Jerky movements: definition, characteristics and functions

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

Whether this is true or not, what we can discover from someone’s gaze is where they look. If he pays attention to us, looks us in the face, or on the contrary he is absent looking elsewhere.

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

These types of movements are jerky, which, although it may sound simple, has been studied extensively and plays a very important role in orientation through physical space. Let’s take a closer look at what they consist of.

    What are saccadic movements?

    Jerky movements, also called pull-ups, are rapid, simultaneous movements that both eyes perform when 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 seeing how people read in silence. These movements are controlled at the cortical level by the frontal eye fields and subcortical by the superior colliculus.

    The movements performed during reading are not one, but consist of several small fixations that allow you to read an entire sentence. The same thing happens when you look at a picture or when you look at a room to look for the keys.

    function

    Humans and many animal species do not see just by looking at a particular point statically. In order to capture as much information as possible and not miss any detail, you need to move your eyes. With jerky movements, it is possible to scan the environment, find interesting data and mentally draw a three-dimensional map.

    Another important function of these movements has to do with the way the photoreceptor cells are distributed. The central part of the retina, that is to say the fovea, is a place where there is a high concentration of cones, cells responsible for color vision. For this reason, the eye, being static, is only able to perceive in detail between 1 and 2 degrees out of the 164 total degrees of human vision. The rest of the retina has rods, cells that are good at capturing movement.

    By moving both the head and the eyes, it is possible to make the fovea capable of capturing more detail, which allows the brain to have a higher percentage of the scene with high visual resolution. It’s worth noting that both cones and rods need these jerky movements, as they are cells that are activated by changes in light intensity. If there is no change in the light they receive, there is a shutdown of the stimuli sent to the brain.

    Duration

    The saccadic movements are very fast. In fact, they are one of the fastest movements the human body is able to perform.

    In the human species, the angular speed of the eyes when performing shots can exceed 900 degrees per second. The start time of jerky movements in the face of an unexpected stimulus can be as short as 200 milliseconds., And last between 20 and 200 milliseconds depending on the amplitude.

    Types of strokes

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

    1.visually guided shots

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

    An example of this type of shot would be when a fly appears and we follow it with our eyes to finish it or when we look at a painting, in which we look at the most striking details of the painting.

    2. Antisaccade

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

    3.guided memory reading

    This type of eye movement is very curious, because it consists of moving the eyes to a memorized 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 subconsciously move our eyes looking at the details of the painting, even though what we are seeing is totally mental.

    4. Predictive saccades

    The eyes, which see a real object or stimulus, move in a way that predicts the behavior of the observed object.

    For example, while watching an airplane fly, you can follow the path of the airplane assuming it is going to do so in a straight line.

    Relationship with reading

    The human eye is able to read, continuously and without stopping, an entire line of text. Jerky movements are necessary when reading, because if you keep your gaze fixed on a certain letter, the fovea perceives only those closest to it, only being able to see clearly between four and five other letters.

    When read, the eyes fix their gaze on one word, extract the information and move on to the next, allowing chained movements that allow them to read verbatim or sets of them and understand the text. Thus, the reading process consists of continuously gluing and unhooking the gaze on the observed page. When the detachment occurs, the fovea ceases to act and becomes the task of the peripheral retina, which follows where the next jump is to be made. Once the point in question is located, the fovea acts again.

    A fairly common problem in children with reading problems is when the jerking movements aren’t necessary or don’t last as long as they should. This makes them unable to correctly identify the letter (confuse the letter “d” with the “b”) or to extract the information completely. As a result, children need the help of a finger to read, move their head more than they normally need, skip sentences and paragraphs, or read slowly.

    associated pathologies

    In ADHD, there has been an increase in errors due to antisaccadic movements, while visually guided shots, which are used to watch stimuli and pay attention to detail, are delayed.

    Nystagmus is a condition in which eye movements occur involuntarily, making the eyes appear as if they are vibrating. This problem results in impaired and reduced vision, because when the eyes are in constant motion, the fovea captures the environment in an anarchic manner. This makes it impossible to see clearly what you want, by not being able to fix your gaze on a point.

    saccadic masking

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

    This means that if we stand in front of a mirror and look at one eye first and then the other constantly, we don’t see eye movement, but neither do we feel that we have stopped seeing in one. given the process.

    Bibliographical references:

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

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