Many depth cues are monocular. So why do we have two eyes?

7 July 2016

Human beings perceive a three-dimensional world however the image projected to the retina is two dimensional in form (Howard 1995). A number of visual cues allow human beings to perceive depth in the world around them letting the brain to distinguish between objects that are close, and those that are far away. Binocular cues, such as stereopsis and convergence, afford depth information when both eyes are viewing a scene. Monocular cues on the other hand afford depth information when a scene is viewed with one eye.

Since human beings have two eyes instead of one there must be an advantage to having two eyes. This essay will describe some binocular cues and their functions and secondly describe monocular cues and their functions. It will be argued that there is a definite advantage having binocular depth cues (Ciuffeda, 2002). Depth perception is important because it means that human beings can distinguish the distance between objects (Zijiang & Teng ,2000).

Many depth cues are monocular. So why do we have two eyes? Essay Example

For example, people need to be able to perceive depth when they cross a road without being hit by an approaching car. Convergence is a binocular depth cue that allows people to perceive absolute depth (McFadden, 1994). For example, when a bowler in a game of cricket has to determine the distance between them and the batsman in a game of cricket, they would use convergence. Primary cues such as convergence relate to a physiological process of the visual system.

When the eye is fixated at an object and that object is getting closer to the nose, both eyes would converge inwards. To evaluate whether convergence is an advantage for having two eyes because of this action an individual would be able to judge an object is coming closer to them. For instance a bee coming towards a person face, the person would be able to judge the actual distance of how close the bee is to them. However, a limitation of this cue is that it only estimates close distance and not far distance accurately.

Retinal disparity is a binocular depth cue that occurs because the image viewed by the left and right eye is at slightly different angles due to their positioning 6. 6 cm apart from each other. The two images are fused together creating stereopsis. This allows the human being to perceive relative depth because the person only has a rough estimate of the difference from themselves and the object. When objects are at a crossed disparity they appear closer than the object being focused on whilst objects at uncrossed disparities appear further away than the object being focused on.

When an object appears at crossed disparity the image seen by the right eye is moved to the left and left image that is viewed in the left eye is moved to the right. Objects at uncrossed disparity have the opposite effect, this is when both eyes are fixated on the nearby object and the object in the background has separate images In this case, the object that is viewed by the right eye is moved to the right and the object that is viewed by the left eye is moved to the left.

In all, this gives a relative estimate of the distance of the object and the individual (Snowden, Thompson, & Troscianko, 2012, page 201). This helps a person to estimate if an object is further away from them and then demonstrates the disparity of each eye is sending a different message and then overlap these images to be seen as one three dimensional image. This is another important advantage of having two eyes as both crossed and uncrossed disparity helps an individual to understand the relative distance between them and an object. The last depth cue that will be discussed is parallax.

The appearances of different objects at different distances are perceived differently because both eyes project slightly differences of the object (Rogers & Graham, 1979). For instance, when driving a car the nearby telephone poles go past faster than the trees in the distance. Not all depth cues require the use of both eyes. Monocular depth perception also provides cues to absolute and relative depth. An example of a monocular absolute depth cue is accommodation. When the object moves closer to the eye this leads to a feeling of strain in the eye, this indicates the depth of an object is coming closer.

This is also a primary cue as it relates to the physiology of the visual system Mather, (2009) page 268. This is important on how the physiological system in a person body is a benefit for an individual to adapt in to the environment. This involves the lens of the eye changing shape in order to have a more accurate image of the retina. There are four monocular cues that relate to relative depth. Firstly, linear perspective occurs when small objects seem to project a far image on to the retina compared to those object that are nearer (Zelanski, & Fisher, 1988).

For instance, would be railway tracks; it seems the two parallel tracks are merging with each other at the distance. Secondly clarity, that the nearer the object is the more clearly it looks to an individual compared to something further away as it would look more blurry and unclear. Such as looking at a landscape, the mountains seem to be blurred while the near objects seem to look clear. This is important depth as it gives a relative indication of the distant to the person what is near and distant to them.

Furthermore, this could also be without the need of the cue, if a person was to see a blurry picture, the person’s judgement of the picture would be it was taken from a distance far away. However, with this perception there is no actual depth information is provided, if the image is taken close or further away (tilt shift perception. ) However, a limitation of this depth cue is that it could cause driving in fog to be more dangerous because the driver might think it is far away from braking but we not really judging from the frog. Another depth cue is occlusion.

This is when the object is blocking another object that objects is perceived to be nearer. This is important depth cue because an individual knows when some part of the object is not showing due to the overlap of the other object it is not displayed. For example when a person sees another friend move behind a table and would able only to see the front half of the body, the person would still assume the friend still had two legs due to prior experience. However, a limitation of this depth cue it does not provide how far the distance is from the person to the object.

The final monocular depth cue that will discussed is relative size is another depth cue, if an object seems further away from the eye the image seems to be smaller, in this situation person would perceive the object as smaller than the object coming close as it would perceive it as bigger. One limitation of this cue is only useful when the person knows the actual size of the object. Finally, there is also monocular motion parallax, the perception of depth would be perceived differently if one eye was closed and whilst moving (Rogers & Graham, 1979).

For instance if a person is fixated on a finger with both eye and then shuts one eye and moved the depth of the finger would change. The benefit of having two eyes as opposed to one is not entirely clear. Firstly, with two eyes due to retinal disparity a person is able to detect better acuity of the depth of the object. This is because there is more depth information coming in compared to single eye. This can be demonstrated by holding a pencil length on its side of each hand, with one eye closed they would try and touch both pencils together; the person is more likely to miss.

This suggests with both eyes open, this would give a better depth perception. Supporting this was a study from Melmoth and Grant (2006) they analysed the natural reaching and grasping of objects of monocular conditions (one eye sighted) and binocular condition (two eyes present. ) In this task participants had to reach and grasp list items of normal house hold objects under normal lighting conditions. The results demonstrated that participants with more binocular cues were more accurate and made fewer errors than monocular cues for reaching and holding the objects.

They concluded due to binocular disparity, they have the depth information from both eyes; participants were more accurate with these motor movements compared to monocular. With two eyes a person has the advantage of a wider field of vision (Ogle, 1964); this would help the individual to adapt to the environment especially on practical tasks such as driving. A person with two eyes is better to identify an object in the dark by 40% better compared to one eye closed (Rodgers, 1983). This could be interpreted as an evolutionary advantage and necessary for survival.

Furthermore, having two eyes compensates for having a blind spot. This disparity increases information for the brain to interpret the surroundings, therefore leading to better depth perception. For instance, a person perceiving with one eye had a situation that they could not be able to see clearly, for example due to dryness on one eye other eye can take over and see things more clearly. Lastly, a person with two eyes would have the advantage if one eye was damaged then would have a spare eye to able to use (Ciuffreda, 2002).

Therefore, binocular cues are more of an advantage than monocular cues and this would be demonstrated in the task if a person was shooting a basketball. It is likely to be more accurate in the long run with two eyes open compared to one. Stereopsis allows a person to view three-dimensional films cinemas due to retinal disparity people get from both of their eyes. This is produced by getting two views in the same scene and by projecting this by special system in the audience glasses).

This sends information to the right and left eye and the brain perceive (not sure diffuses is the right word here) them together giving the viewer a strong illusion of depth in and out of the screen (DeAngelis, 2000). This is important because having two eyes provides a person to see the world in 3D. However, people who have stereoblindness, caused by a strabismus can adapt to the environment perfectly due to the monocular depth cues. This perhaps suggests the use of binocular depth cues is not necessary and therefore two eyes are not better than one. Many artists use monocular cues to provide depth in their paintings.

Furthermore, artists prefer looking at paintings with the use of only one eye due to better perceived depth as binocular vision could lead to cue conflict this would flatten the scene (Pirenne, 1970). This is because binocular vision sees all objects in similar depths whereas monocular vision shows a wide range of depths. To summarise, the essay began by describing binocular cues and how important these are for depth perception. Convergence is important for an individual to judge the absolute depth of an object in order to realise when it is closer to them.

Retinal disparity is important to judge relative depth of objects when they are further or nearer to the person. On the other hand, there are many monocular cues to provide absolute (accommodation) and relative depth such as linear perspective. It was agreed in the end having binocular vision does have a greater advantage compared to monocular vision. This is because binocular vision provides wider vision field, this is an advantage when adapting to an environment, for instance driving a car. Furthermore, binocular vision would have greater advantage when identifying objects in the dark.

Lastly the advantage for a person to have two eyes would be if one was eye was damaged they would have a spare eye to use. However, in some circumstances monocular vision could be seen as more beneficial over binocular vision. For instance, artists prefer to look at a painting with one eye compared to two eyes due to the cue conflict with binocular disparity. Overall both depth cues play an important role in allowing the person to perceive depth. Whilst binocular vision has a greater advantage one cannot ignore the importance of monocular cues as they are equally as important for a person to perceive depth perception.

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