Posts tagged vision

April 24th, 2012

Researchers at the University of Sydney have thrown new light on the tricks the brain plays as it struggles to make sense of the visual and other sensory signals it constantly receives.

In this tilt illusion, the lines in the centre of the image appear tilted counterclockwise, but they are actually vertical. Image adapted from University of Sydney image.

The research has implications for understanding how the brain interprets the world visually and how the brain itself works.

People rely on their eyes for most tasks – yet the information provided by our visual sensing system is often distorted, unreliable and subject to illusion.

In a just published article in Proceedings of the National Academy of Science, Dr Isabelle Mareschal and Professor Colin Clifford, from the University’s School of Psychology and The Vision Centre, report a series of groundbreaking experiments tracing the origins of the tilt illusion to the cells of the primary visual cortex. This is where the first stage of vision processing takes place before the conscious mind takes over.

“We tend to regard what we see as the real world,” said Dr Mareschal.

“In fact a lot of it is distortion, and it is occurring in the early processing of the brain, before consciousness takes over. Our work shows that the cells of the primary visual cortex create small distortions, which then pass on to the higher levels of the brain, to interpret as best it can.”

A common example of this that is often exploited by artists and designers is known as the tilt illusion where perfectly vertical lines appear tilted because they are placed on an oriented background.

“We wanted to test at what level the illusion occurs in the brain, unconscious or conscious – and also to see if the higher brain is aware of the illusions it is receiving and how it tries to correct for them,” she explains.

“The answer is that the brain seeks more contextual information from the background to try to work out the alignment of the object it is seeing.”

The team subjected volunteers to a complex test in which they indicated the orientation of a vertical line, perceived as constantly tilting from side to side, against a fuzzy background that was also changing.

“These illusions happen very fast, perhaps in milliseconds,” Dr Mareschal says. “And we found that even the higher brain cannot always correct for them, as it doesn’t in fact know they are illusions.”

This is one reason why people’s eyes sometimes mislead them when looking at objects in their visual landscape.

Normally, Dr Mareschal explains, it doesn’t matter all that much – but in the case of a person driving a car fast in traffic, an athlete performing complex acrobatic feats, a pilot landing an aircraft or other high-speed uses of sight, the illusion may be of vital importance by causing them to misinterpret the objects they ‘see’.

The brain uses context, or background, to interpret a host of other visual signals besides the orientation of objects. For example, it uses context to tell colour, motion, texture and contrast. The research will help study how the brain understands these visual cues adding to our overall understanding of brain function.

Source: Neuroscience News

April 3rd, 2012

Working in mice, scientists at Washington University School of Medicine in St. Louis have devised a treatment that prevents the optic nerve injury that occurs in glaucoma, a neurodegenerative disease that is a leading cause of blindness.

Researchers increased the resistance of optic nerve cells to damage by repeatedly exposing the mice to low levels of oxygen similar to those found at high altitudes. The stress of the intermittent low-oxygen environment induces a protective response called tolerance that makes nerve cells — including those in the eye — less vulnerable to harm.

The study, published online in Molecular Medicine, is the first to show that tolerance induced by preconditioning can protect against a neurodegenerative disease.

Stress is typically thought of as a negative phenomenon, but senior author Jeffrey M. Gidday, PhD, associate professor of neurological surgery and ophthalmology, and others have previously shown that the right kinds of stress, such as exercise and low-oxygen environments, can precondition cells and induce changes that make them more resistant to injury and disease.

Scientists previously thought tolerance in the central nervous system only lasted for a few days. But last year Gidday developed a preconditioning protocol that extended the effects of tolerance from days to months. By exposing mice to hypoxia, or low oxygen concentrations, several times over a two-week period, Gidday and colleagues triggered an extended period of tolerance. After preconditioning ended, the brain was protected from stroke damage for at least 8 weeks.

“Once we discovered tolerance could be extended, we wondered whether this protracted period of injury resistance could also protect against the slow, progressive loss of neurons that characterizes neurodegenerative diseases,” Gidday says.

To find out, Gidday turned to an animal model of glaucoma, a condition linked to increases in the pressure of the fluid that fills the eye. The only treatments for glaucoma are drugs that reduce this pressure; there are no therapies designed to protect the retina and optic nerves from harm.

Scientists classify glaucoma as a neurodegenerative disease based on how slowly and progressively it kills retinal ganglion cells. The bodies of these cells are located in the retina of the eye; their branches or axons come together in bundles and form the optic nerves. Scientists don’t know if damage begins in the bodies or axons of the cells, but as more and more retinal ganglion cells die, patients experience peripheral vision loss and eventually become blind.

For the new study, Yanli Zhu, MD, research instructor in neurosurgery, induced glaucoma in mice by tying off vessels that normally allow fluid to drain from the eye. This causes pressure in the eye to increase. Zhu then assessed how many cell bodies and axons of retinal ganglion cells were intact after three or 10 weeks.

The investigators found that normal mice lost an average of 30 percent of their retinal ganglion cell bodies after 10 weeks of glaucoma. But mice that received the preconditioning before glaucoma-inducing surgery lost only 3 percent of retinal ganglion cell bodies.

“We also showed that preconditioned mice lost significantly fewer retinal ganglion cell axons,” Zhu says.

Gidday is currently investigating which genes are activated or repressed by preconditioning. He hopes to identify the changes in gene activity that make cells resistant to damage.

“Previous research has shown that there are literally hundreds of survival genes built into our DNA that are normally inactive,” Gidday says. “When these genes are activated, the proteins they encode can make cells much less vulnerable to a variety of injuries.”

Identifying specific survival genes should help scientists develop drugs that can activate them, according to Gidday.

Neurologists are currently conducting clinical trials to see if stress-induced tolerance can reduce brain damage after acute injuries like stroke, subarachnoid hemorrhage or trauma.

Gidday hopes his new finding will promote studies of tolerance’s potential usefulness in animal models of Parkinson’s disease, Alzheimer’s disease and other neurodegenerative conditions.

“Neurons in the central nervous system appear to be hard-wired for survival,” Gidday says. “This is one of the first steps in establishing a framework for how we can take advantage of that metaphorical wiring and use positive stress to help treat a variety of neurological diseases.”

Source: Neuroscience News

March 21, 2012

Scientists from Germany discovered new functions of brain regions that are responsible for seeing movement.

When observing a fly buzzing around the room, we should have the impression that it is not the fly, but rather the space that lies behind it that is moving. After all, the fly is always fixed in our central point of view. But how does the brain convey the impression of a fly in motion in a motionless field? With the help of functional magnetic resonance imaging (fMRI) scientists from the Werner Reichardt Centre for Integrative Neuroscience and the Max Planck Institute for Biological Cybernetics in Tübingen have identified two areas of the brain that compare the movements of the eye with the visual movements cast onto the retina so as to correctly perceive objects in motion.

The two areas of the brain that are particularly good at reacting to external movements, even during eye movements, are known as V3A and V6. They are located in the upper half in the posterior part of the brain. Area V3A shows a high degree of integration: it reacts to movements around us regardless of whether or not we follow the moving object with our eyes. But the area does not react to visual movements on the retina when eye movements produce them. Area V6 has similar characteristics. In addition, it can perform these functions when we are moving forwards. The calculations the brain has to perform are more complicated in this case: the three-dimensional, expanding forward movement is superimposed onto the two-dimensional lateral movements that are caused by eye movements.

The scientists Elvira Fischer and Andreas Bartels from the Werner Reichardt Centre for Integrative Neuroscience and the Max Planck Institute for Biological Cybernetics have investigated these areas with the help of functional magnetic resonance imaging (fMRI). fMRI is a procedure that can measure brain activity based on local changes in blood flow and oxygen consumption. Participants in the study were shown various visual scenarios whilst undergoing fMRI scanning. For example, they had to follow a small dot with their eyes while it moved across a screen from one side to the other. The patterned background was either stationary or moved at varying speeds, sometimes slower, faster or at the same speed as the dot. Sometimes the dot was stationary while only the background moved. In a total of six experiments the scientists measured brain activity in more than a dozen different scenarios. From this they have been able to discover that V3A and V6, unlike other visual areas in the brain, have a pronounced ability to compare eye movements with the visual signals on the retina. “I am especially fascinated by V3A because it reacts so strongly and selectively to movements in our surroundings. It sounds trivial, but it is an astonishing capability of the brain”, explains Andreas Bartels, project leader of the study.

Whether it is ourselves who move or something else in our surroundings is a problem about which we seldom think, since at the subconscious level our brain constantly calculates and corrects our visual impression. Indeed, patients who have lost this ability to integrate movements in their surroundings with their eye movements can no longer recognize what it is that ultimately is moving: the surroundings or themselves. Every time they move their eyes these patients feel dizzy. Studies such as this bring us one step closer to an understanding of the causes of such illnesses.

Provided by Max-Planck-Gesellschaft

Source: medicalxpress.com

March 21, 2012

(Medical Xpress) — People who lose their sight at a later stage in life have a greater spatial awareness than if they were born blind, according to scientists at Queen Mary, University of London.

The study, published in the journal Neuroscience and Biobehavioral Reviews, examined research which looked at the spatial skills of sighted and blind people and found that some spatial tasks need visual experience.

Co-author on the study, Dr. Michael Proulx from Queen Mary’s School of Biological and Chemical Sciences, said: “Numerous studies have tested how humans use vision for knowing the spatial locations of things yet few have examined the other senses and whether people with a visual impairment use the same strategies.

“In reviewing research already available, we found visual experience is necessary for the brain to develop the ability to process multisensory information. We use vision and the other senses to create a mental map of where objects are in relation to other objects and the environment.

“Our findings suggest that there is a sensitive period during which visual experience is necessary for the brain to develop those neurons that can represent the world in this way.”

Lead author Dr. Achille Pasqualotto, also from Queen Mary’s School of Biological and Chemical Sciences, said: “Blindness reveals how well humans can function using the remaining senses, even in a world designed by sighted people for sighted people.

“The brain develops spatial abilities that relate an object’s location to the individual. This makes sense given that a visually impaired person does not see objects at a distance in an environment, but instead acquires their location by personally approaching and identifying them.”

The team is building on their findings now by testing sighted and blind people on a variety of spatial tasks that will explicitly test these findings.

They hope this research will not only reveal the psychological and neural basis for spatial cognition, but also translate into better services for blind persons, such as the development of better navigational tools.

Dr. Proulx said: “We are actively recruiting blind people to participate in our research and we are particularly keen to involve people who have been blind since birth, yet people who lost vision later in life would be welcome to contact us too.”

Provided by Queen Mary, University of London

Source: medicalxpress.com