Posts tagged fovea
Articles and news from the latest research reports.
How the brain leads us to believe we have sharp vision
We assume that we can see the world around us in sharp detail. In fact, our eyes can only process a fraction of our surroundings precisely. In a series of experiments, psychologists at Bielefeld University have been investigating how the brain fools us into believing that we see in sharp detail. The results have been published in the scientific magazine ‘Journal of Experimental Psychology: General.’ Its central finding is that our nervous system uses past visual experiences to predict how blurred objects would look in sharp detail.
"In our study we are dealing with the question of why we believe that we see the world uniformly detailed," says Dr. Arvid Herwig from the Neuro-Cognitive Psychology research group of the Faculty of Psychology and Sports Science. The group is also affiliated to the Cluster of Excellence Cognitive Interaction Technology (CITEC) of Bielefeld University and is led by Professor Dr. Werner X. Schneider.
Only the fovea, the central area of the retina, can process objects precisely. We should therefore only be able to see a small area of our environment in sharp detail. This area is about the size of a thumb nail at the end of an outstretched arm. In contrast, all visual impressions which occur outside the fovea on the retina become progressively coarse. Nevertheless, we commonly have the impression that we see large parts of our environment in sharp detail.
Herwig and Schneider have been getting to the bottom of this phenomenon with a series of experiments. Their approach presumes that people learn through countless eye movements over a lifetime to connect the coarse impressions of objects outside the fovea to the detailed visual impressions after the eye has moved to the object of interest. For example, the coarse visual impression of a football (blurred image of a football) is connected to the detailed visual impression after the eye has moved. If a person sees a football out of the corner of her eye, her brain will compare this current blurred picture with memorised images of blurred objects. If the brain finds an image that fits, it will replace the coarse image with a precise image from memory. This blurred visual impression is replaced before the eye moves. The person thus thinks that she already sees the ball clearly, although this is not the case.
The psychologists have been using eye-tracking experiments to test their approach. Using the eye-tracking technique, eye movements are measured accurately with a specific camera which records 1000 images per second. In their experiments, the scientists have recorded fast balistic eye movements (saccades) of test persons. Though most of the participants did not realise it, certain objects were changed during eye movement. The aim was that the test persons learn new connections between visual stimuli from inside and outside the fovea, in other words from detailed and coarse impressions. Afterwards, the participants were asked to judge visual characteristics of objects outside the area of the fovea. The result showed that the connection between a coarse and detailed visual impression occurred after just a few minutes. The coarse visual impressions became similar to the newly learnt detailed visual impressions.
"The experiments show that our perception depends in large measure on stored visual experiences in our memory," says Arvid Herwig. According to Herwig and Schneider, these experiences serve to predict the effect of future actions ("What would the world look like after a further eye movement"). In other words: "We do not see the actual world, but our predictions."
Off with Your Glasses
TAU researchers discover a link between sharp vision and the brain’s processing speed
Middle-aged adults who suddenly need reading glasses, patients with traumatic brain injuries, and people with visual disorders such as “lazy eye” may have one thing in common — “visual crowding,” an inability to recognize individual items surrounded by multiple objects. Visual crowding makes it impossible to read, as single letters within words are rendered illegible. And basic cognitive functions such as facial recognition can also be significantly hampered. Scientists and clinicians currently attribute crowding to a disorder in peripheral vision.
Now Prof. Uri Polat, Maria Lev, and Dr. Oren Yehezkel of Tel Aviv University’s Goldschleger Eye Research Instituteat the Sackler Faculty of Medicine have discovered new evidence that correlates crowding in the fovea — a small part of the retina responsible for sharp vision — and the brain’s processing speed. These findings, published in Nature’s Scientific Reports, could greatly alter earlier models of visual crowding, which emphasized peripheral impairment exclusively. And for many adults lost without their reading glasses, this could improve their vision significantly.
"Current theories strongly stress that visual crowding does not exist in the fovea, that it’s a phenomenon that exists only in peripheral visual fields," said Prof. Polat. "But our study points to another part of the eye altogether — the fovea — and contributes to a unified model for how the brain integrates visual information."
A trained eye
According to Prof. Polat, vision is dynamic and changes rapidly, but it takes time for the brain to process this visual information. Rapidly moving tickers on TV, or traffic signs seen as the driver speeds past, are difficult for anyone to read. However, given enough time, someone with excellent vision can fully recognize the words. Those with slower processing speeds — usually the result of poor perceptive development or age — may not be able to decipher the tickers or the traffic signs. In the study, Prof. Polat employed his expertise in improving vision by retraining the brain and the foveal part of the eye, using exercises in which speed is a key element.
"Training adults to reduce foveal crowding leads to improved vision. A similar training we conducted two years ago allowed adults to eliminate their use of reading glasses altogether, using a technology provided by the GlassesOff company. Other patients who had lost sharp vision for whatever reason were also able to benefit from the same training and improve their processing speed and visual capabilities," said Prof. Polat.
Maria Lev, who performed the study as a part of her doctoral thesis, said one young subject had experienced significant limitations in school for years and had been unable to obtain a driver’s license due to severe visual impairment from foveal crowding. After undergoing training that emphasized a foveal rather than a peripheral focus, he was able to overcome the handicap.
"He finally managed to learn to read properly and found his way forward," said Lev. "I’m proud to say that today he is not only eligible for a driver’s license, he’s also been able to earn his master’s degree."
Prof. Polat and his team are currently exploring how visual integration and foveal crowding develop in various clinical cases.
(Source: aftau.org)
More Than Meets the Eye
Many studies suggest that pushing your brain to multitask — writing emails, for instance, while watching the day’s latest news and eating breakfast — leads to poorer performance and lower productivity. But for at least one everyday task — visual sampling (the act of picking up bits of visual information through short glances) — multitasking is not a problem for the brain. A collaboration between researchers at the UC Santa Barbara and the University of Bristol in the UK has shown that during visual sampling, the brain can handle various visual functions simultaneously.
“We might not realize it, but human vision is rather limited,” said Miguel Eckstein, professor in the Department of Psychological and Brain Sciences at UCSB. “We only see clearly in a small region around our specific focus.” Eckstein’s study, “Foveal analysis and peripheral selection during active visual sampling,” appears in the early Proceedings of the National Academy of Sciences Plus edition.
Human eye movements for vision are remarkably adaptable
When something gets in the way of our ability to see, we quickly pick up a new way to look, in much the same way that we would learn to ride a bike, according to a new study published in the Cell Press journal Current Biology on August 15.
Our eyes are constantly on the move, darting this way and that four to five times per second. Now researchers have found that the precise manner of those eye movements can change within a matter of hours. This discovery by researchers from the University of Southern California might suggest a way to help those with macular degeneration better cope with vision loss.
"The system that controls how the eyes move is far more malleable than the literature has suggested," says Bosco Tjan of the University of Southern California. "We showed that people with normal vision can quickly adjust to a temporary occlusion of their foveal vision by adapting a consistent point in their peripheral vision as their new point of gaze."
The fovea refers to the small, center-most portion of the retina, which is responsible for our high-resolution vision. We move our eyes to direct the fovea to different parts of a scene, constructing a picture of the world around us. In those with age-related macular degeneration, progressive loss of foveal vision leads to visual impairment and blindness.
In the new study, MiYoung Kwon, Anirvan Nandy, and Tjan simulated a loss of foveal vision in six normally sighted young adults by blocking part of a visual scene with a gray disc that followed the individuals’ eye gaze. Those individuals were then asked to complete demanding object-following and visual-search tasks. Within three hours of working on those tasks, people showed a remarkably fast and spontaneous adjustment of eye movements. Once developed, that change in their “point of gaze” was retained over a period of weeks and was reengaged whenever their foveal vision was blocked.
Tjan and his team say they were surprised by the rate of this adjustment. They note that patients with macular degeneration frequently do adapt their point of gaze, but in a process that takes months, not days or hours. They suggest that practice with a visible gray disc like the one used in the study might help speed that process of visual rehabilitation along. The discovery also reveals that the oculomotor (eye movement) system prefers control simplicity over optimality.
"Gaze control by the oculomotor system, although highly automatic, is malleable in the same sense that motor control of the limbs is malleable," Tjan says. "This finding is potentially very good news for people who lose their foveal vision due to macular diseases. It may be possible to create the right conditions for the oculomotor system to quickly adjust," Kwon adds.
(Source: eurekalert.org)