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09 October 2015

What you Probably Didn’t Know about How your Eyes See

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«What the eye doesn’t see and the mind doesn’t know, doesn’t exist,» said D. H. Lawrence. And he was right, because vision is the result of teamwork between the eyes and the brain, which together create a complete and complex picture of the world around us. This is the latest thing that neurobiologists, physicists and optical experts around the world have learned about how we see.

Photoshop of the human eye

The resolution of the image captured by the human eye is limited by the number of cells that fit in the retina, which operate just like pixels. It is estimated that a “snapshot” obtained by the eye has scarcely one megapixel. However, the brain is able to increase the resolution of the images before processing them, as would a digital camera or mobile phone.

A mammalian eye has about 70 kinds of cells. Credit: B.W. Jones, R.E. Marc / University of Utah

This happens in the lateral geniculate nucleus, a brain region that receives the image from the eye and, before sending it to the cortex for analysis, applies a similar photo editing technique that involves interpolating or filling in the missing information with data derived from an algorithm. The result is a retinal image with more pixels and, therefore, greater apparent resolution, as discovered by Luis Martinez-Otero, researcher at the Institute of Neurosciences of Alicante, and his colleagues, according to the journal Neuron.

Better contrast than any camera

There is something that no camera, whether analog or digital, has managed to surpass in the human eye, and that is its amazing ability to obtain contrast in the images without losing detail, whether in light or in shadow. Scientists at the University of California have investigated this and have come to the conclusion that it’s thanks to cellular teamwork.

The eye uses many layers of nerve cells to convert light into vision. Credit: Wei Li, National Eye Institute

What the digital camera and the eye have in common is that both pixels and light receptor cells (rods and cones) generate an electrical response proportional to the intensity of the light they receive. But there is one crucial difference: when the photoreceptors of the eye are excited, they send signals to dozens of neighboring cells, inhibiting some and stimulating others. And this positive and negative feedback is what allows good contrast and good definition to exist at the same time.

A dominant eye

Human beings are not symmetrical, and neither are our eyes – there is always a dominant eye. Specifically, in two-thirds of the population the right eye prevails over the left. But what are the implications? The most immediate one is that if we are shown texts —on a screen or on paper— to both the left and right eyes, we first read those on the side of our dominant eye.

But more interesting is that those people with a dominant left eye perceived more symbols of a fragment of text on their dominant side, and therefore read faster. Experts suggest that, just as there are left-handed scissors and tools, we could start developing books, video games and electronic devices for visual lefties. Meanwhile, some exercises to determine your dominant eye are becoming popular.

The blind spot can “shrink”

All eyes have a blind spot. This is because the optic nerve that sends visual signals to the brain has to cross the retina of the eye at some point, and in doing so it creates a hole in the light-sensitive tissue. If an image is projected just right there, we don’t see it. The good news is that, while it is impossible to eliminate the blind spot, with proper training you can make it “shrink”, according to a study published in Current Biology. To prove this, for 20 days scientists trained a dozen subjects at detecting the movement and the color of a series of waves that were partially projected onto the blind spot of one eye. Upon completion of the training, the subjects were better able to see the waves with the trained eye. All the indications are that this training improves the sensitivity of neurons whose perceptual fields partly overlap with the blind spot. This technique could be used to treat some forms of blindness.

Retinal rods in the fovea (colored blue). Credit: University of Rochester

Two operating as one

Our vision is binocular, meaning that the brain combines images from two eyes, which are in different positions, into just one, in order to offer a vision of the world similar to what the Cyclops of Greek mythology would experience with its single central eye. Not long ago, Bas Rokers and his colleagues at the University of Wisconsin-Madison discovered the exact spot in the brain where the two images are combined. As reported in the journal Current Biology, this finding will help treat diseases such as lazy eye.

On the other hand, we must bear in mind that all eyes suffer from optical imperfections that make projections on the retina have some blurring, though we perceive them as clear because the visual system auto-calibrates. Researchers at the Institute of Optics Daza de Valdés of the CSIC (Spain) have found that when each eye has a different level of blur, the brain uses the image projected through the eye with fewer imperfections as the sharpness reference. In other words, having two options gives us the best view of the world that surrounds us.


Opening our eyes wide when something frightens us, and narrowing them when faced with an unpleasant image, is not accidental. This universal response has to do with regulating the entry of light into the eyes, according to Adam Anderson, a neuroscientist at the University of Toronto (Canada). If something scares us, the eyes are opened to increase sensitivity, expand the field of view and better detect danger. To a stimulus that disgusts us, on the contrary, we block the light and focus our view very precisely only on the source of our displeasure, reducing the visual field. This is pure evolutionary adaptation, according to Anderson.

The eye processes much more information than we thought. Credit: University of Melbourne

Subjectivity in colors

If we had to depend only on our eyes, over the years the colors of the world that we see would lose their intensity, just like in an old photograph, eventually turning almost black and white. This is because the receptors that perceive the color, called cones, lose their sensitivity with age and become less effective. Luckily, we can rely on our brain to offset this decline and keep our view of the world from acquiring a vintage tone so that we continue to perceive skies of vibrant blue and meadows with intense green tones throughout life. “The visual brain is recalibrated as we age,” says Sophie Wuerger, co-author of a study published in PLoS ONE.

And the perception of color is subjective. We see colors as more gray and blue, (with less yellow component) if we feel sad and depressed than under more emotionally neutral situations, as Psychological Science informs us.

The bionic eye does work

In early 2015, Allen Zderad, who had become completely blind a decade earlier by retinitis pigmentosa, recovered his vision thanks to a bionic eye implanted in the Mayo Clinic. He was not the first to undergo this operation. There are now more than a hundred blind people who have recovered their sight through these implants, made up of a series of chips and electrodes placed on the back of the eye that receive the images of a microscopic camera located on a pair of glasses worn by the patient.

They never stay still

Although this usually goes unnoticed, our eyes never stop making quick, tiny corrections in the direction we look. These involuntary, jerk-like movements, called Microsaccades, occur on the order of 3 to 4 times every second, faster than the heartbeat. Until recently, nobody knew what purpose these corrections served, but scientists at the University of Tubingen (Germany) seem to have found the key.

According to the researchers, at each instant only a very small area of ​​our visual field is perceived clear and focused. With these rapid movements of the eye, we redirect the viewing angle to the various points of interest that seem interesting. The brain then joins these points together and builds a complete image. In addition, these constant readjustments allow us to experience greater peripheral vision and maintain a watchful eye, even when focusing our gaze on something, in case threats emerge.

You can see with the ear

What about the visual cortex in the brain of a sightless person? Well, far from being obsolete due to the lack of sight, the neurons are recycled and are dedicated to sharpening two other senses: hearing and touch. Lacking visual stimuli, brain plasticity allows these cells to be used to analyze the data received from other senses, according to a study carried out with functional magnetic resonance imaging, published in the journal Neuron. Put another way, if our eyes do not provide the information, our brain resorts to plan B and collects data from the sense of hearing and touch in order to build a mental spatial picture of the world around us.

By Elena Sanz for Ventana al Conocimiento


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