Saccadic movements: definition, characteristics and functions

It is said that the eyes are the mirror of the soul, that a person's look can reveal many facts about them.

Whether this is true or not, what we can find out from someone's gaze is where they are looking. If he is paying attention to us, looking at our faces, or, on the contrary, he is absent looking towards another place.

As we watch, the eyes make hundreds of rapid movements with which we can see different details of the object, animal, person or scene in front of us.

These types of movements are saccades, something that, although it may seem simple, has been widely studied and plays a very important role in orientation through physical space. Let's take a deeper look at what they consist of.

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What are saccadic movements?

Saccadic movements, also called saccades, are rapid and simultaneous movements that both eyes perform when they are looking at a point in the same direction.

These movements were described by the French ophthalmologist Louis Émil Javal in 1880, who was able to observe them experimentally by watching people read silently. These movements are controlled cortically by the frontal eye fields and subcortically by the superior colliculus.

The movements that are made during the reading are not just one, but rather consist of several small fixations that allow a whole sentence to be read. The same thing happens when you are looking at a painting or when you are looking at a room to find the keys.

Function

Both humans and many animal species do not see only by looking at a specific point in a static way. In order to capture as much information as possible, and not to miss any detail, it is necessary to move your eyes. With saccadic movements it is possible to scan the environment, find interesting data and mentally create a three-dimensional map.

Another of the important functions of these movements has to do with how the photoreceptor cells are distributed. The central part of the retina, that is, the fovea, is a place where there is a high concentration of cones, cells responsible for color vision. Due to this, the eye, being static, is only capable of perceiving in detail between 1 and 2 degrees of the total 164 degrees that human vision has. The rest of the retina has rods, cells that are effective in capturing movement.

By moving both the head and the eyes it is possible to make the fovea able to capture more details, which allows the brain to have a greater percentage of the scene with high visual resolution. It should be said that both the cones and the rods need these saccadic movements, since they are cells that are activated by changes in the intensity of light. If there are no changes in the light they receive, the stimuli sent to the brain.

Duration time

Saccadic movements are very fast. In fact, they are one of the fastest movements capable of the human body.

In the human species, the angular velocity of the eyes when performing saccades can exceed 900 degrees per second. The onset time of saccades when faced with an unexpected stimulus can be as little as 200 milliseconds., and last between 20 and 200 milliseconds depending on the amplitude.

Types of saccades

Saccadic movements can be classified into four types depending on the purpose for which they are made.

1. visually guided saccades

The eyes move towards a scene. It can be done exogenously, because a stimulus has been seen that has appeared in the visual scene, or done endogenously, in order to scan what is being seen.

An example of this type of saccades would be when a fly suddenly appears and we follow it with our eyes to put an end to it or when we are looking at a painting, in which we look at the most striking details of the paint.

2. Antisaccade

As its name suggests, an antisaccade is an eye movement that moves away from the stimulus.

3. memory guided saccada

This type of eye movement is very curious, since it consists of moving the eyes towards a remembered point, without actually seeing a stimulus.

For example, when we close our eyes and try to remember the famous painting by Leonardo da Vinci, La Gioconda, we unconsciously move our eyes looking at the details of the painting, despite the fact that what we are doing is totally mental. We're seeing.

4. predictive saccades

The eyes, which are seeing a real object or stimulus, move in ways that predict how the observed object will behave.

For example, while you are watching an airplane fly, you can follow the trajectory of the aircraft assuming that it will do so in a straight line.

Relationship with reading

The human eye is capable of reading, continuously and without stopping, a whole line of text. Saccadic movements are necessary in reading, since if you keep your gaze fixed on a specific letter, the fovea only perceives those closest to it, and it is only possible to clearly see between four and five more letters.

When reading, the eyes gaze at one word, extract the information, and move on to the next, allowing chained movements that allow reading word by word or groups of them and understanding the text. Thus, the reading process consists of continually engaging and disengaging the gaze on the page that is being observed. When the disengagement occurs, the fovea stops acting and becomes the task of the peripheral retina, which tracks where the next jump has to be made. Once the point in question is located, the fovea reactivates.

A fairly common problem in children with reading problems occurs when the saccadic movements are not precise or do not last as long as they should. This means that they are not able to correctly identify the letter (confuse the letter 'd' with the 'b'), or that they cannot extract the information completely. As a result, children need the help of a finger to read, move their heads more than normally necessary, skip sentences and paragraphs, or read slowly.

Associated pathologies

In it ADHD there has been an increase in errors due to antisaccadic movements, while saccades visually guided, which are those used to look at stimuli and pay attention to details, are given in a retarded.

Nystagmus is a condition in which eye movements occur involuntarily, causing the sensation that the eyes are vibrating. This problem causes vision to be affected and reduced, since the eyes are in constant movement, the fovea captures the environment in an anarchic way. This means that what you want cannot be seen clearly, since you cannot fix your gaze on one point.

saccade masking

Many people believe that when saccadic movements are performed, the optic nerve does not transmit information.. This belief is not true. What happens is that the brain selectively blocks visual processing while eye movements are taking place.

This means that, if we stand in front of a mirror and look first at one eye and then at the other constantly, we do not the eye movement is seen, but also do not give the feeling that it has stopped seeing at some point in the process.

Bibliographic references:

• Quevedo, L.; Aznar-Casanova, JA and da Silva, J.A. (2016) Dynamic Visual Acuity: a critical review International Journal of Psychological Reviews, 1(1): 7-21
• Amador-Campos, J.A.; Aznar-Casanova, JA.; Bezerra, I.; Torro-Alves, N. and Moreno-Sanchez, M. (2015) Attentional blink in children with ADHD” in ADHD. Brazilian Journal of Psychiatry, 32(2), 133-138.