question: Why is it easy to cross your eyes to your nose, but hard to make them move outwards?
comment: You can focus on one thing only. If you hold out your hand and focus on your finger - and then bring that finger slowly closer till it touches your nose, your eyes will naturally go crosseyed while following. But when you hold out both hands and hold up two fingers - you can't even start to get each eye focussed on a different finger, let alone follow your fingers around. That's because you can't control your eyes independently. They always work in tandem.
passages: "Cross-fixation congenital esotropia", also called "Cianci's syndrome" is a particular type of large-angle infantile esotropia associated with tight medius rectus muscles. With the tight muscles, which hinder adduction, there is a constant inward eye turn. The patient cross-fixates, that is, to fixate objects on the left, the patient looks across the nose with the right eye, and vice versa. The patient tends to adopt a head turn, turning the head to the right to better see objects in the left visual field and turning the head to the left to see those in the right visual field. Binasal occlusion can be used to discourage cross-fixation. However, the management of cross-fixation congenital esotropia usually involves surgery.
   The relative strength of these two forces depends on which way the eye is looking. When the eye is "adducted" (looking toward the nose), the force of depression increases. When the eye is "abducted" (looking away from the nose), the force of intorsion increases, while the force of depression decreases. When the eye is in the primary position (looking straight ahead), contraction of the superior oblique produces depression and intorsion in roughly equal amounts.
   Three antagonistic pairs of muscles control eye movements: the lateral and medial rectus muscles, the superior and inferior rectus muscles, and the superior and inferior oblique muscles. These muscles are responsible for movements 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 movements are controlled entirely by the medial and lateral rectus muscles; the medial rectus muscle is responsible for adduction, the lateral rectus muscle for abduction. Vertical movements require the coordinated action of the superior and inferior rectus muscles, as well as the oblique muscles. The relative contribution of the rectus 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 movements. 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 rectus 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 movements.
   The relative strength of these two forces depends on which way the eye is looking. When the eye is "adducted" (looking toward the nose), the force of depression increases. When the eye is "abducted" (looking away from the nose), the force of intorsion increases, while the force of depression decreases. When the eye is in the primary position (looking straight ahead), contraction of the superior oblique produces depression and intorsion in roughly equal amounts.
   During the first days in orbit, the asymmetry of vertical eye movements in response to moving visual scenes is inverted. A return to symmetry of the vestibulo-ocular and optokinetic reflexes is then observed. Some studies have shown increased latencies and decreased peak velocities of saccades, while others have found just the opposite. It is possible that these conflicting results depend on when the measures were obtained during the mission. There is also a serious disruption of smooth pursuit eye movements, especially in the vertical plane.
   This summary of the superior oblique muscle describes its most important functions. However, it is an oversimplification of the actual situation. For example, the tendon of the superior oblique inserts "behind" the equator of the eyeball in the frontal plane, so contraction of the muscle also tends to "abduct" the eyeball (turn it outward). In fact, each of the six extraocular muscles exerts rotational forces in "all three planes" (elevation-depression, adduction-abduction, intorsion-extorsion) to varying degrees, depending on which way the eye is looking. The relative forces change every time the eyeball moves – every time the direction of gaze changes. The central control of this process, which involves the continuous, precise adjustment of forces on twelve different tendons in order to point both eyes in exactly the same direction, is truly remarkable.
   The approximate field of view of an individual human eye (measured from the fixation point, i.e., the point at which one's gaze is directed) varies by facial anatomy, but is typically 30° superior (up, limited by the brow), 45° nasal (limited by the nose), 70° inferior (down), and 100° temporal (towards the temple). For both eyes combined (binocular) visual field is 135° vertical and 200° horizontal. It is an area of 4.17 steradians or 13700 square degrees for binocular vision. When viewed at large angles from the side, the iris and pupil may still be visible by the viewer, indicating the person has peripheral vision possible at that angle.
   The optimum eye relief distance also varies with application. For example, a rifle scope needs a very long eye relief to prevent recoil from causing it to strike the observer.
   To maintain stereopsis and singleness of vision, the eyes need to be pointed accurately. The position of each eye in its orbit is controlled by six extraocular muscles. Slight differences in the length or insertion position or strength of the same muscles in the two eyes can lead to a tendency for one eye to drift to a different position in its orbit from the other, especially when one is tired. This is known as phoria. One way to reveal it is with the cover-uncover test. To do this test, look at a cooperative person's eyes. Cover one eye of that person with a card. Have the person look at your finger tip. Move the finger around; this is to break the reflex that normally holds a covered eye in the correct vergence position. Hold your finger steady and then uncover the person's eye. Look at the uncovered eye. You may see it flick quickly from being wall-eyed or cross-eyed to its correct position. If the uncovered eye moved from out to in, the person has esophoria. If it moved from in to out, the person has exophoria. If the eye did not move at all, the person has orthophoria. Most people have some amount of exophoria or esophoria; it is quite normal. If the uncovered eye also moved vertically, the person has hyperphoria (if the eye moved from down to up) or hypophoria (if the eye moved from up to down). Such vertical phorias are quite rare. It is also possible for the covered eye to rotate in its orbit. Such cyclophorias cannot be seen with the cover-uncover test; they are rarer than vertical phorias.
   The majority of autostereograms, including those in this article, are designed for divergent (wall-eyed) viewing. One way to help the brain concentrate on divergence instead of focusing is to hold the picture in front of the face, with the nose touching the picture. With the picture so close to their eyes, most people cannot focus on the picture. The brain may give up trying to move eye muscles in order to get a clear picture. If one slowly pulls back the picture away from the face, while refraining from focusing or rotating eyes, at some point the brain will lock onto a pair of patterns when the distance between them matches the current convergence degree of the two eyeballs.
   A near object (for example, a computer screen) appears large in the field of vision, and the eye receives light from wide angles. When moving focus from a distant to a near object, the eyes converge. The ciliary muscle constricts making the lens thicker, shortening its focal length. The pupil constricts in order to prevent strongly diverging light rays hitting the periphery of the cornea and the lens from entering the eye and creating a blurred image.
  
  Section::::Pathway.
   Section::::Neuroscience behind face superiority.
  
  Evidence from neurophysiology studies with humans and monkeys also support face superiority. Neuroimaging and electrophysiological studies in humans shows the effects of holistic face recognition. In particular, when humans are shown normal upright faces, neuroimaging displays higher brain activity and response rates in the middle fusiform gyrus (MFG), and the inferior occipital gyrus (IOG) than when shown scrambled or inverted faces. Additionally, experiments computing event-related scalp potentials (ERPs) reveal higher brain responses 180ms after presenting the normal face, than in inverted/scrambled conditions.
   Disgraced by nature, he one day harangued the audience by throwing "Gentlemen, it is easier for you to accustom yourself to my face, that for me to change it."
   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.
   BULLET::::8. Lens fogging technique. The subject fixates a distant object with both eyes open and appropriate correction in place. A +2.00 or +2.50 lens is alternately introduced in front of each eye, which blurs the distant object. The subject is then asked to state in which eye is the blur more noticeable. This is the dominant eye.
  
  BULLET::::9. A dichoptic motion coherence threshold test yields a quantified indication of ocular dominance.
  
  Forced choice tests of dominance, such as the Dolman method, allow only a right or left eye result.
  
  Section::::See also.
  
  BULLET::::- Ocular dominance column
  
  BULLET::::- Right- and left-hand traffic
   Faces are processed in separate areas of the brain to other stimuli, such as scenes or non-facial objects. For example, the fusiform face area (FFA) is a face-selective region in the brain that is only used for facial processing. The FFA responds more to upright but not inverted faces, demonstrating that inverted faces are not detected the same way that upright faces are.
   One example from Miller & Cohen involves a pedestrian crossing the street. In the United States, where cars drive on the right side of the road, an American learns to look left when crossing the street. However, if that American visits a country where cars drive on the left, such as the United Kingdom, then the "opposite" behavior would be required (looking to the right). In this case, the automatic response needs to be suppressed (or augmented) and executive functions must make the American look to the right while in the UK.
   To perform Bienfang’s test, the examiner should ensure that the patient is seated comfortably with head and eyes in primary gaze. The examiner is positioned 2–3 feet in front of the patient’s face to allow for clear observation of the patient’s eyes. The patient is instructed to tightly squeeze his or her eyelids shut for five to ten seconds. This not only relaxes the levator palpebrae superioris but actively inhibits it. Simultaneously, it fatigues the orbicularis oculi. The patient then opens his or her eyes and fixates on a target directly ahead, maintaining primary gaze. Hence, the balance between these two opposing muscles may be shifted to a higher lid position.
   In order to understand the actions of the superior oblique muscle, it is useful to imagine the eyeball as a sphere that is constrained – like the trackball of a computer mouse – in such a way that only certain rotational movements are possible. Allowable movements for the superior oblique are (1) rotation in a vertical plane – looking down and up ("depression" and "elevation" of the eyeball) and (2) rotation in the plane of the face ("intorsion" and "extorsion" of the eyeball).
   There are three basic techniques, referred to as X-shaped, Y-shaped, and single strip support. In X-shaped and Y-shaped, the arms run the risk of the being pulled medially, which would press on the optic nerve and could result in optic nerve atrophy. In single strip support, the material covers the posterior pole vertically between the optic nerve and insertion of the inferior oblique muscle. Often, this method is preferred, since it is the easiest method for placement, provides the widest area of support, and reduces the risk of optic nerve interference.
  
  Section::::Materials.
   Section::::Vision.:Extraocular muscles.
  
  Each eye has six muscles that control its movements: the lateral rectus, the medial rectus, the inferior rectus, the superior rectus, the inferior oblique, and the superior oblique. When the muscles exert different tensions, a torque is exerted on the globe that causes it to turn, in almost pure rotation, with only about one millimeter of translation. Thus, the eye can be considered as undergoing rotations about a single point in the center of the eye.
  
  Section::::Vision.:Rapid eye movement.
   BULLET::::2. The Porta test. The observer extends one arm, then with both eyes open aligns the thumb or index finger with a distant object. The observer then alternates closing the eyes or slowly draws the thumb/finger back to the head to determine which eye is viewing the object (i.e. the dominant eye).
   This summary of the superior oblique muscle describes its most important functions. However, it is an oversimplification of the actual situation. For example, the tendon of the superior oblique inserts "behind" the equator of the eyeball in the frontal plane, so contraction of the muscle also tends to "abduct" the eyeball (turn it outward). In fact, each of the six extraocular muscles exerts rotational forces in "all three planes" (elevation-depression, adduction-abduction, intorsion-extorsion) to varying degrees, depending on which way the eye is looking. The relative forces change every time the eyeball moves—every time the direction of gaze changes. The central control of this process, which involves the continuous, precise adjustment of forces on twelve different tendons in order to point both eyes in exactly the same direction, is truly remarkable.
   Perhaps finding Bates' concepts of "shifting" and "swinging" too complicated, some proponents of vision improvement, such as Bernarr Macfadden, suggested simply moving the eyes up and down, from side to side, and shifting one's gaze between a near-point and a far-point.
  
  Section::::Treatments.:Sunning.
   Scott Fahlman first suggested the use of the colon with the left parenthesis to iconically represent a frowning face on the Internet in what has become a well-known emoticon. In this form the frown is entirely presented as a curve of the lips facing away from the eyes.
  