EYES : THOSE PRETTY LITTLE LIARS

Can we trust our eyes? Sometimes our vision plays tricks on us; we can either ‘see’ things that are not even there or not see something that is right before our eyes. Learn how this is possible.



Playing hide-and-seek
Sometimes we try to find something, look for it everywhere to finally find it at the very first spot we had checked. It happens often, doesn’t it? When there are more people with us, we can even blame them for hiding the object. ‘That could only be the work of that mischief friend’, we might think. If we are alone, things are even worse, as we risk believing in supernatural forces. Before deciding to leave an apple outside for goblins, so they stop making jokes on us, we must remember we cannot believe our eyes.



Human vision
Humans have a visual field that covers almost 180°, divided in central and a peripheral vision field. The central visual field is where the eyes’ gaze rest, whereas the peripheral visual field comprises from the surroundings of the central vision to the end of the visual field.

Only the central visual field is sharp, with good visual accuracy do detect color and details of the objects. However, peripheral vision is useful to detect movement and see in dim light.

The human eye works similarly to an old photograph machine, with a dark chamber. The light enters through an orifice, represented by the pupil in the eyes and the shutter in the machine. The image forms in the back part of the eye, the retina, a light-sensitive tissue layer that corresponds to the photographic film in the camera.

In the retina, there are two types of cells able to detect light: cones and rods. These cells are known as photoreceptors. However, differently from a photographic film, the distribution of photoreceptors in the retina is not uniform. In the central part of the retina (fovea), there is a high concentration of cones and no rods. In the peripheral retina, few cones are present, but rods predominate. Cones perceive color and rods, only black and white.

The path to the brain
In the central retina, each photoreceptor usually has more than one neuron to send information to the brain. In the peripheral retina, though, several photoreceptors share the same neuron to transmit information. Hence, many neurons signalize information from the central retina, but only few neurons signalize information from the peripheral retina.

In the brain, this difference sharpens. In the visual cortex, there is a larger area to process information from the central retina than from the peripheral retina. Hence, the peripheral vision brings poorer information than the central vision.

The blind spot
To make matters worse, in both of our eyes, we have a blind spot located at the periphery of the visual field, near the nose. The blind spot is the area where the optic nerve and blood vessels attach to the eyeball (optic disk). In the blind spot, the retina is not sensitive to light, as it has no photoreceptor. Therefore, we do not see whatever falls in those areas of our visual field.

Testing the blind spot
It is possible to prove the existence of the blind spot with two simple tests:
1. Close your right eye. Fix the gaze of your left eye into the ‘X’ of the white circle. Stay at approximately 50 cm from the screen. Slowly move yourself a little bit back and forth until you notice changes in the picture. What happened to the black circle?

Eyes_testing blind spot

2. Repeat the same protocol proposed for the previous exercise. What happened to the gap in the black line?

Eyes_testing blind spot2

In these two exercises, the brain fills in the missing information based on expectations of what things should be. In both cases, our eyes are tricking us, either showing things that are not there or making things disappear, as our neurons generate a representation of the missing area. In the first test, the black circle disappears when it falls exactly on our blind spot. The background is white, so, the missing information is filled in as white. In the second test, the gap in the black line disappears as it falls on our blind spot, as it is more likely that the line continues instead of being broken considering its representation in neighbor areas.

Unnoticed blindness
Although we do not see objects that fall in our blind spot and have a poor peripheral vision, we hardly notice it. How is that possible? Humans have two mechanisms to compensate these deficiencies in vision. First, eye movements change the focus of our central vision constantly, so that most of our surroundings pass through it. Second, the brain fills the gaps in perception based on previous information stored.

Improving perception
It is possible to improve our visual perception. Recently, scientists from the University of Queensland, Australia, discovered that it is possible to reduce the size of the functional blind spot after only eight days of training. Perceptual training improved sensitivity to motion and color in volunteers, but the benefits were restricted to the trained eye. Training included exercises to stimulate areas at the periphery of the blind spot. The improved performance of the trained eye can be related to an increase in sensitivity to weak signals around the blind spot.

This recent finding goes beyond an extra help to find our lost keys or earrings. If exercises decrease the functional area of the blind spot, they may also be useful in the recovery of gained blind spots resulting from injuries.

References
Miller, P.A., Wallis, G., Bex, P.J. and Arnold, D.H., 2015. Reducing the size of the human physiological blind spot through training. Current Biology, 25(17), pp.R747-R748.
Ramachandran, V.S., 1992. Blind spots. Scientific American, 266(5), pp.86-91.
Wandell, Brian A. 1995. Foundations of vision. Sinauer Associates.

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