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Binocular vision is experienced whenever one is looking at a distance with a close vertical object in the foreground. Individuals with binocular vision see through the object.
In the course of providing eye care, ophthalmologists are aware that the majority of our patients are using both eyes simultaneously; that is, they have binocular vision. However, we also have patients who are monocular, relying solely on one eye.
When thinking about how the visual functioning of the two groups differs, one may turn to a standard description of the advantages of our natural state of binocular vision. The functional benefits most often mentioned are depth perception/stereopsis and a wider field of view.
We would like to propose that a third major aspect of binocular vision be included in the standard definition, a feature we are calling “the see-through phenomenon.”
Because of the lateral separation of our two eyes when we are upright, we are able to “see-through” a vertical object in the foreground when looking in the distance.
The closer the near vertical object, and the farther the area of distance viewing, the more this phenomenon is in effect. If the intervening object has a horizontal orientation, it is not seen through.
Conversely, when we are lying on our side with our eyes separated vertically, we see through a horizontal object, but not a vertical one.
One easy way to observe the see-through phenomenon is as follows. An individual with binocular vision covers one eye and places his or her index finger vertically in front of the unoccluded eye. When looking at a distant object, all or part of the object will be obscured. When the second eye is uncovered, the distant object can be seen fully. The finger is being “seen through.”
Another way this aspect of binocular vision can be appreciated is by looking at a distant object, with both eyes open, while moving one’s hand at a medium pace in front of the eyes back and forth, from right to left, then left to right, continually. It will be noted that the distant object remains essentially visible the entire time.
However, when performing the same maneuver with one eye covered, the hand will block the view of the object as it moves across.
Yet another way to experience this phenomenon is to look in the distance through a window that has dividing strips between the window panes. Choose an area in the distance that has a vertical strip in the foreground. It will be noted that the vertical strip does not appear to be blocking the view in the distance.
However, when one eye is then covered, the strip will block the area behind it. The closer one is to the window, and the farther the area viewed, the more noticeable is this phenomenon. It will also be noted that the horizontal strips are not seen through.
Depth perception/stereopsis and a wider field of vision are two major advantages of binocular vision that are well recognized.
The see-through aspect of binocular vision as described here, however, is rarely mentioned, even though it is experienced in daily life, for we are often confronted with objects near and far. We are benefiting from it when looking through a window, as discussed above, when viewing something across a room with an intervening lampstand, or when there is a street post in front of us as we are walking outdoors.
Since many similar situations exist, it seems that this see-through aspect of binocular vision-and the absence of it with monocular vision-deserves emphasis.
Our observation is not a new one. A similar, although not exact, observation was noted in 150 CE by the Greek astronomer Claudius Ptolemy, who described physiologic diplopia due to lateral displacement of the two globes in his book, Optics.1
An individual with binocular vision can notice this phenomenon by holding a pencil directly in front of his or her head. He or she then fixates on a distant object beyond the pencil, and the pencil will be seen as double.
Consistent with our discussion, Leonardo da Vinci described an obstacle becoming “transparent” with binocular viewing.2 Fahle, in 1987, included da Vinci’s observation in his discussion of the advantages of having two eyes.3 Currently, however, it seems this aspect of binocular vision is often overlooked.
Depth information is necessary for navigation and tasks requiring hand-eye coordination.4 While visuospatial cues embedded in the environment (linear perspective, shadows, textures, and gradients) may still allow monocular individuals to accomplish routine reaching and grasping tasks directed at common objects,5 fast and accurate performance requires more direct information about the precise metrics of the goal object and its environment.6
A frequently studied application of stereopsis is prehension. This task involves using the hands and fingers to grasp, pinch, or pick up an object.
It has been studied extensively in individuals with reduced binocularity.
Kinematic analyses of normal adult prehension-especially in the final approach to the target and in grasping it-have consistently demonstrated slower and less accurate performance with monocular versus binocular vision.7,8 This disadvantage has been largely attributed to the absence of retinal image disparity and vergence cues in monocular vision.9–12
Given their inferior performance, children and adults with severely reduced or absent binocularity may be more accident prone when required to respond rapidly to unexpected situations.6 This especially has implications for monocular individuals who have diminished ambulation or disability, and for professionals such as pilots and truck drivers.
Field of vision is increased by binocular vision. The maximum horizontal field of vision that one can appreciate is more than 200°.13 Roughly 120° of this is the binocular visual field of view.14 Depending on facial anatomy, one eye alone can see up to 60° nasally, 107° temporally, 70° superiorly, and 80° degrees inferiorly.15
Other aspects of binocular vision perhaps have slightly less functional advantage: binocular summation, binocular luster, and retinal rivalry-although the latter is playing a role in the phenomenon we are describing.
Binocular vision is better in seeing faint distant objects due to the phenomenon of binocular summation.16 In reading performance, the binocular advantage over monocular vision is relatively small. However, comparing monocular and binocular performance at different stimulus contrasts showed that binocularity contributes increasingly to the performance of reading as the stimulus contrast decreases.17
Binocular luster is the phenomenon of an object appearing darker in one eye’s view and lighter in the other eye’s view, relative to the background.18
Retinal rivalry is the alternation in perception that occurs when different images are presented to the two eyes.19 When one image is presented to the eye and a different image is presented to the other eye, one image is seen for a few moments, then the other is seen, and they keep alternating.20 It is believed that the way we cope with this everyday situation is through viewing the target of interest with one eye-usually the dominant eye.21
Finally, it should be noted that the complexity of binocular vision creates an open window for possible anomalies or disorders that can lead to diplopia, asthenopia, headache, blurred vision,22 amblyopia, and loss of depth perception.23
The see-through phenomenon should have a prominent place in the standard definition and understanding of the advantages of binocular vision, alongside depth perception/stereopsis and wider field of view.
This aspect of binocular vision is experienced whenever one is looking at a distance with a close vertical object in the foreground. Individuals with binocular vision see through the object, while those with monocular vision are not able to do so.
Ali AbdulkarimE: firstname.lastname@example.org
Dr. Abdulkarim is a student at the Faculty of Medicine, Cairo University, Egypt.
Merve Gurakar, MPH
Dr. Gurakar is a fourth-year medical student at Virginia Commonwealth University School of Medicine, Richmond, VA.
Gislin Dagnelie, PhDE: email@example.com
Dr. Dagnelie is associate director, Lions Vision Research and Rehabilitation Center and associate professor of ophthalmology, Johns Hopkins University School of Medicine, Baltimore.
Andrew F. Kolker, MDE: firstname.lastname@example.org
Dr. Kolker is in private practice in Clinton, MD.
Richard J. Kolker, MDE: email@example.com
Dr. Kolker is assistant professor of ophthalmology, Division of Comprehensive Eye Care, Wilmer Eye Institute, Johns Hopkins University School of Medicine, Baltimore.
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