You can not see even with healthy eyes: when blindness depends on the brain

Not always the malfunction of vision comes from physical eye problems. In fact, the sense of sight does not end with the eyes, which simply play the role of a lens and a sensor in a camera, but also involves certain areas of the brain which are responsible for processing the signals sent by the eyes. For this reason details brain injuries can impair the brain’s ability to interpret images, sometimes leading to “forget” a part of the world or, in other cases, to replace it with incredible alternative scenarios. Here are some conditions cerebral blindness in which patients cannot see properly despite having perfectly healthy eyes.

How the brain allows us to see and which areas control vision

The eyes regulate the amount of incoming light, focus images and, thanks to specialized cellsI am able to convert the light waves in the language of the brain, that is, in electrical signals.

However, what catches the eye it is not yet “seen”: at this level, images are electrical signals without a form, a bit like the set of code strings in the movie Matrix.

From the eyes visual information begins a journey into the brain, passing through specialized areas that interpret specific aspects of what we see. There first stage it’s there visual cortexin the occipital lobe, which has the task of creating a “basic assembly” of the imageanalyzing their shapes, colors and orientation. Subsequently, the information follows two paths:

• there street of the “where”through the parietal lobe, processes movements and spatial details.
• There via the “what”directed to the temporal lobe, recognizes faces and objects.

Our ability to perceive and interpret the world it depends on the perfect coordination of these areas. But what happens if one of them suffers damage?

There prosopagnosiawhen we can’t recognize faces

Would you ever trade the face of a loved one for a hat? As absurd as it seems, this is what happened to patient P. who, after a stroke, developed prosopagnosiaa form of brain blindness that prevents you from recognizing faces. The cause often lies in injuries to the fusiform cortexa region of the “via del cosa” specialized in their recognition. When this area is damaged, the brain loses the very idea of ​​a face and a familiar face can become a set of disconnected details, to the point of being confused with any object, even… a hat.

Akinetopsia, when we cannot perceive movements

Imagine this scene: you are crossing the street, you notice a car in the distance, but an instant later you find it a few meters away, without having perceived its movement. Creepy, right? Yet, those who suffer from akinetopsia he wouldn’t be surprised. This rare form of blindness is caused by lesions in the brain areas that process movements. Consequently, for these patients the world does not flow smoothly, but appears as a sequence of frames. Consider that the SCI patient found it impossible to even pour a glass of water: to his eyes, the liquid looked like an immobile column, making it impossible for him to understand when to stop before overflowing!

One-sided spatial negligence, when we only see “half” of the world

Imagine waking up one morning and not recognizing your face in the mirror or, again, seeing the world in half. This is what happened to Anton Räderscheidta twentieth-century German painter, who suddenly lost the ability to see the world as a whole, perceiving only what was to the right of his field of vision. However, if you are thinking that Anton had been struck by an illness in one of his eyes you are on the wrong track.

The painter had suffered a stroke in a parietal lobea fundamental area for perceiving the space around us, developing a disorder called unilateral spatial neglect. Those affected by it, reading a text, will only notice the words on half of the page. If invited to draw a clock, he will depict the numbers on only one half of the dial. It is as if a part of the world, although visible to the eye, ceased to exist, “erased” by brain damage.

Anton’s syndrome, when we don’t know we’re blind

If it is the one who suffers damage visual cortexthe brain loses the ability to process images and the person becomes blind, as if the problem was with the eyes. However, if you met someone suffering from Anton’s syndromea form of blindness caused by damage to the visual cortex, you probably wouldn’t notice your condition right away. Indeed, the patient could precisely describe his surroundings, just as if he could see. Soon, however, you would realize that the objects he talks about do not exist: it is all the result of an illusion built by the brain! Patients with Anton syndrome, in fact, deny their blindness (a condition known as visual anosognosia) and tend to confabulatethat is, to invent details and descriptions to fill the gaps left by the loss of sight.