It is said that the eyes are the mirror of the soul, that a person’s gaze can reveal many facts about him or her.
Whether this is true or not, what we can find out from someone’s eyes is where they are looking. Is he paying attention to us, looking at our faces, or on the contrary, is he absent looking somewhere else?
As we observe, 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 the saccadic ones, something that although it may seem simple has been widely studied and they take a very important role in orientation through physical space. Let’s take a closer look at what they are.
What are saccadic movements?
Saccadic movements, also called saccades, are rapid and simultaneous movements that both eyes make when they are looking towards 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 seeing how people read in silence. These movements are controlled at the cortical level by the frontal eye fields and subcortically by the upper eye.
The movements that are made during the reading are not just one, but consist of several small fixations that allow a whole sentence to be read. The same happens when you are looking at a painting or when you are looking at a room to find the keys.
Both humans and many animal species do not see only by looking at a particular point in a static way . In order to capture as much information as possible, and not let any detail escape, 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 phovea, is a place where there is a high concentration of cones, cells responsible for colour vision. Because of this, the eye, being static, is only able to perceive 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 phovea able to capture more detail, which allows the brain to have a greater percentage of the scene with high visual resolution. It should be said 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, there is a cessation of stimuli sent to the brain.
The saccadic movements are very fast. In fact, they are one of the fastest movements the human body is capable of.
In the human species, the angular velocity of the eyes when performing saccades can exceed 900 degrees per second. The start time of saccadic movements when faced with an unexpected stimulus can be only about 200 milliseconds , and last between 20 and 200 milliseconds depending on the amplitude.
Types of sacks
Saccadic movements can be classified into four types depending on the purpose for which they are performed.
1. Visually Guided Drawings
The eyes move towards a scene. It can be exogenous, 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 extraction would be when a fly appears suddenly and we follow it with our eyes to finish it off or when we are looking at a painting, in which we look at the most striking details of the painting.
As the name suggests, an anti-sacralization is an eye movement that moves to the opposite location of the stimulus.
3. Guided memory retrieval
This type of eye movement is very curious, since it consists of moving the eyes to 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 to look at the details of the painting, even though it is totally mental what we are seeing.
4. Predictive Drawings
The eyes, which are seeing an actual object or stimulus, move in a way that predicts how the observed object will behave.
For example, while you are watching an airplane fly, you can follow the path of the aircraft assuming it is going to fly straight.
Relationship to reading
The human eye is capable of reading a whole line of text at once without having to stop. Saccadic movements are necessary in reading, since if you keep your eyes fixed on a particular letter, the camera only perceives the ones closest to it, and it is only possible to see clearly four or five more letters.
When reading, the eyes look at one word, extract the information and move on to the next, allowing chained movements that allow reading word by word or sets of words and understanding the text. Thus, the process of reading consists of continually engaging and disengaging the gaze on the page being viewed. When the release is given, the phovea 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 has been located, the phovea acts again.
A fairly common problem in children with reading problems occurs when saccadic movements are not accurate or do not last as long as they should. This causes them to be unable to correctly identify the letter (confuse the letter ‘d’ with ‘b’), or to extract the information completely. As a result, children need the help of a finger to read, move their heads more than usually necessary, skip sentences and paragraphs, or read slowly.
In ADHD there has been an increase in errors due to anti-saccadic movements, while visually guided pulls, which are used to look at stimuli and notice details, are delayed.
Nystagmus is a condition in which eye movements occur involuntarily, giving the sensation that the eyes are vibrating. This problem causes vision to be affected and reduced, as the eyes are constantly moving, the phovea captures the environment in an anarchic way. This makes it impossible to see clearly what you want, as you cannot fix your gaze on one point.
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 being made.
This means that if you stand in front of a mirror and look at one eye first and then the other eye constantly, you will not see the eye movement, but neither will you have the feeling that you have stopped seeing at some point in the process.
- Quevedo, L.; Aznar-Casanova, JA y 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. y Moreno-Sanchez, M. (2015) Attentional blink in children with ADHD” en ADHD. Revista Brasileira de Psiquiatria, 32(2), 133-138.