Monocular infers that the individual has the use of one eye only. Although many individuals are truly monocular due to the loss of one eye due to trauma or disease, most so called monocular individuals still have some vision in the weaker eye. As early as 1938 Livingston distinguished between the monocular individual (two eyes present, with loss of central vision in one eye) and the uniocular individual (one eye present) (Wright et al., 2013). Typically uniocular/monocular, or individuals with binocular visual problems will normally not be considered for any aspects of aviation training. However, it is the individuals with acquired monocular vision which cause concern. Invariably these individuals’ monocular vision arise later in life due to injury and or pathology and clinicians are left with difficult decisions regarding their fitness to fly, work as air traffic controllers, or in other aspects of the aviation industry.
Binocular vision is seeing with two eyes simultaneously. Normal binocular vision implies fusion or the blending of sight from the two eyes to form a single percept and implies the most acute kind of depth perception or stereopsis, based on the horizontal separation of the two eyes in the skull (normally 65mm). An individual with stereopsis is able to construct a three dimensional percept of two two-dimensional retinal images, in other words a percept of “solid” depth (Hart, 1992). Other benefits of binocular vision is larger visual fields – approximately 30° larger than the monocular field (Griffin et al., 2010), better visual acuity – approximately one line better on the Snellen chart (Griffin et al., 2010), better contrast sensitivity – approximately 40% better than monocular contrast sensitivity (Griffin et al., 2010), and obviously the fact that if one does lose one eye, there will always be a spare (Griffin et al., 2010). Individuals with binocular vision also fare better at certain tasks such as card filing, needle threading and speed of word decoding, indicating that binocular vision and stereopsis provides a performance advantage for many different vocations. This hold true, particularly for those who require near-point eye-hand coordination such as surgeons, machinists, pilots, and cartographers (Sheedy et al., 1986). Furthermore, motor and fine motor control (motor movements were faster and more accurate) is improved under binocular conditions as opposed to monocular conditions (Jackson et al., 1997).
Although binocular vision is required for stereopsis and therefore depth perception, depth perception is a summation of both monocular and binocular depth cues. Monocular depth cues include; apparent size – real objects do not change in size and therefore small retinal sizes are interpreted as distant objects and vice versa, interposition – relatively nearer objects tend to conceal or overlay more distant objects, aerial perspective – water vapour, dust and smoke in the atmosphere scatter light and make distant objects indistinct and relatively colour desaturated, shading – light falling on solid objects causes shadows and graduation of the intensity of the shadow on curved surfaces. The brighter of the two objects are interpreted as nearer, geometric perspective – parallel lines converge toward the vanishing point at the horizon, relative velocity – image velocity of a moving target in the distance is lower than the image velocity of the same target when it is nearby, and finally motion parallax – translations of the head cause images of near objects to move opposite to the head movement and vice versa (Hart, 1992). Artists, video game developers, and cinematographers masterfully use these monocular clues to create an illusion of three-dimensionality, and it is these cues that enable one-eyed individuals to perform tasks such as surgery and flying, that are believed to require precise degrees of stereopsis (Hart, 1992).
Monocular cues are psychological in nature and are learned skills based on life experiences. They can therefore be improved with training, but they are also prone to illusion (Cibis, 1952, Wright et al., 2013). In contrast, stereopsis is a binocular physiological cue that results from simultaneous fusion of two disparate retinal images resulting is a “solid” perception of three-dimensions. It develops early in life and is immune to misinterpretation (Braddick et al., 1980, Fox et al., 1980, Held et al., 1980). To summarise: monocular cues provide indirect information on depth that may be influenced by environmental or intrinsic factors, while stereopsis is a direct measurement that is not prone to illusion (Wright et al., 2013).
It is important to consider the range of distances that provide useful stereoscopic information and how they relate to vocations that require stereopsis. Some authors believe that it may be a short as 10 – 20 feet (Gregory, 1997, Nagata, 1991), while others estimate that stereopsis is useful up to 1200 meters (Hirsch and Weymouth, 1947). However, the consensus suggest that stereopsis is useful up to around 500 meters (Wright et al., 2013). This may explain why experienced pilots can and do cope well after recovery from an event that resulted in monocular vision (pilots without stereopsis). It is interesting to note that although no differences were reported in landing accuracy between monocular and binocular pilots, monocular approaches were flown higher and steeper and importantly rated as more difficult and cognitively demanding by the pilots (Wright et al., 2013). Speed and judgement when taxiing, wingtip and rotor blade clearances when manoeuvring in a confined space, and general cockpit tasks such as reading a map and selecting radio frequencies can also be adversely affected in monocular/uniocular aviators (Wright et al., 2013).
Currently the definition contained in the SACAA visual standards document defines monocularity as either an eye that is absent (true uniocular individual) or its visual acuity cannot be corrected to better than 6/24. No mention is made of the visual fields or stereopsis. There is also a definition for substandard visual acuity in one eye while the other eye meets the required standards for a particular class of licence. Class 1 requires corrected central visual acuity better than 6/24 but worse than 6/9 with normal visual fields. Class 2 requires corrected central visual acuity better than 6/24 but worse than 6/18 with normal visual fields, and Class 3 requires corrected central visual acuity better than 6/24 but worse than 6/12 with normal visual fields. No mention is made of stereopsis. This “substandard visual acuity in one eye” group is probably better classified as individuals with defective or reduced stereopsis exhibiting monofixation syndrome. Monofixation syndrome is characterised by a central suppression scotoma, which may be constant or intermittent, with peripheral fusion (Parks, 1969, Epstein and Tredici, 1973). Currently there is disagreement between international aviation governing bodies with regard to what level of stereopsis is required to be considered “fit to fly” or to work in the air traffic control environment. Interestingly the US Army and Navy require stereopsis of 40”, the US Airforce 25”, the FAA, SACAA has no standards, and the Royal Airforce standard is 120“.
Monovision, a situation where one eye is corrected for distance vision and the other for near vision usually with contact lenses or surgery, has a significant effect on stereopsis and therefore the individual has to rely on monocular depth cues which, as previously discussed, can be influenced by environmental and intrinsic factors causing illusions (Evans, 2007). Aviators and air traffic controllers as well as other individuals involved in aviation should not use this modality to correct their refractive errors and presbyopia.
It is not impossible for uniocular, monocular individuals, or individuals with defective stereopsis to work in the aviation industry. However, each of these individuals needs to be assessed on a case to case basis in order to find the cause of the visual problem, to rule out active pathology and diplopia (double vision), to assess the visual fields, as well as the individual’s capability to cope with the “monocularity” in his/her work environment. Air traffic controllers (ATC’s) work mainly in a near distance environment with complex visual presentations requiring good hand-eye coordination, fine motor skills, contrast sensitivity, visual acuity and visual fields. These skills are all enhanced by and depend on stereopsis requiring binocular vision. With the advent of augmented reality displays and virtual environments for ATC (air traffic control), good binocular vision and stereopsis is essential for ATC, probably more so than for pilots (Ellis, 2006). It is therefore suggested that individuals with central visual acuity below the stated visual standards may be considered fit for ATC duties providing that binocular visual fields are within normal limits and the underlying pathology is acceptable/inactive according to ophthalmic assessment by an accredited Eye Care Practitioner skilled in the evaluation of binocular vision problems.
Screening recommendations for optometrists dealing with aviators
Class 1 – 3 medicals to pass a stereopsis test – Stereo Fly or Randot – with best spectacle correction in place for the test distance. Minimum requirement of 40“ of arc (Class 4 medical standards can be less stringent with lower stereopsis requirements). Alternately a Worth-Four-Dot-Test can be used to determine binocularity. Subjects wear red/green goggles while a target consisting of four lights, red at the top, white at the bottom and green on each side is presented at a minimum distance of 6 meters. Pass is identifying four lights, one red, two green and one white. If one eye is supressed the individual will either see three green lights or two red lights (Griffin et al., 2010). Individuals failing one or both of these tests should undergo a comprehensive eye evaluation to determine the cause of the loss of depth perception. The evaluation must include a binocular vision work-up and threshold visual field examination of both eyes. After the comprehensive visual evaluation pilots and ATC’s will also undergo a practical examination convened by SACAA, conducted by suitably qualified individuals, in their respective work environment to ensure that the monocularity does not affect their ability to perform critical tasks. The final decision whether the individual is “fit to fly” or work as an ATC remains with the SACAA department of aviation medicine.
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