Posts tagged saccades
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."
Neuroscientists disprove idea about brain-eye coordination
By predicting our eye movements, our brain creates a stable world for us. Researchers used to think that those predictions had so much influence that they could cause us to make errors in estimating the position of objects. Neuroscientists at Radboud University have shown this to be incorrect. The Journal of Neuroscience published their findings – which challenge fundamental knowledge regarding coordination between brain and eyes – on 15 April.
You continually move your eyes all day long, yet your perception of the world remains stable. That is because the brain processes predictions about your eye movements while you look around. Without these predictions, the image would shoot back and forth constantly.
Errors of estimation
People sometimes make mistakes in estimating the positions of objects – missing the ball completely during a game of tennis, for example. Predictions on eye movements were long held responsible for such localization errors: if the prediction does not correspond to the eventual eye movement, a mismatch between what you expect to see and what you actually see could be the result. Jeroen Atsma, a PhD candidate at the Donders Institute of Radboud University, wanted to know how that worked. ‘If localization errors really are caused by predictions, you would also expect those errors to occur if an eye movement, which has already been predicted in your brain, fails to take place at the very last moment.’ Atsma investigated this by means of an ingenious experiment.
Localizing flashes of light
Atsma asked test subjects to look at a computer screen where a single small ball appeared at various positions at random. The subjects followed the balls with their eyes while an eye-tracker registered their eye movements. The experiment ended with one last ball on the screen, followed by a short flash of light near that ball. The person had to look at the last, stationary ball while using the computer mouse to indicate the position of the flash of light. However, in some cases, a signal was sent around the time the last ball appeared, indicating that the subject was NOT allowed to look at the ball. In other words, the eye movement was cancelled at the last moment. The person being tested still had to indicate where the flash was visible.
Remarkable findings
Even when test subjects heard at very short notice that they should not look at the ball – in other words when the brain had already predicted the eye movement – they did not make any mistakes in localizing the flash of light. ‘That demonstrates you don’t make localization errors solely on the basis of predictions’, Atsma explained. ‘So far, literature has pretty much suggested the exact opposite. That is why we repeated the experiment several times to be sure.’
The findings of the neuroscientists in Nijmegen are remarkable because they challenge much of the existing knowledge about eye-brain coordination. Atsma: ‘This has been an issue ever since we started studying how the eyes function. For the first time ever our experiment offered the opportunity to research brain predictions when the actual eye movement is aborted. Therefore I expect our publication to lead to some lively discussions among fellow researchers.’
Faster eye responses in Chinese people not down to culture
New research from University of Liverpool scientists has cast doubt on the theory that neurological behaviour is a product of culture in people of Chinese origin.
Scientists tested three groups – students from mainland China, British people with Chinese parents and white British people – to see how quickly their eyes reacted to dots appearing in the periphery of their vision.
These rapid eye movements, known as saccades, were timed in all of the participants to see which of them were capable of making high numbers of express saccades – particularly fast responses which begin a tenth of a second after a target appears.
The findings, published in the journal PLoS One, revealed that similar numbers of the British Chinese and mainland Chinese participants made high numbers express saccades, with the white British participants made far fewer. Culturally the British Chinese participants were similar to their white British counterparts and different to the mainland Chinese students.
Therefore in terms of eye movement patterns, Chinese ethnicity was more of a factor than culture. This is contrary to several previous reports from other research groups which looked at behaviour in Asian and white participants and concluded that culture explained behavioural differences between groups.
Neurophysiologist, Dr Paul Knox, from the University’s Institute of Ageing and Chronic Disease, led the study. He said: “Examining saccades from different populations is revealing a lot about underlying brain mechanisms and how we think.
"Many scientists believe that the eye movement patterns you develop are due to where you live – the books you read and the influence of your family, peers and community – your culture."
"Our research has shown that this cannot be the case, at least for saccade behaviour. What this leaves is the way we’re made, perhaps our genetics. And this may have a bearing on the way the brains in different groups react to injuries and disease."
All of the participants completed questionnaires which evaluated their cultural values. They then wore a headset and looked at a plain white board on which lights appeared. The headset measured the time it took for participants’ eyes to react to the lights as they appeared in different places on the board.
Twenty-seven percent of Chinese participants responded with high proportions of express saccades, similar to 22% of the British Chinese, but many more than the 10% of white British participants.
Dr Knox concluded: “From a situation where 80% of our understanding of neuroscience was derived from tests on US psychology undergraduates, we’re now showing how the human brain is not just amazingly complex in general, but also highly variable across the human population.”
Noisy brain signals: How the schizophrenic brain misinterprets the world
People with schizophrenia often misinterpret what they see and experience in the world. New research provides insight into the brain mechanisms that might be responsible for this misinterpretation. The study from the Montreal Neurological Institute and Hospital – The Neuro - at McGill University and McGill University Health Centre, reveals that certain errors in visual perception in people with schizophrenia are consistent with interference or ‘noise’ in a brain signal known as a corollary discharge. Corollary discharges are found throughout the animal kingdom, from bugs to fish to humans, and they are thought to be crucial for monitoring one’s own actions. The study, published in the April 2 issue of the Journal of Neuroscience, identifies a corollary discharge dysfunction in schizophrenia, which could aid with diagnosis and treatment of this difficult disorder. It was carried out in collaboration with researchers Veronica Whitford, Gillian O’Driscoll, and Debra Titone in the Department of Psychology, McGill University.
“A corollary discharge is a copy of a nervous system message that is sent to other parts of the brain, in order to make us aware that we are doing something,” said Dr. Christopher Pack, neuroscientist at The Neuro and lead investigator on the study. “For example, if we want to move our arm, the motor area of the brain sends a signal to the muscles to produce a movement. A copy of this command, which is the corollary discharge, is sent to other regions of the brain, to inform them of the impending movement. If you were moving your arm, and you didn’t have the corollary discharge signal, you might assume that someone else was moving your arm. Similarly, if you generated a thought, and you had an impaired corollary discharge, then you might assume that someone else placed the thought in your mind. Corollary discharges ensure that different areas of the brain are communicating with each other, so that we are aware that we are moving our own arm, talking, or thinking our own thoughts.”
Schizophrenia is a disorder that interferes with the ability to think clearly and to manage emotions. People with schizophrenia often attribute their own thoughts and actions to external sources, as in the case of auditory hallucinations. Other common symptoms include delusions and disorganized thinking and speech.
Recent research has suggested that an impaired corollary discharge can account for some of these symptoms. However, the nature of the impairment was unknown. In their study, Dr. Pack and his colleagues (including Dr. Alby Richard, neurology resident at The Neuro) used a test called a perisaccadic localization task, to investigate corollary discharge activity. In this test, subjects are asked to make quick eye movements to follow a dot on a computer screen. At the same time they are also asked to localize visual stimuli that appear briefly on the screen from time to time. In order to perform this task accurately, subjects need to know where on the screen they are looking – in other words they use corollary discharges signals that arise from the brain structures that control the eye muscles.
The results showed that people with schizophrenia were less accurate in figuring out where they were looking. Consequently they made more mistakes in estimating the position of the stimuli that were flashed on the screen. “What is interesting and potentially clinically important is that the pattern of mistakes made by the patients correlated with the extent of their symptoms,” said Dr. Pack. “This is particularly interesting because the circuits that control eye movements include the best-understood structures in the brain. So we are optimistic that we can work backward from the behavioral data to the biological basis of the corollary discharge effects. We have already started to do this with computational modeling. Mathematically we can convert the corollary discharge of a healthy control into the corollary discharge of a patient with schizophrenia by adding noise and randomness. It is not that people with schizophrenia have no corollary discharge, or a corollary discharge with delayed or weaker amplitude. Rather the patients appear primarily to have a noisy corollary discharge signal. This visual test is very easy thing to do and quite sensitive to individual differences.“
The study shows that patients with schizophrenia make larger errors in localizing visual stimuli compared to controls. These results could be explained by a corollary discharge signal, which also predicts patient symptom severity, suggesting a possible basis for some of the most common symptoms of schizophrenia. This work was supported by The Natural Sciences and Engineering Research Council of Canada, The Brain & Behavior Research Foundation (NARSAD) and the EJLB Foundation.
Fast eye movements: A possible indicator of more impulsive decision-making
Using a simple study of eye movements, Johns Hopkins scientists report evidence that people who are less patient tend to move their eyes with greater speed. The findings, the researchers say, suggest that the weight people give to the passage of time may be a trait consistently used throughout their brains, affecting the speed with which they make movements, as well as the way they make certain decisions.
Caption: Despite claims to the contrary, the eyes of the Mona Lisa do not make saccades. Credit: Leonardo da Vinci
In a summary of the research to be published Jan. 21 in The Journal of Neuroscience, the investigators note that a better understanding of how the human brain evaluates time when making decisions might also shed light on why malfunctions in certain areas of the brain make decision-making harder for those with neurological disorders like schizophrenia, or for those who have experienced brain injuries.
Principal investigator Reza Shadmehr, Ph.D., professor of biomedical engineering and neuroscience at The Johns Hopkins University, and his team set out to understand why some people are willing to wait and others aren’t. “When I go to the pharmacy and see a long line, how do I decide how long I’m willing to stand there?” he asks. “Are those who walk away and never enter the line also the ones who tend to talk fast and walk fast, perhaps because of the way they value time in relation to rewards?”
To address the question, the Shadmehr team used very simple eye movements, known as saccades, to stand in for other bodily movements. Saccades are the motions that our eyes make as we focus on one thing and then another. “They are probably the fastest movements of the body,” says Shadmehr. “They occur in just milliseconds.” Human saccades are fastest when we are teenagers and slow down as we age, he adds.
In earlier work, using a mathematical theory, Shadmehr and colleagues had shown that, in principle, the speed at which people move could be a reflection of the way the brain calculates the passage of time to reduce the value of a reward. In the current study, the team wanted to test the idea that differences in how subjects moved were a reflection of differences in how they evaluated time and reward.
For the study, the team first asked healthy volunteers to look at a screen upon which dots would appear one at a time –– first on one side of the screen, then on the other, then back again. A camera recorded their saccades as they looked from one dot to the other. The researchers found a lot of variability in saccade speed among individuals but very little variation within individuals, even when tested at different times and on different days. Shadmehr and his team concluded that saccade speed appears to be an attribute that varies from person to person. “Some people simply make fast saccades,” he says.
To determine whether saccade speed correlated with decision-making and impulsivity, the volunteers were told to watch the screen again. This time, they were given visual commands to look to the right or to the left. When they responded incorrectly, a buzzer sounded.
After becoming accustomed to that part of the test, they were forewarned that during the following round of testing, if they followed the command right away, they would be wrong 25 percent of the time. In those instances, after an undetermined amount of time, the first command would be replaced by a second command to look in the opposite direction.
To pinpoint exactly how long each volunteer was willing to wait to improve his or her accuracy on that phase of the test, the researchers modified the length of time between the two commands based on a volunteer’s previous decision. For example, if a volunteer chose to wait until the second command, the researchers increased the time they had to wait each consecutive time until they determined the maximum time the volunteer was willing to wait — only 1.5 seconds for the most patient volunteer. If a volunteer chose to act immediately, the researchers decreased the wait time to find the minimum time the volunteer was willing to wait to improve his or her accuracy.
When the speed of the volunteers’ saccades was compared to their impulsivity during the patience test, there was a strong correlation. “It seems that people who make quick movements, at least eye movements, tend to be less willing to wait,” says Shadmehr. “Our hypothesis is that there may be a fundamental link between the way the nervous system evaluates time and reward in controlling movements and in making decisions. After all, the decision to move is motivated by a desire to improve one’s situation, which is a strong motivating factor in more complex decision-making, too.”
(Source: eurekalert.org)
Eye movement rhythm important to eye-tracking diagnoses
Quick eye movements, called saccades, that enable us to scan a visual scene appear to act as a metronome for pushing information about that scene into memory.
Scientists at Yerkes National Primate Research Center, Emory University, have observed that in monkeys exploring images with their eyes, the onset of a saccade resets the rhythms of electrical activity (theta oscillations) in the hippocampus, a region of the brain important for memory formation.
Tracking eye movements is already a promising basis for diagnosing brain disorders such as Alzheimer’s disease and schizophrenia. A deeper understanding of how the rhythm of eye movements orchestrate memories could bolster the accuracy and power of eye-tracking diagnoses.
The findings were published this week in Proceedings of the National Academy of Sciences, Early Edition.
Senior author Elizabeth Buffalo was a researcher at the Yerkes National Primate Research Center and an associate professor of neurology at Emory University School of Medicine and is currently associate professor of physiology and biophysics at Universpity of Washington in Seattle. The first author of the paper is postdoctoral fellow Michael Jutras„ who is now an instructor at the University of Washington.
Theta oscillations are cycles of electrical activity in the brain occurring between 3 to 12 times per second. Scientists have previously seen theta oscillations in the hippocampus in rodents, when the rodents were actively exploring, sniffing or feeling something with their whiskers.
"Both animals and humans seem to take in sensory information at this theta rhythm," Buffalo says. "But one striking difference between rodents and primates is the way they gather information about the external world. Rodents are much more reliant on the senses of smell and touch."
She says the actions that are most comparable to rodents’ sniffing and whiskering in primates are saccades. When our eyes scan text or explore a picture, the eyes’ focus tends to jump from point to point several times per second.
Buffalo and Jutras examined electrical signals in the hippocampi of two rhesus monkeys while the monkeys were looking at a variety of pictures and the researchers tracked their eye movements. The researchers observed that after a saccade, the electrical signals in the hippocampus display a more coherent rhythm.
The rhythm reset a saccade imposes may be a way to ensure the hippocampus is receptive to new sensory information, the researchers propose.
“The eye movements are acting like the conductor of the hippocampal orchestra,” Jutras says, “The phase reset might be a mechanism to ensure the ongoing theta rhythm is in sync with incoming visual information.”
Scientists have previously hypothesized that theta oscillations in the hippocampus set the stage for memory formation. The researchers tested this idea by presenting the monkeys each image twice during a viewing session. Because all primates have an innate preference for novelty, monkeys tend to spend a longer time looking at new images and less time looking at repeated ones. The researchers inferred that the monkeys had a stronger memory of a given picture if, upon second viewing, they looked through it quickly. The theta rhythm reset was more consistent during the viewing of images that the monkeys remembered well.
"Based on this finding, we concluded that this resetting of the theta rhythm is an important part of the memory process," Jutras says.
"This study has given us a better understanding of the function of the hippocampal theta rhythm, which has been well characterized in rodents but isn’t well understood in primates," he says. "A future goal is to investigate the relationship between hippocampal theta and eye movements during memory formation and navigation in humans. This could be possible with epilepsy patients who undergo monitoring of hippocampal activity as part of their treatment."
(Source: news.emory.edu)