brain.
1.2 Eye. Types of Eye Movement
1.2.1 Oculomotor Muscles and Field of View
Human eyes are located in the depressions of the skull and are controlled by three pairs of muscles, called oculomotor or oculomotor muscles (Figure 1.2.1). Eye movements allow us to keep images of moving objects on the retina, to monitor objects without turning our heads. The central fossa, the macula and the peripheral part of the retina are involved in the process of vision, so our eye is a wide-angle optical system [Shiffman, 2008], [Kassan, 2011]. The field of view of one eye is downwards 70°, upwards 60°, to nose 60° and to temple 90° when observed with fixed eyes, and the sharpness of the image is provided only by the area of yellow spot within 6° – 8°.
The eyes of animals of many species are unable to move autonomously. For example, the eyes of a night owl are so large for her small skull that they almost touch each other. Because of this, the owl’s eyes are motionless, and in order to receive visual information, she has to turn her head.
1.2.2 Visual Acuity
Visual acuity refers to the ability to recognize small parts. There are five main types of acuity: detection, localization, resolution, recognition and dynamic acuity. The detection acuity is characterized by the detection of an object in the field of view. It is usually necessary to detect a small object of a certain size against a darker background. The acuteness of localization lies in the ability to distinguish between two lines, the ends of which are in contact with each other, as one single solid line (Figure 1.2.2, left). The sharpness of the resolving power of the eye is the ability to perceive the boundary between discrete elements (Figure 1.2.2, right); as it approaches the visual stimuli merge into one.
Figure 1.2.1 Human oculomotor muscles.
Testing the sharpness of resolution is usually performed using Landolt rings (Figure 1.2.3, right). To determine the acuity of recognition, a letter-based test is used, known to us as the Snellen table, which we encounter in an office of an optometrist (Figure 1.2.3, left). Dynamic acuity is determined by the ability to detect a moving stimulus and track its movement. With increasing speed of movement of the visual stimulus, it decreases.
The process of recognition of a visual stimulus depends on the size of the stimulus and on the distance to it. These two parameters are determined by the concept of the angle of view, the angle of view is the magnitude of the projection of the stimulus on the retina. The angle of view is measured in degrees; you can also use another subjective parameter – the rule of thumb. The width of the thumb of average size at arm’s length has the value of the angle of view of the order of 2°. Using this rule, it is quite easy to determine the approximate angles of view of objects that have been removed; for example, the moon or the sun make an angle of about 0.5°.
Figure 1.2.2 Determination of the sharpness of the resolution of the eye.
Figure 1.2.3 Snellen table and Landolt rings.
Figure 1.2.4 The stylized Landolt rings without the noise and with different noise level [Eremina, Shelepin, 2015].
The work of [Eremina, Shelepin, 2015] is dedicated to the study of visual perception by patients after cardiosurgical operations. The article describes the results of research of visual perception peculiarities and cognitive processing of visual images by the patients with ischemic heart disease. One hundred and seven middle-aged patients (62 years old) took part in the study. It was demonstrated that the capability to recognize the fragmented images declined immediately after the surgery (exemplified by Landolt rings, Figure 1.2.4). However, such capability is not only recovered but also surpasses the pre-surgical level in three months. Moreover, the patients undergoing surgery cope better with the offered tasks in three months after the surgery than patients getting non-surgical treatment.
1.2.3 Types of Eye Movement
Our brain usually tries to avoid emptiness, monotony, and an aggressive visual environment. The visual system highlights certain areas of the surrounding scene and controls the eye to recognize their details. The angle of view of a clear vision is about 2° of the visual field, and the oculomotor (oculomotor) muscles constantly make various movements to search for small objects. These movements allow you to direct the view in such a way that the visual stimulus is projected onto the central fossa, since this part of the retina is characterized by the most acute vision. Our eyes can both be fixed on the object and move continuously. Consider some publications in this area that describe the main types of eye movement.
The article of [Butenko, 2016] presents a brief description of types of eye movements and analyzes the methods and systems registering the oculomotor activity. The movement of the eyes is the natural component of visual perception. The eyes make micromovements even in the case of relative fixity of look. It is generally known that there are eight types of eye movements which belong to the micro- and macromovements.
The macro eye movements characterize the change of eye direction and can be controlled. They are divided into macrosaccades (sudden change of eye direction), accompanying movements (the sweeping drift of gaze after the moving fixation object), vergence eye movements (coupling and decoupling of visual axes), nystagmus (vibratory eye movements together with the accompanying movements) and torsional movements (rolling eye movements about the visual axis). The micro eye movements are the natural background of oculomotor activity that cannot be controlled. The micro eye movements are divided into the tremor (frequent eye vibrations), drift (sweeping drift of gaze interrupted by the micro jumps) and microsaccades (rapid eye movement occurring in the case of fixation points change).
Oculography (eye tracking) is a determination of eye position and meeting point of the eye bulb optical axis with the plane of observable object. The eye tracker is a device used to determine the orientation of the eye bulb optical axis in space. The eye trackers are applied in the course of study of the visual system as well as in psychology and cognitive linguistics.
The tracking methods can be divided into two groups: contact and noncontact. The first group is represented by the electro-oculography, photooptic and electromagnetic
