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Assessment Of Stereopsis Among Commercial Drivers In Owerri Municipality




This research work determined the stereoscopic vision of commercial drivers. It was a descriptive  study carried out by assessing  and testing four hundred  drivers from several commercial vehicle parks in Owerri Municipality with their different  age range. With the aid of age distribution table, the age group with the highest number of subjects with positive stereopsis result with respect to age showed that out of 400 subject responded with positive presence of stereopsis representing 91.5%. While 4 subjects representing 8.5% responded negatively to presence of stereopsis. It was found that drivers in age groups 25-33yrs, 34-42yrs and 43-51yrs recorded presence of stereopsis. While 4 subjects  representing  8.5% showed  negative result for stereopsis.



1.1 Background to the Study

Stereopsis from the Greek word OTEPEO- stere-o- meaning “solid”, and OPISIS, “ Appearance, sight” is a term  that is most often used to refer to the perception of depth and 3 dimensional structure obtained on the basis of visual information derived from the two eyes by individuals with normally developed binocular vision. Stereopsis was first explained by Charles Wheatstone in 1838.

The mind perceives an object of three dimensions by means of the two dissimilar pictures projected by it on the two retinas. He recognized that because each eyes image differs from the other, objects at different distances from the eyes project images in the two eyes that differ in their horizontal disparity also known as retinal disparity and as binocular disparity. Wheatstone (2008) showed that this was an effective depth cue by creating the illusion of depth from flat pictures that deferred only in horizontal disparity. To display his pictures separately to the two eyes, Wheatstone invented the stereoscope which is a device by which each eye can be presented with different images allowing stereopsis to be stimulated with two pictures, one for each eye.

According to Panum et al. (1960) research into stereopsis was dedicated to exploring its limits and its relationship to singleness of vision.

Puerta (1989), demonstrated with photographs that retinal images with no parallel disparity but with different shadows are fused stereoscopically, impacting depth perception to the image seen. He named the phenomenon “shadow stereopsis“. Shadows are therefore an important stereoscopic cue for depth perception. He showed how effective the phenomenon is by taking two photographs of the moon at different times, and therefore with different shadows making the moon to appear in 3 dimensions stereoscopically, despite the absence of any other stereoscopic cue.

The human eyes are located at different lateral positions of the head. Binocular vision results in two slightly different images projected to the relative horizontal positions of objects in the two images. These positional differences are referred to as horizontal disparities or more generally, binocular disparities which are proposed in the visual cortex of the brain to yield Depth Perception. While binocular disparities are naturally present when viewing a real 3 dimensional scene with eyes, they can also be stimulated by artificially presenting two different images separately to each eye using a method called Stereoscopy which is a technique for creating or enhancing the illusion of depth of an image by means of stereopsis.

1.1.1 Geometrical basis

Stereopsis appears to be processed in the visual cortex of mammals in binocular cells having receptive fields in different horizontal positions in the two eyes such as a cell is only active when its preferred stimulus is in the correct position in the left eye and in the correct position in the right eye, making it a disparity detector.

When a person stares at an object, the two eyes converge so that the object appears at the center of the retina in both eyes; other objects around the main object appear shifted in relation to the main object. Because each eye is in a different horizontal position, each has a slightly different retinal image, normally two images are not observed, but rather a single view of the scene, a phenomenon known as singleness of vision, nevertheless, stereopsis is possible with double vision. The form of stereopsis was called qualitative stereopsis (Ogle, 1950).

If the images are very different (such as by going crossed eyed or by presenting different images in a stereoscope) then one image at a time may be seen as a phenomenon known as binocular rivalry. There is a hysteresis effect associated with stereopsis. Once fusion and stereopsis have stabilized, fusion and stereopsis can be maintained even if the two images are pulled apart slowly and symmetrically to a certain extent in the horizontal direction. In the vertical direction, there is a similar but smaller effect.

According to Derek Fender et al. (1967), Journal of the optical society of American. This effect first demonstrated on a random dot stereogram was initially interpreted as an extensive of panum’s fusional area, and that stereoscopic depth can be perceived in random- line stereograms despite the presence of cyclo disparities of about 15 degree and this has been interpreted as stereopsis with diplopia.

According to Horace et al. (1960), they found neurons in the cat visual cortex that had their receptive fields in different horizontal positions in the two eyes; this established the rural basis of stereospis. Their findings were disputed by David Hubel and Torsten Wiesel, although they eventually conceded when they found similar neurons in  monkeys brains that responded to the depth of random- dot stereogram’s .

A stereoscope is a device by which each eye can be presented with different images allowing stereopsis to be stimulated with two pictures, one for each eye.  This has led to various crazies for stereopsis, usually prompted by new sorts of stereoscopes. In Victorian time, it was the prism stereoscope (allowing stereo photograph to be viewed), while in the 1920s it was red-green glasses, (allowing stereo movies to be viewed). In 1939 the concept of the prism stereoscope was reworked into the technologically more complex view- master, which remains in production today. In the 1950s, it was the polarizing glasses allowing stereopsis of coloured movies to be viewed. In 1990s magic Eye pictures (auto-stereogram) which did not require a stereoscope but relied on viewers using a form of free fusion so that each eye views using a form of free fusion so that each eye views different images introduced. 

1.2 Types of Stereoscope

1.2.1 Wheatstone Stereoscope

1)       The first or earliest type of stereoscope was invented by Sir Charles Wheatstone in 1838. It used a pair of mirrors at 45 degree angles to the user’s eyes, each reflecting a picture located off to the side. It demonstrated the importance of binocular depth perception by showing that when two pictures stimulating left eye & right eye views of the same object are presented, so that each eye sees only the image designed for it, but apparently in the same location, the brain will fuse the two and accept them as a view of one solid three dimensional objects.

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Wheatstone stereoscope was introduced in the year before the first practical photographic process became available. So drawings were used. This type of stereoscope has the advantage that the two pictures can be very large if desired.

1.2.2 Brewster Stereoscope

Brewster’s personal contribution was the suggestion to use Lenses for uniting the dissimilar pictures in 1849 and accordingly the Lenticular stereoscope (lens based) may fairly be said to be his invention.

This allowed a reduction in size, creating hand-held devices, which was admired by Queen Victoria when they were demonstrated at the Great Exhibition of 1851.

1.2.3. Holmes Stereoscope

In 1861, Oliver Wendell Holmes created and deliberately did not patent a handled streamlined, much more economical view than had seen available before the stereoscope which dates from the 1950s. It consisted of two prismatic lenses and a wooden stand to hold the stereo card. This type of stereoscope remained in production for a century and there are still companies making them in limited production currently.

1.2.4. Lens or Prism Stereoscope

In this type of stereoscope two lenses or narrow angled prisms are used. In mirror stereoscope, mirrors are used; it allows greater area of the photograph to be covered.

A simple stereoscope is limited in the size of the image that may be used. A more complex stereoscope uses a pair of horizontal periscope like devices, allowing the use of larger mages that can present more detailed information in a wider field of view. The stereoscope is essentially an instrument in which two photographs’ of the same object, taken from slightly different angles are simultaneously presented one to each eye. This recreates the way which in natural vision,  each eye is seeing the object from a slightly different angle, since they are separated by several inches which is what gives humans natural depth perception. Each picture is focused by a separate lens and by showing each eye a photograph taken several inches apart from each other and focused in the same point, it recreates the natural effect of seeing things in three dimensions.

A stereoscope offers several advantages

  1. Using positive curvature (magnifying) lenses; the focus point of the image is changed from its short distance (about 30 to 40cm) to a virtual distance at infinity. This allows the focus of the eyes to be consistent with the parallel lines of sight greatly reducing eye strain.
  2. The card image is magnified; this offers a wider field of view and the ability to examine the detail of the photograph.
  3. The viewer provides a partition between the images, avoiding a potential distraction to the user.

A stereo transparency viewer is a type of stereoscope that offers similar advantages, e.g. the viewer- master.

Disadvantages of stereo cards, slides or any other hard copy or print are that, the two images are likely to receive differing wear, scratches and other decay. This results in stereo artifacts when the images are viewed. These artifacts compete in the mind resulting in a distraction from the 3 dimension effect, eye strain and headaches.

1.3. Types of Stereopsis

  1. Coarse stereopsis and
  2. Fine stereopsis
  3. Static depth perception

They provide depth information of different degrees of spatial and temporal precision

  • Coarse Stereopsis (also called gross stereopsis) appears to be used to judge stereoscopic motion in the periphery. It provides the sense of being immersed in one’s surrounding and is therefore sometimes also referred to as qualitative stereopsis. Coarse stereopsis is important for orientation in space while moving for example when descending a flight of stairs.
  • Fine Stereopsis: This is mainly based on static differences; it allows the individual to determine the depth of objects in the central visual area (panum’s fusional area) and is therefore also called quantitative stereopsis. It is typically measured in random-dot tests, persons having coarse but no fine stereopsis are often unable to perform on random-dot tests, also due to visual crowding which is based on interaction effects from adjacent visual contours. Fine stereopsis is important for fine-motor tasks such as threading a needle.

Fine stereopsis requires both eyes to have a good visual acuity in order to detect small spatial differences and is easily disrupted by early visual deprivation. There are indications that in the course of the development of the visual system in infants, coarse stereopsis may develop before fine stereopsis and that coarse stereopsis guides the vergence movement which are needed in order for fine stereopsis to develop in a subsequent stage. Furthermore, there are indications that coarse stereopsis is the mechanism that keeps the two eyes aligned after strabismus surgery.

  • Static depth perception (or static stereo motion perception), some individuals who have strabismus and show no depth perception using static stereo-tests  in particular, using litmus tests, do perceive  motion in depth when tested using dynamic random dot stereo-grams to measure stereopsis in strabismic patients. A study by Meier   et al (2014), one study according to the American orthotic journal, found the combination of motion stereopsis and no static stereopsis to be present only in exotropes not in esotropes.
  • Prevalence And Impact Of Stereopsis In Humans

Not everyone has the same ability to see using stereopsis. A study shows that 97.3% are able to distinguish at horizontal   disparities of 2:3 minutes of arc or smaller and at least 80% could distinguish depth at horizontal differences of 30 seconds of arc.

According to Coutant et al. (1993), stereopsis has a positive impact on exercising practical tasks such as needle- threading, ball- catching (especially in fast ball games), pouring liquids, and other professional activities may involve operating stereoscopic instruments, such as a binocular microscope. While some of these tasks may profit from compensation of the visual system, by means of other depth cues, there are some roles for which stereopsis is imperative. Occupations requiring the precise judgment of distance sometimes include; a requirement to demonstrate some level of stereopsis; In particular, there is such as requirement for airplane pilots.

According to Biddie et al. (2014), surgeons normally demonstrate high stereo acuity, as to car driving, a study found positive impact of stereopsis in specific situation at intermediate distances only.

A study on elderly persons found that glare, visual field loss and useful field of view were significant predictors of crash involvement, where as the elderly persons values of visual activity, contrast sensitive and stereo-acuity were not associated with crashes. Binocular vision has further advantages aside from stereopsis, in particular the enhancement of vision quality through binocular summation; (even those who have no double vision); have lower scores of binocular summation, and it appears to incite persons with strabismus. It has long been recognized that full binocular vision, including stereopsis is an important factor in the stabilization of post- surgical outcome of strabismus corrections. Many persons lacking stereopsis have (or have had); visible strabismus which is known to have a potential socio-economic impact on children and adults. In particular, both larger angle and small angle strabismus can negatively affect self-esteem as it interfered with normal eye contact often causing embarrassment, anger and feelings of awkwardness.

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It has been noted that with the growing introduction of 3D display technology in entertainment and in medical and scientific imaging, high quality binocular vision including stereopsis may become a key capability for success in modern society.

Nonetheless, there are indications that the lack of stereo vision may lead persons to compensate by other means in particular, stereo- blindness may give people on advantage when depicting a scene using monocular depth cues of all kinds, and among artists there appear to be a disproportionately high number of persons lacking stereopsis.

1.5.Computer Stereo Vision

Computer stereo vision is a part of the field of computer vision. It is sometimes used in mobile robotics to detect obstacles, two cameras take pictures of the same scene, but they are separated by a distance exactly like our eyes. A computer compares the image, while shifting the two images together over top of each other, to find the parts that match the shifted amount. This is called the disparity at which objects in the image best match is used by the computer to calculate their distance.

For a human, the eyes change their angle according to the distance from the observed object. To a computer, this represents significant extra complexity in the geometrical calculations. In fact the simplest geometrical case is when the camera image planes are on the same image plane. This is called image rectification. Computer stereo vision with many cameras under fixed lighting is called structure from motion. Techniques using a fixed camera and known lighting are called photometric stereo techniques or “shape from shading”.

  • Test For Stereopsis

In stereopsis test, slightly different images are shown to each eye, such that a 3 Dimensional image is perceived in case stereo-vision is present. This can be achieved by means of vectographs (visible polarized glasses); Analglyphs visible with–red-green lenticular glasses. The type of changes from one eye to the other may differ depending on which level of stereo-acuity is to be detected.

There are two types of common clinical tests for stereopsis and stereoacuity: Random stereotest and contour stereotests.

  • Random Dots Stereotests

The ability of stereopsis can be tested by, for example, the long stereotest which consists of a random-dot stereogram upon which a series of parallel ships of cylindrical lenses are imprinted in certain shapes, which separate the news seen by each eye in these areas, similar to a hologram. Without stereopsis, the image looks like a field of random dots, but the shapes become discernible with increasing stereopsis and generally consist of a cat (indicating that there is ability of stereopsis of 1200 seconds of arc of retrial disparity). A star (600 seconds of arc) and a car (550 seconds of arc). To standardize the results; the image should be received at a distance from the eye of 40cm and exactly in the front Parallel plane. There is no need to use special spectacles for such tests thereby facilitating use in young children.

  • Contour Stereotests

Examples of the contour stereotests are the Titmus stereotests, the most well known example being the Titmus fly stereotest where a picture of a fly is displayed with disparities on the edges. The patient uses a 3-D glasses to look at the pictures and determine whether 3-D image varies such as 400-100 seconds of arc and 800-40 seconds of arc. Other test includes;

  • The two pencil test: The test is a method of comparing binocular and monocular stereopsis, it furnishes information on vision as used in everyday life. Good binocular performance is sometimes still possible in cases of monocular impairment of vision such as cataract or maculopathy, as well as harmonious anomalous retinal correspondence, even though more sophisticated stereo-tests may be negative.

Procedure- The patient places a vertical pencil on top of pencil held by the examiner. Repeated with one eye occluded (deviating eye), if the binocular single vision performance is better, stereopsis is present but if monocular and binocular single vision are of similar performance, stereopsis is absent. The Performances improve upon repetition.

  • The Titmus stereotest: It is a commonly used test for stereopsis. The test consists of a booklet containing polarized test stimuli for use at a 40cm testing distance and requires the use of cross polarized lenses. The test includes a stereofly, which serves as a gross test for stereopsis, producing approximately 3000 seconds (almost 1 degree) of retinal disparity at the 40cm testing distance.

The stereo-fly is a particularly good test of stereopsis for preschool children.

According to Park (1979), patients with central fusion have 40 seconds of stereopsis or better at near, whereas patients with peripheral fusion but lacking central fusion have stereo-acuity ranging from 3000 to 60 seconds. Averaging 200 seconds, because stereopsis requires good visual acuity, the patient’s corrective lenses should be worn along with the Polaroid filters.

  • The Bernell-Stereo Reinder Test: Like the Titmus stereotest is a polarized test providing stimuli for both gross and fine stereopsis testing. The test has the advantage that the examiner can make the reinders nose wiggle by rotating the picture slightly.
  • The American optical vectographic project-o- chart slide, this test provides a convenient method of testing stereopsis at 6m. This slide has five rows of circles for each row of circles, the patient is asked to report which circle appears to be closer than the others in the row. The retinal disparity stimulated by this test ranges from 240 seconds for the top row to 30 seconds for the bottom row.
  • The frisby stereotests: They are simple and effective screener and assessment test of stereopsis vision. It has three transparent plates, one of those shown on each presentation to the subject.

The “target” that the patient has to detect is printed on one side of each plate. This target is a cluster of randomly arranged arrow heads of different sizes, on other side of each plate is printed a background of singular texture. An observer with normal binocular stereo vision can readily detect the target because it appears to stand out from the background. Such an observer is essentially seeing the thickness of the plate by virtue of the texture element printed on the two sides.

For an observer lacking normal binocular stereo vision, to be able to detect the target as it can be distinguished. Only on the basics of binocular disparity cues to depth, the test box has a fold down flap that provides a white background against which the plate can be viewed. The three plates are of different thickness 6mm, 3mm & 1.5mm. Thus, they present different sizes of disparity cues, thereby enabling the stereoacuity of the observer to be measured. The plate can be presented at different distance to further vary the disparity cues.

  • The ramdot Stereotest: This is aimed for adults stereotesting. Adult patients are asked to identify six geometric shapes during testing. This test helps to test a patients depth perception along with normal stereo vision. Adult testing involves 400-20secs of arc and pediatric testing involves 400-100 seconds of arc. It is frequently used for detecting ambyopia, strabismus and suppression and for assessing monocular blur than real depth stereotests such as the frisby test.
  • The Lang Stereotest: It is based on two principles, random dots and cylindrical graphing. Random dots are currently used in stereograms. In the cylindrical graphing, the images of the two eyes are separated by a system parallel cylindrical strip of pictures, one seen by the right eye and the other seen by the left eye.
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1.6.9.. Synoptophore: for gross qualitative stereopsis (yellow slides) assessment can be obtained using two images of the same object, hypothetically taken from slightly different angles, to indicate depth perception. The slides are inserted into the slide holders with the controls of each slide positioned towards or away from the subject. The patient is then required to describe the apparent effect i.e. a swing moving towards or away from them. The test should be repeated, with the slide controls changing position, to ensure the stereoscopic effect is authentic and the patient is not guessing.     SLIDE PROJECTOR

All grades of Binocular Single Vision (BSV) can be assessed with the Synoptophore including stereopsis.

  • Deficiency Of Stereopsis

Deficiency in stereopsis can be complete (then called stereo-blindness) or more or less impaired. Causes include blindness in one eye, amblyopia and strabismus. Stereo-blindness is the inability to see in 3 dimension using stereopsis or stereo vision resulting in an inability to perceive stereoscopic depth by combining and comparing images from the two eyes.

Individuals with only one functioning eye always have this condition, the condition also results when two eyes do not function together properly. Stereoblind persons with two healthy eyes do employ binocular vision to some extent, albeit less than persons with normally developed eye sight. This was shown in a study in which stereo-blind subjects were posed with the task of judging the direction of rotation of a stimulated transparent cylinder, the subjects performed better when using two eyes than when using their preferred eye. They appeared to judge the direction of rotation from the images in each eye separately and then to combine these judgments, rather than relying on differences between the images in the two eyes Christa M, (2011). In some cases each eye can contribute to peripheral vision for one side of the field of view Jeremy, (1980).

Scientist has suggested that more artists seem to have stereo-blindness when compared with a sample of people with stereo – acuteness (normal stereo vision). British neurologist Oliver Sacks lost his stereoscopic vision in 2001due to a malignant tumor in his right eye and had no remaining vision in that eye. His loss of stereo vision was recounted in his book “the mind’s eye” published in October 2019.

1.8. Treatment Of Stereopsis

Vision therapy is one of the treatments for stereopsis. It is used to improve vision skills such as eye movement control, eye coordination, contrast sensitivity and perception and the training of stereopsis vision training involves a sense of procedures carried out in both home and office settings usually under  professional supervision by an optometrist. According to the Canadian association of optometrists in “vision therapy”, vision therapy will allow individuals to enhance their vision through several exercises such as by strengthening and improving eye movement. There is recent evidence that stereo-acuity maybe improved in persons with amblyopia by means of perceptual learning  which is the repetitive practice of a demanding visual task in human adults living  deprived of normal binocular vision by the use of a training paradigm  that combines monocular cues that were corrected perfectly with the disparity cues. Following perceptive learning (thousands of trials) with stereopsis, both on psychophysical tests with stimuli that contained no monocular cues and on clinical testing.

Driving requires several sets of abilities, which include sensory ability (mainly visual), mental ability, motor ability, and compensatory abilities. Though 95% of the sensory requirement for driving have been shown to be visual Taylor H, (1982). For this reason, drivers with good stereoscopic vision have advantage over those with poor stereoscopic vision.  Stereopsis is important for driving Omolase et al. It is needed to accurately judge distances, overtake vehicles and change lane which could lead to the occurrence of road traffic accidents; especially in busy traffic. This pre supposes that drivers with defective stereopsis may have difficulty judging distances in their quest to overtake other vehicles and change lanes which could lead to the occurrence of road traffic accident (RTAs).

The relevance of stereoscopic vision and depth perception parameters among commercial drivers cannot be underestimated, there is need to investigate, assess the relationship between stereopsis, age and duration of commercial drivers and provide early intervention to avoid continuous consequences of abnormal stereopsis among commercial drivers in Owerri municipality.

1.9. Aims And Objectives

1.9.1.  Aim Of The Study

The aim of the study is to assess the stereoscopic vision of commercial drivers in Owerri Municipality

1.9.2. Objectives Of The Study

1) To determine the association between age and stereopsis of commercial drivers in Owerri Municipality.

2) To determine the association between duration of driving experience and stereopsis of commercial drivers in Owerri Municipality.

1.10. Statement of the Problem

Driving requires several sets of abilities, which includes sensory ability mainly(visual), mental, motor and compensatory  abilities. Although 95% of the sensory requirements for driving have been shown to be visual (Taylor, 2002), For this reason, stereopsis play a significant role in driving. So drivers with good stereoscopic vision have advantages over those with poor stereoscopic vision. Stereopsis is important for driving (Omplase et al., 2005). It is needed to accurately judge distances, overtake vehicles and change lane. Which could lead to the occurance of road traffic accidents; especially in busy traffic. Therefore, there is need for regular assessment of stereopsis among drivers, especially commercial  drivers.

1.11. Significance of the study

  • The outcome of this study will be documented to supplement as literature and a source of additional reference information to subsequent research on this topic and related topics.
  • The outcome of this study will help the optometrist in assessing drivers with abnormal stereopsis.

1.12. Scope and delimitation of study

This study will be carried out among drivers of the Young Shall Grow, Peace mass, Imo transport company, Okigwe park, and ABC Transport Companies, at Egbu Road in Owerri Municipality. This study is limited to commercial drivers in the above mentioned Transport Companies in Owerri Municipality. Private drivers are excluded from this study.

Pages:  64

Category: Project

Format:  Word & PDF

Chapters: 1-5

Material contains Table of Content, Abstract and References.

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