Saccadic movements: definition, characteristics and functions

These rapid eye movements allow us to capture more information from the environment.

It is said that the eyes are the mirror of the soul, that a person's gaze can reveal a lot about him or her.

Whether this is true or not, what we can find out from someone's gaze is where they are looking. If they are paying attention to us, looking at our face, or on the contrary, if they are absent, looking somewhere else.

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

This type of movements are the saccadic movements, something that although it may seem simple has been widely studied and plays a very important role in the orientation in physical space. Let's see in more depth what they consist of.

• Recommended article: "The 11 parts of the eye and their functions".

What are saccadic movements?

Saccadic movements, also called saccades, are rapid, simultaneous movements made by both eyes when they are looking in the same direction. made by both eyes 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 how people read in silence. These movements are controlled cortically by the frontal eye fields and subcortically by the superior colliculus.

The movements made during reading are not just one, but consist of several small fixations that allow reading a whole sentence. The same occurs when looking at a painting or when looking around a room for the keys.

Function

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

Another important function of these movements has to do with how the photoreceptor cells are distributed. The central part of the retina, i.e., the fovea, is a place where there is a high concentration of cones, cells responsible for color vision. Because of this, the eye, being static, is only able to perceive in detail between 1 and 2 degrees of the total 164 degrees of human vision. 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 capture more details, which allows the brain to have a greater percentage of the scene with high visual resolution. It should be noted that both cones and rods need these saccadic movements, since they are cells that are activated by changes in light intensity. If there are no changes in the light they receive, the stimuli sent to the brain cease.

Duration time

Saccadic movements are very fast. In fact, they are one of the fastest movements capable of being performed by 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 saccadic movements in response to an unexpected stimulus can be as short as 200 milliseconds and last between 20 and 200 milliseconds.and last between 20 and 200 milliseconds depending on the amplitude.

Types of saccades

The saccadic movements can be classified into four types according to the purpose for which they are performed.

1. Visually guided saccades

The eyes move towards a scene. It can be exogenously, due to seeing a stimulus that has appeared in the visual scene, or 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 in order to kill it or when we are looking at a painting, in which we look at the most striking details of the painting.

2. Antisaccade

As its name suggests, an antisaccade is an eye movement that moves to the opposite place where the stimulus is.

3. Memory-guided saccade

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 Leonardo da Vinci's famous painting, La Gioconda, we unconsciously move our eyes, focusing on the details of the painting, even though it is entirely mental what we are seeing.

4. Predictive saccades

The eyes, which are looking at a real object or stimulus, move in a way that predicts how the observed object will behave.

For example, while watching an airplane fly, one can follow the trajectory of the aircraft assuming that it will fly straight ahead.

Relation to reading

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

When reading, the eyes fix their gaze on a word, extract the information and move on to the next word, allowing chained movements that make it possible to read word by word or sets of words and understand the text. Thus, the reading process consists of continuously engaging and disengaging the gaze on the page being observed. When disengagement occurs, the fovea ceases to act and it becomes the task of the peripheral retina, which tracks where the next jump has to be made. Once the point in question has been located, the fovea starts working again.

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

Associated pathologies

In ADHD, an increase in errors due to antisaccadic movements has been seen, while visually guided saccades, which are those used to look at stimuli and fixate on details, are delayed.

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

Saccadic masking

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

This means that if we stand in front of a mirror and look first at one eye and then at the other eye constantly, we do not see the eye movement, but neither do we get the sensation that we have 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. Revista Brasileira de Psiquiatria, 32(2), 133-138.