research suggested that the two areas of the brain responsible for color processing received input only from cone photoreceptors – the parts of the retina used in central, normal daylight vision for things like reading and seeing details and colors in a scene,” she said.

However, Brewer and co-author Brian Barton, a postdoctoral researcher in cognitive sciences, employed functional MRI to determine that rod photoreceptors, which are only active under very low light, also play a role in the color experience and use the same neural pathways that cones do.

“This is surprising because there are no rods in the central part of the retina, the part we use to see fine details,” Brewer said. “We are functionally blind in the center of our vision under low light, something we call a ‘rod scotoma.’”

To compensate for this vision loss, people look at objects under low light at an angle that accesses the rod receptors.

This adaptation gives researchers an opportunity to track how the brain responds to what the eye sees without using central vision – similar to the way individuals with retinal damage interpret what they see.

Brewer and Barton had test subjects sit in a completely dark room for 30 minutes and then view checkerboard stimuli under very low light while their brain activity was measured with fMRI. In addition to the neural pathway finding, they discovered that the brain adapts immediately to required shifts in vision – a process previous work had said could take months.

“The amount and timing of the brain’s ability to reorganize to compensate for a loss of visual input is very important for us to understand what types of rehabilitation can help recovery,” Brewer said. “The temporary and reversible rod scotoma from low-light conditions provides an excellent way for us to study how the brain reacts and recovers from vision loss, something we found to be immediate rather than long-term.”

“By being able to accurately track how the brain responds to retinal damage, we can begin to create new rehabilitation techniques that could help restore vision,” she added.

Early Retina Cell Changes in Glaucoma Identified
Mouse study points to the specific structural features and cell types in the retina that may act as key factors in glaucoma progression

Glaucoma, the second leading cause of blindness, usually stems from elevated eye pressure, which in turn damages and destroys specialized neurons in the eye known as retinal ganglion cells. To better understand these cellular changes and how they influence the progression and severity of glaucoma, researchers at University of California, San Diego School of Medicine and Shiley Eye Institute turned to a mouse model of the disease. Their study, published Feb. 10 in The Journal of Neuroscience, reveals how some types of retinal ganglion cells alter their structures within seven days of elevated eye pressure, while others do not.

“Understanding the timing and pattern of cellular changes leading to retinal ganglion cell death in glaucoma should facilitate the development of tools to detect and slow or stop those cellular changes, and ultimately preserve vision,” said Andrew D. Huberman, PhD, assistant professor of neurosciences, neurobiology and ophthalmology. Huberman co-authored the study with Rana N. El-Danaf, PhD, a postdoctoral researcher in his lab.

Retinal ganglion cells are specialized neurons that send visual information from the eye’s retina to the brain. Increased pressure within the eye can contribute to retinal ganglion cell damage, leading to glaucoma. Even with pressure-lowering drugs, these cells eventually die, leading to vision loss.

In this study, Huberman and El-Danaf used a mouse model engineered to express a green fluorescent protein in specific retinal ganglion cells subtypes. This tool allowed them to examine four subtypes of retinal ganglion cells. The different cell types differ by the location in the eye to which they send the majority of their dendrites (cellular branches). Within seven days of elevated eye pressure, all retinal ganglion cells that send most or all of their dendrites to a region of the eye known as the OFF sublamina underwent significant rearrangements, such as reductions in number and length of dendritic branches. Retinal ganglion cells with connections in the ON part of the retina did not.

“We are very excited about this discovery,” Huberman said. “One of the major challenges to the detection and treatment of glaucoma is that you have to lose a lot of cells or eye pressure has to go way up before you know you have the disease. These results tell us we should design visual field tests that specifically probe the function of certain retinal cells. In collaboration with the other researcher members of the Glaucoma Research Foundation Catalyst for a Cure, we are doing just that and we are confident these results will positively impact human patients in the near-future.” 

Pictured: Example of retinal ganglion cells with dendrites in the retina of a healthy eye.