Движения глаз: механика и функции

Movement of the eyes

Eye movement includes the voluntary or involuntary movement of the eyes. Eye movements are used by a number of organisms (e. g. primates, rodents, flies, birds, fish, cats, crabs, octopus) to fixate, inspect and track visual objects of interests. A special type of eye movement, rapid eye movement, occurs during REM sleep. The eyes are the visual organs of the human body, and move using a system of six muscles. The retina, a specialised type of tissue containing photoreceptors, senses light. These specialised cells convert light into electrochemical signals. These signals travel along the optic nerve fibers to the brain, where they are interpreted as vision in the visual cortex. Primates and many other vertebrates use three types of voluntary eye movement to track objects of interest: smooth pursuit, vergence shifts and saccades. These types of movements appear to be initiated by a small cortical region in the brain's frontal lobe. This is corroborated by removal of the frontal lobe. In this case, the reflexes (such as reflex shifting the eyes to a moving light) are intact, though the voluntary control is obliterated.

Six extraocular muscles facilitate eye movement. These muscles arise from the common tendinous ring (annulus of Zinn) in the orbit (eye cavity), and attach to the eyeball. The six muscles are the lateral, medial, inferior and superior recti muscles, and the inferior and superior oblique muscles. The muscles, when contracting, cause movement of the eyeball, by pulling the eyeball towards the muscle. For example, the lateral rectus is on the lateral side of the eyeball. When it contracts, the eyeball moves so that the pupil looks outwards. The medial rectus causes the eyeball to look inwards; the inferior rectus downwards and outwards, and the superior rectus upwards and outwards. The superior oblique muscle and inferior oblique muscle attach at angles to the eyeball. The superior oblique muscle moves the eye downwards and inwards whereas the inferior oblique muscle moves the eye upwards and outwards. Three antagonistic pairs of muscles control eye movement: the lateral and medial recti muscles, the superior and inferior recti muscles, and the superior and inferior oblique muscles. These muscles are responsible for movement of the eye along three different axes: horizontal, either toward the nose (adduction) or away from the nose (abduction); vertical, either elevation or depression; and torsional, movements that bring the top of the eye toward the nose (intorsion) or away from the nose (extorsion). Horizontal movement is controlled entirely by the medial and lateral recti muscles; the medial rectus muscle is responsible for adduction, the lateral rectus muscle for abduction. Vertical movement requires the coordinated action of the superior and inferior recti muscles, as well as the oblique muscles. The relative contribution of the recti and oblique groups depends on the horizontal position of the eye. In the primary position (eyes straight ahead), both of these groups contribute to vertical movement. Elevation is due to the action of the superior rectus and inferior oblique muscles, while depression is due to the action of the inferior rectus and superior oblique muscles. When the eye is abducted, the recti muscles are the prime vertical movers. Elevation is due to the action of the superior rectus, and depression is due to the action of the inferior rectus. When the eye is adducted, the oblique muscles are the prime vertical movers. Elevation is due to the action of the inferior oblique muscle, while depression is due to the action of the superior oblique muscle. The oblique muscles are also primarily responsible for torsional movement. The muscles are supplied by the oculomotor nerve, with the exception of the superior oblique, which is supplied by the trochlear nerve, and the lateral rectus, supplied by the abducens nerve.

When reading, the eye moves continuously along a line of text, but makes short rapid movements (saccades) intermingled with short stops (fixations). There is considerable variability in fixations (the point at which a saccade jumps to) and saccades between readers and even for the same person reading a single passage of text.

Eye movement in music reading is the scanning of a musical score by a musician's eyes. This usually occurs as the music is read during performance, although musicians sometimes scan music silently to study it, and sometimes perform from memory without score. Eye movement in music reading may at first appear to be similar to that in language reading, since in both activities the eyes move over the page in fixations and saccades, picking up and processing coded meanings. However, music is nonlinguistic and involves a strict and continuous time constraint on an output that is generated by a continuous stream of coded instructions.

Eye movement in scene viewing refers to the visual processing of information presented in scenes. A core aspect of studies in this area is the division of eye movements into the rapid movement of the eyes (saccades), and the focus of the eyes on a point (fixations). Several factors can influence eye movement in scene viewing, including the task and knowledge of the viewer (top down factors), and the properties of the image being viewed (bottom up factors). Typically, when presented with a scene, viewers demonstrate short fixation durations and long saccade amplitudes in the earlier phases of viewing an image. This is followed by longer fixations and shorter saccades in the latter phases of scene viewing processing. It has also been found that eye movement behaviour in scene viewing differs with levels of cognitive development fixation durations are thought to shorten and saccade amplitudes lengthen with an increase in age.

Where eye movements fixate is affected by both bottom up and top down factors. Even an initial glimpse of a scene has an influence on subsequent eye movements. In bottom up factors, the local contrast or prominence of features in an image, such as a large contrast in luminance or a greater density of edges, can affect the guidance of eye movements. However, the top down factors of scenes have a greater impact in where eyes fixate. Areas containing more meaningful features, or areas where colour aids the discrimination of objects, can influence eye movements. Images which are related to previous images shown can also have an effect. Eye movements can also be guided towards items when they are heard verbally at the same time as seeing them. Cross culturally, it has been found that Westerners have an inclination to concentrate on focal objects in a scene, whereas East Asians attend more to contextual information.

Average fixation durations last for about 330 ms, although there is a large variability in this approximation. This variability is mostly due to the properties of an image and in the task being carried out, which impact both bottom up and top down processing. The masking of an image and other degradations, such as a decrease in luminance, during fixations (factors which affect bottom up processing), have been found to increase the length of fixation durations. However, an enhancement of the image with these factors also increases fixation durations. Factors which affect top down processing (e. g. blurring) have been found to both increase and decrease fixation durations.

Patients with eye movement disorders may report diplopia, nystagmus, poor visual acuity or cosmetic blemish from squint of the eyes.