Dennis R. Ankrum
Director of Human Factors Research
Nova Solutions, Inc.
10007 San Luis Trail,
Austin, TX. 78733-1253
The Visual System
The visual system is an information gatherer. It picks up information contained in the light that enters our eyes and sends it to the brain. The higher the quality of that information, the better our perception and ability to function. The demands on the visual system are great. To be efficient, the visual system must adapt to a wide range of light levels and viewing distances.
Visual ergonomics adapts the workplace to conform to the visual capabilities of workers. An understanding of the human visual system can help us design workplaces that reduce eye fatigue, improve performance and even reduce some of the risk factors for cumulative trauma disorders.
Sustained near visual work, such as reading and computer operation, imposes demands that are relatively new to the human visual system. When we view a monitor, or any other close object, our eyes accommodate and converge, both of which can contribute to eye strain (Fischer, 1977, Collins, O'Meara and Scott, 1975). While accommodation and convergence are not new, it is only in the last few centuries (since the invention of the printing press) that sustained "near work" has become so widespread.
Accommodation is the process by which the eyes adapt to maintain clear focus as visual targets get closer. When we look at close objects, a small muscle in each eye called the ciliary muscle changes the shape of the lens capsule. With proper accommodation, the lens bends the incoming light rays so that they strike the retina at a single point, allowing a sharp image to be interpreted by the brain. If the point at which the rays converge is too far in front of or behind the retina, the result is blur. The brain reacts to this blur and signals the ciliary muscle to alter the shape of the lens capsule, bringing the object into sharper focus.
The ciliary muscle is a smooth muscle. However, it has the type of dense innervation and ability to rapidly contract and relax that makes it more like skeletal muscles such as the biceps, which do fatigue (Kaufman, 1992). Other qualities make the ciliary muscle unique among smooth muscles. Studies (Ehrlich, 1987, Owens and Wolf-Kelly, 1988) indicate that the ciliary muscle may be susceptible to fatigue. Some studies have suggested that the work done by the ciliary muscle can be a factor in eye discomfort at computers (Krueger, 1984, Jaschinski-Kruza, 1988).
Resting Point of Accommodation
The eyes have a resting point of accommodation. This is the distance at which the eyes focus when there is no object on which to focus. Accommodation is more or less "relaxed." The resting point of accommodation differs among individuals, but the average is around 31 1/2 inches for young people and increases with age (Krueger, 1984). Jaschinski-Kruza (1988) found that uninterrupted viewing of a computer monitor closer than the resting point of accommodation contributed to eye strain. The ciliary muscle must work two and a half times harder to focus on a monitor 12 inches away than it does to focus at 30 inches (Fischer, 1977).
The closest distance at which an object can be brought into sharp focus is called the near point of accommodation. As we get older, the lens undergoes changes which cause the near point to recede.
A 16-year-old can focus as close as 3 inches. At age 32 the near point averages 4.7 inches; at 44 it is 9.8 inches; at 50 it is 19.7 inches; and at 60 it reaches 39.4 inches (Grandjean, 1987). At around age 40, many people with otherwise adequate vision begin to hold reading material farther and farther from their eyes until their "arms get too short." They then require reading glasses.
When determining viewing distance, optometrists recommend not requiring more than half of the eyes" ability to accommodate (Boorish, 1970).
Computer users generally have more accommodative disorders than the rest of the population (Sheedy, 1990). Computer users with accommodative problems complain of blurred vision when looking at the screen. They sometimes have difficulty in shifting focus. The problems generally get worse later in the day and can continue after work. Blurred vision looking at distances after working hours also occurs.
Making Accommodation Easier
The first step in reducing the demands on accommodation is simply to do less of it. Since a closer screen requires more accommodation, moving the screen back will reduce the load on accommodation. The only practical limit on how far away the screen can be is the size of the letters: if we can comfortably interpret what we're looking at, the screen is not too far away. Fortunately many software programs allow users to change the font size. Editing can be done with a larger font and then changed before printing.
Another way to reduce visual stress is to take "vision" breaks, looking at something at a farther distance or closing our eyes. Interspersing computer work with other tasks such as filing can also help the eyes recover, not to mention the benefits to the musculo-skeletal system.
Lowering the monitor also reduces the demand on the accommodative system. When the eyes gaze downward, the near point of accommodation moves inward. Ripple (1952) found that subjects over age 42 increased their ability to accommodate by an average of 25.5 percent by directing their eyes downward in the "usual reading position."
Try this for yourself: Hold a business card at arm's length in front of your eyes. Bring it toward you until the letters just begin to blur. Without moving your head, gradually lower the business card in an arc, keeping it the same distance from your eyes. You should notice the letters getting sharper. Your eyes have improved their ability to accommodate simply by looking downward.
Convergence also contributes to eye strain. When we view close objects, our eyes converge, or turn inward towards the nose. That projects the image of the object to the same relative place on each retina. Without accurate convergence, we would see double images. The closer the object, the greater the strain on the muscles that converge the eyes (Collins, 1975).
Resting Point of Vergence
The eyes also have a resting point with regard to convergence. With nothing to look at, the eyes converge to a distance called the "resting point of vergence."
Jaschinski-Kruza (1988) found that convergence plays a larger role in eye strain at computer workstations than accommodation. He found that those subjects with "far" resting points of vergence experienced more eye strain working at a computer 20 inches away than those subjects with "close" resting points of vergence. But even those subjects with close resting points of vergence had less eye strain viewing the monitor at 40 inches.
As we might suspect, the resting point of vergence also changes with gaze angle (Heuer and Owens, 1989). Horizontally, that resting point averages about 45 inches. Looking upward 30-degrees, the resting point of vergence goes out to about 53 inches. But with a 30-degree downward gaze angle, it moves inward to 35 inches.
The obvious sign of too much stress on the convergence system is double vision. Other symptoms are headaches, irritated eyes and general fatigue.
Making Convergence Easier
Just as placing the monitor farther away reduces the demand on accommodation, it also reduces the demand on convergence. The farther away a visual target is, the less convergence is required.
In setting eye-to-screen distances, the limitation on distance is determined by the character height of the letters on the screen and the furniture layout. The principle is to use as little as possible of the distance inside the resting point of vergence. If an individual's resting point of vergence is 45 inches, it's extremely unlikely that a viewing distance of 44 inches would cause a problem -- or even 40 inches, for that matter. But, in general, the more the eyes must converge, the more stress on the vergence system.
Because the resting point of convergence moves inward with a downward gaze angle, lowering the monitor reduces the demand on the vergence system. A monitor would have to be 45 inches from the eyes at a horizontal gaze angle to reduce the effort of convergence to the same level it would be at 32 inches with a 40-degree downward gaze angle. As Krimsky (1948) noted, "When looking upwards, the eyes tend to diverge . . . when they look down, the effort to converge is much easier."
Depth of Focus
When the viewing distance changes, the eyes must refocus to maintain a sharp image. The depth of focus is that range of distances that do not require the eyes to refocus. For younger people, refocusing does not pose much of a problem. For people over 40 years of age, whose eyes have begun to lose the ability to focus at close distances, a larger depth of focus allows them to see clearly at distances closer than would otherwise be possible.
Depth of Focus Problems
The depth of focus can be a factor for the over-40 computer user who has to work with both a screen and documents. If the user has lenses set for one viewing distance, both the screen and document must be at approximately the same distance.
If the letters are large enough, both the screen and document can be placed at a far distance. Most of the time, however, the document has smaller characters than the screen. Moving the documents farther back could make the letters too small to read. But moving the screen closer increases the stresses on convergence and accommodation.
Depth of Focus Solutions
One obvious solution is to make the font sizes on the documents larger. Both screen and document can then be placed at a farther viewing distance. Usually, however, the user has no control over the hard copy.
Constricting the pupil increases the depth of focus, which can be demonstrated by squinting. Because the pupils automatically constrict with higher light levels, we can increase the depth of focus by increasing the luminance levels of the screen and documents. The monitor software can be switched to positive polarity (dark letters on a light background). Positive polarity has been found to increase performance and reduce errors.
Avoid increasing the overall room lighting because that can increase glare and veiling reflections on the screen. Strategically placed, variable- intensity task lighting can increase the brightness of the source documents without introducing glare. Using non-glossy, white copy paper instead of "greenbar" will improve contrast.
As the viewing distance decreases, depth of focus becomes more critical. We don't have to change focus when alternating between viewing objects at 100 feet and 200 feet, but we do have to change focus when the viewing distance changes from 30 inches to 15 inches. Placing both document and screen at farther viewing distances reduces the demand for a large depth of focus.
The Near Triad
Accommodation, convergence and depth of focus (known as the Near Triad) all work together. If the object is closer, the eyes will have to both converge and accommodate. When one occurs, the other will be stimulated. Since an increased depth of focus is most valuable for viewing at close distances, the pupils automatically constrict to increase the depth of focus.
In the visual environment. as we look down, the objects we're looking at are usually closer than when we are looking straight ahead. It's usually easy to find something at least 50 feet away at a horizontal or upward gaze angle. But, unless you live on the edge of the Grand Canyon or gaze out a window, it's unusual to look at far distances with a downward gaze angle. The visual system anticipates this, and downward gaze angles stimulate accommodation, convergence and pupillary constriction.
The thin layer of tears that covers the surface of our eyes dries out when exposed to the air. Blinking spreads a new tear layer. If that tear layer evaporates, the result can be dry eye syndrome. Sufferers of dry eye syndrome complain of burning, itchy eyes, eye irritation and "red eyes." That can be caused by a combination of infrequent blinking and dust particles or other irritants in the air.
A letter to the New England Journal of Medicine (Tsubota and Nakamori, 1993 ) reported a study on tear evaporation. Researchers found that tears evaporate at a faster rate when we gaze straight ahead at a monitor than when we look downward in a reading position. The authors also found that subjects blinked less and had an increased rate of tear evaporation when using a VDT than under relaxed conditions.
Their report confirms an unpublished study by Bruce Rupp (1987). Rupp found that when subjects looked downward at a 45-degree angle, they exposed over 40 percent less of the eyeball's surface than when they looked straight ahead.
Dry Eye Solutions
Some of the remedies for dry eye syndrome are eliminating airborne irritants (change the air filter in the HVAC system),. increasing the humidity, and applying a solution of artificial tears. Contact lens wearers may want to consider a pair of glasses for computer work.
Our eyelids partially close when we look downward. That reduces the surface of the eyeball exposed to the atmosphere and helps counteract the effects of reduced blinking. Because looking straight ahead or up at a monitor exposes more of our eye's surface than when we look downward, Tsubota and Nakamori suggest that computer users lower the monitor and tilt the screen upward.
The Vertical Horopter
An important, but little known characteristic of the visual system is the vertical horopter. It may not be well known, but it influences postural and visual comfort at VDTs. The word horopter, loosely translated, means the horizon of vision. The horopter is the locus, or grouping, of points in our field of view that appear to us as single images. Points that are anywhere else in space appear as double images.
Horizontally, the horopter is curved, with the sides coming closer to the observer. The vertical horopter, however, starts somewhere between the viewer's waist and feet. It projects outward, intersecting the point of fixation and continuing in a straight line. If an observer looks at the center of a vertical wire at a close distance in front of them, both ends of the wire will be seen as double in peripheral vision until the wire is tilted backward, with its top farther away from the observer.
The development of a backward-tilted vertical horopter corresponds to another characteristic of our visual environment: the orientation of surfaces. Look at something in the distance, or focus at a spot on the ground. What's below the spot you're looking at will usually be closer to you, and what's above that spot will usually be farther away.
Vertical Horopter Problems
If the top of the screen is closer to the eyes than the bottom of the screen, the result is a viewing condition that is opposite to the developed abilities of the eyes.
Ankrum, Hansen and Nemeth (in press) compared postural and visual discomfort at three monitor tilts allowed by ANSI HFS-100 (1988). They found greater increases in postural and visual discomfort when the top of the monitor was in the forward tilt position as compared to when the monitor was tilted back with its top farther from the eyes than the bottom. Subjects also preferred monitor tilts in which the top of the monitor was farther away from the eyes than the bottom. At lower gaze angles it is especially important for the monitor to be tipped backward.
Vertical Horopter Solutions
The obvious solution is to tilt the monitor backward. That can be a challenge. Although many monitors come with swivel bases, often they will not allow the monitor to tilt back far enough. Tilting back the momitor becomes increasingly difficult as the monitor is lowered to take advantage of the benefits to accommodation and convergence.
Another problem sometimes encountered when tipping the monitor back is glare from ceiling lights. Indirect lighting can often be combined with a positive polarity screen (dark letters on a light background) to reduce glare problems. A polarizing glare filter with an anti-reflective coating can reduce reflected glare, and supplementary task lighting can allow some of the ceiling lights to be turned off.
Each lighting problem is unique and may require a combination of solutions. Solving the Problem of VDT Reflections (Rea, 1991) provides excellent guidance for lighting the computerized office.
Preferred Gaze Angles
Studies of preferred gaze angles, measured from the horizontal with unspecified posture, have reported preferences varying from the -9 degrees at computer workstations reported by Grandjean (1983) to the -38 degrees for reading (Lehmann and Stier, 1961). Grandjean explained the difference between his findings and Lehmann and Stier's by saying that "VDT operators definitely prefer a smaller viewing angle." However, factors other than the difference between VDT viewing and reading could explain the difference in their findings. Grandjean did not control for glare in his study. In fact, one of the benefits found for the adjustable workstation (which included adjustable monitor tilt) was a lower incidence of "strong" annoying reflections compared to the old workstations. But, even at the preferred screen inclination, 12 percent of the subjects still reported "strong" annoying reflections. It is entirely possible that glare avoidance had a major influence on the monitor placement preferences found in Grandjean's study.
It would be inappropriate to consider the monitor adjustments and gaze angles observed in office environments as reflecting preferred settings because of the limitations of the equipment and lighting systems. In many offices, lowering and tilting the monitor back would result in unacceptable glare from ceiling lights.
Vision and Posture
Vision and posture interact. The location of the visual target often determines the range of postures we can comfortably assume. Sometimes we strike a balance between postural and visual discomfort. For instance, if a visual target is to our extreme left or right side, we may twist our neck's and trunk's into uncomfortable positions. In order to see clearly, we put up with postural discomfort. Generally, head movement occurs with horizontal eye excursions of greater than six to eight degrees (Fischer, 1925, cited in Von Noorden, 1985).
Vision and Posture Problems
Some VDT guidelines have recommended placing the top of the monitor at or slightly below eye level. The justifications include minimizing the postural load on the muscles of the neck and preventing the user from tilting the head too far back or forward.
In attempting to force users to adopt a natural and comfortable posture, those recommendations can actually produce the opposite results: Placing the monitor at eye level can limit our ability to comfortably change neck postures.
Motion is critical to postural comfort. If we are forced to maintain the same position for a long time, we feel uncomfortable. We want to encourage motion at computer work, but there is a difference between voluntary and task-required motion. We want to encourage voluntary motion and reduce task- required motion.
Eye level is often determined with the user "sitting tall." But is this the way we really sit? In normal, upright sitting (without a visual target), subjects studied by Hsiao and Keyserling (1991) tilted their head and neck an average of 13 degrees forward from the upright position. In fact, hardly anyone sits in the "upright" posture often depicted in some guidelines.
If the monitor is set to eye level as described in the preceding paragraph, the user is presented with a choice: either assume a more erect head/neck posture than preferred, or look up at the monitor. Hill and Kroemer (1986) found that when users in an upright seated posture were shown targets 50 and 100 cm away, they preferred to gaze an average of 40 degrees below the Ear- Eye Line. (The Ear-Eye Line is an imaginary reference line that passes through the center of the ear hole and the outer slit of the eyelid. When the head tilts forward, the Ear-Eye Line tilts with it.)
Alternative Neck Postures
When the head erect posture becomes tiring, users have few alternative neck postures from which to chose. One option is to tilt the head backward (extension). Because the Ear-Eye Line moves with the head, tilting the head back (without moving the monitor) slightly reduces the cost to the visual system. Bending the neck backward, however, greatly increases the cost to the postural system. In a study that simulated bifocal use, Kumar (1994) found that EMG activity in neck and back muscles increased as neck extension increased. He also found greater discomfort.
Another alternative posture for computer users with eye-level monitors is the forward head position. That is where the head remains erect while jutting forward from the trunk. Users sometimes assume a forward head posture in a counterproductive attempt to relieve muscle tension caused by contracted neck muscles (Mackinnon and Novak, 1994). Forward head postures have been associated with cervical headaches (Watson and Trott, 1993), increased fatigue (Urbanowicz, 1991) and thoracic outlet syndrome (Mackinnon and Novak, 1994). The increased postural discomfort far outweighs the low cost to the visual system.
The last alternative neck posture available with an erect trunk posture is flexion, or forward bending. Chaffin (1973) found that 15 degrees of sustained neck flexion for a long period (six hours with 10-minute breaks each hour) resulted in no elevated EMG readings or subjective reports of discomfort. Sustained neck tilts of 30 degrees and more, however, greatly increased neck extensor fatigue rates.
The cost to visual comfort when both flexing the neck and looking up at a monitor is high. Each degree of flexion requires one degree of elevation in the line of sight. Looking downward at close objects (relative to 11 degrees below the Ear-Eye Line) is more comfortable than looking at objects at an equal, but upward angle (Menozzi, 1992).
With an eye-level monitor and an upright backrest, computer users have been known to assume a variety of awkward postures when moving from an "upright," 90-degree posture. For example, they sometimes round their shoulders and upper back and slump forward (thoracic kyphosis). Looking up at the monitor from that posture requires extreme neck extension. It is even worse when a user leans forward to rest an elbow on the table and props chin in hand.
Lowering the monitor increases the range of neck postures that a computer user can assume while not compromising either visual or postural comfort. With a low monitor, the user can hold his or her head erect and direct the eyes downward. When that neck posture becomes uncomfortable, as eventually any neck posture will, the user can get momentary relief from discomfort by changing the degrees of neck flexion. This can be done at little or no cost to the visual system.
As we saw before, a downward gaze angle also benefits the visual system. As gaze angle tilts downward, convergence and accommodation become easier and the risk of dry eye syndrome is reduced. Also, with a downward gaze angle, reports of headaches (Tyrrell and Leibowitz, 1990), eye strain (Tyrrell and Leibowitz, 1990) and fatigue (Owens and Wolf-Kelly, 1987) decrease.
Implications for Monitor Placement
Near work can stress the visual system. The definition of near work is ambiguous, but it might be defined as work performed closer than the resting point of vergence. One way to reduce the visual stress of computer work is to decrease the vergence effort. That can be accomplished by increasing eye to screen distance, lowering the monitor, or a combination of the two.
If we look at the 13-degree average forward head tilt found by Hsiao and Keyserling, locating the monitor at least 15-degrees below the horizontal eye level would take advantage of the benefits to the visual system and at the same time allow a wide range of comfortable head/neck postures.
When lowering the monitor, the top of the screen should not be closer to the eyes than the bottom (Ankrum, et al., 1995). At the same time, glare and reflections must be controlled or the benefits of a lower gaze angle will be lost.
Some methods of ergonomic evaluation employ checklists in an attempt to ensure good ergonomics. While checklists have their place, they can backfire. For instance, most people would think that if you correct a workstation so that nine out of ten criteria on a check list are met, then that's better than eight out of ten.
As an example, we have discussed how lowering the monitor can improve focusing, reduce the stress on convergence and avoid dry eye syndrome and constrained neck postures. We have also indicated that the monitor should be tipped back so that its top is farther from the eyes than its bottom. We have also stressed the need to satisfactorily address glare and reflections.
We now have three criteria for our checklist. However, if only two out of the three are satisfied, the result could actually be a worse workstation than the one we started with. If the monitor is lowered and glare is eliminated, but the monitor is tipped forward, the result could be increased neck discomfort. If the monitor is lowered and tipped backward without addressing glare, increased visual discomfort can result. The old adage that "Two out of three ain't bad," doesn't hold true.
Symptoms such as blurred or double vision and irritated eyes may indicate problems other than an overtaxed visual system. It is always safest to consult and optometrist or ophthalmologist at the first sign of vision disorders.
VDT work is primarily a visual task. Even keying depends on visual stimuli, either for source material or to confirm accuracy. An understanding of visual principles is critical to the success of any VDT ergonomics program.
Ankrum, D.R., Hansen, E.E., and Nemeth, K.J. (in press). The vertical horopter and the angle of view. Work With Display Units '94, eds. Grieco, A., Molteni, G., Occhipinti, E. and Piccoli, B., Amsterdam: Elsevier
American National Standard for Human Factors National Standard for Human Factors Engineering of Visual Display Terminal Workstations. (1988). Santa Monica: Human Factors Society
Boorish, I.M. (1970). Clinical Refraction. Chicago: The Professional Press.
Chaffin, D.B. (1973). Localized Muscle Fatigue - Definition and Measurement.
Journal of Occupational Medicine, 15, 4, 346-354.
Collins, C., O'Meara, D., and Scott, A.B., (1975). Muscle strain during unrestrained human eye movements. Journal of Physiology, London, 245, 351-369.
Ehrlich, D.L. (1987). Near Vision Stress: Vergence Adaption and Accommodative Fatigue. Ophthalmology & Physiological Optics, 7, 4, p. 353-357.
Fischer, F.P. (1925). Uber die Verwendung von Kopf bewegungen beim Umherschen. I and II. Archives of Ophthalmology, 115, 49.
Fisher, R.F. (1977). The force of contraction of the human ciliary muscle during accommodation. Journal of Physiology, London, 270, 51-74.
Grandjean, E. (1987). Ergonomics in Computerized Offices. London: Taylor & Francis.
Grandjean, E., Hunting, W. and Pidermann, M. (1983). VDT workstation design: preferred settings and their effects. Human Factors, 25, 161-175.
Heuer, H., Owens, D. (1989). Vertical gaze direction and the resting posture of the eyes. Perception, 18, 363-377.
Hill, S.G., Kroemer, K.H.E. (1986). Preferred Declination of the Line of Sight. Human Factors, 28, 2, 127-134.
Hsiao, H. and Keyserling, W.M. (1991). Evaluating posture behavior during seated tasks. International Journal of Industrial Ergonomics. 8, 313-334.
Jaschinski-Kruza, W. (1988). Visual strain during VDU work: the effect of viewing distance and dark focus. Ergonomics, 31, 10, 1449-1465.
Kaufman, P.L. (1992). Accommodation and Presbyopia: Neuromuscular and Biophysical Aspects. Adler's Physiology of the Eye. St. Louis: C.V. Mosby.
Krimsky, E. (1948). The Management of Binocular Imbalance. Philadelphia: Lea and Febiger.
Krueger, H. (1984). Visual Functions in Office- Including VDUs (Introductory paper) Ergonomics and Health in Modern Offices. London: Taylor & Francis.
Kumar, S. (1994). A computer desk for bifocal lens wearers, with special emphasis on selected telecommunication tasks. Ergonomics, 37, 1669-1678.
Lehmann, G., and Stier, F. (1961). Mensch und Geraet (Human and equipment). In Handbuch der gesamten Arbeitsmedizn (Vol.1) p. 718-788. Berlin: Urban und Schwarzenberg.
Mackinnon, S.E., Novak, C.B., (1994). Clinical commentary: Pathogenesis of Cumulative Trauma Disorder. Journal of Hand Surgery. 19A, 5, 873-883.
Menozzi, M., Buol, A.v., Krueger, H., Miege, Ch. and Pedrono, C. (1992). Fitting Varifocal Lenses: Strain as a Function of the Orientation of the Eyes in Ophthalmic and Visual Optics, Technical Digest Series (Vol. 3). p134-137. Optical Society of America: Washington D.C.
Owens, D.A. (1984). The resting state of the eyes. American Scientist, 72, 378-387.
Owens, D.A., Wolf-Kelly, K. (1987). Near Work, Visual Fatigue, and Variations of Oculomotor Tonus. Investigative Ophthalmology and Visual Science. 28, 743-749.
Rea, M.S., 1991, Solving the Problem of VDT Reflections. Progressive Architecture, Oct. p.35-40.
Ripple, P. (1952). Variation of Accommodation in Vertical Directions of Gaze. American Journal of Ophthalmology, 35, 1630-1634.
Rupp, B.A., (1987). Personal communication.
Sheedy, J.E. (1990). Video Display Terminals: Solving the Vision Problems. Problems in Optometry. 2,1, 1-16.
Tsubota, K., Nakamori, K. (1993). Dry Eyes And Video Display Terminals. New England Journal of Medicine, 328, 8, 584.
Tyrrell, R., Leibowitz, H. (1990). The Relation of Vergence Effort to Reports of Visual Fatigue Following Prolonged Near Work. Human Factors, 32, 3, 341-357.
Urbanowicz, M. (1991). Alteration of Vertical Dimension and Its Effect on Head and Neck Posture. Journal of Craniomandibular Practice. 9, 174-179.
Von Noorden, G.K. (1985). Binocular vision and ocular motility. St. Louis, MO: C.V. Mosby.
Watson, D.H. and Trott, P.H. (1993). Cervical headache: an investigation of natural head posture and upper cervical flexor muscle performance