Cues for Depth Perception The dimensions of visual perception include height, width, and depth. Depth perception describes the process of seeing distances between objects (Blake and Sekuler 2006). The image projected on the retina (and deconstructed for further processing in the brain) is two- components of the environment in order to recover the quality of depth. In general cues providing information for depth perception are generally classified as being either binocular cues or monocular cues (Blake & Sekuler, 2006; Eysenck & Keane, 2010; Sternberg & Sternberg, 2011).

Binocular cues include convergence, binocular disparity, and shadow stereopis. Binocular depth cues are based on the receipt of sensory information occurring in both eyes. These cues rely on the relative positioning of the eyes (Sternberg & Sternberg, 2011).

Convergence of the eyes is one process that can help cue the perception of depth. The human eyes are separated by about 6 cm and face forward in order to accurately judge depth, whereas animals that need a larger range of vision typically have their eyes on either side of the head (Blake & Sekuler, 2006). Convergence refers to natural movements of both eyes as objects are closer or further away from the viewer (Schwartz, 2010). As objects get closer the eyes turn slightly inward and the brain interprets these neural signals (strong singles) as indications of depth. As objects get further the eyes most or more outward in these neural singles are weaker, thus the brain interprets these as being farther away. Convergence is an effective depth cue at short distances of up to about 10 meters (Schwartz, 2010).

The different images each eye receives can provide information regarding depth. Each eye receives a slightly different image and combining the two images allows for stereoscopic vision. When objects are further away there is a large discrepancy between the images seen by either eye. The visual system is very sensitive to these differences in the images and interprets these in terms of depth. Points on the hotopter are points in space which are imaged on corresponding sections of the left and right retina (Schwartz, 2010). This retinal disparity increases as objects are further away. Binocular discrepancy (or binocular parallax) is the most important depth cue, especially for objects in medium ranges as even if all other depth cues are removed the human brain can use discrepancy to perceive depth (Eysenck & Keane, 2010; Schwartz, 2010).

Shading and shadows can provide binocular cues for depth. When a visual scene with no binocular disparity has different shadows these shadows are often blended by the information from both eyes with non-shadowed areas into a three-dimensional image (shadow steropis; Puerta, 1989).

Monocular depth cues can be represented in two dimensions and can be perceived by just one eye (Sternberg & Sternberg, 2011). Important monocular cues include accommodation, blur, texture gradient, the relative size of objects, the familiar size of objects, interposition, linear perspective, aerial perspective, location relative to the horizon, shading, motion parallax, and kinetic perspective.

One important monocular cue comes from accommodation. Accommodation refers to the tension that the ciliary muscles exert to change the lens of the eye to adjust for objects at different distances. The lens is made thicker when objects are closer and stretches the lens out when objects are further away to allow the image to be focused on the back of the retina. These changes in tension provide a weak cue for depth perception (typically only effective at less than two meters; Sternberg & Sternberg 2011). Accommodation offers information about a single object in the visual field; however, the degree of blur also offers a cue of depth perception for objects that are further away.

Images that are blurry relative to other images in the visual field are considered to be further away than the more sharply detailed images (Gillam & Borsting, 1988). Thus, photographers can selectively blur images in a picture to create the perception of depth in a two-dimensional picture.

Certain cues from the surface of objects in the visual field can provide monocular cues for depth (texture gradient). The closer an object is the more details regarding the object's surface texture can be visualized. Objects that are perceived as having smoother textures or having smaller grains (closer together) are perceived as being further away, whereas objects with rougher textures or larger grains (further apart) are perceived as being closer to the person (Gillam & Borsting, 1988).

The sizes of objects that are in the visual field provide monocular cues for depth. Larger images are perceived as being closer; whereas smaller images are perceived as being further away (Eysenck & Keane, 2010). The relative size...

The brain compares the sensed size of the object to this real conceptualization (familiar size) in order to determine information regarding the distance of objects from each other (Gillam and Borstingm 1988). Objects appearing smaller than their familiar size are perceived as being further away, whereas those larger are perceived as being closer.
The way that objects are positioned in relation to one another provides monocular cues for depth (interposition). Objects that overlap can provide cues regarding depth. If an object partly obstructs or overlaps another object it is perceived as being closer or in front of the other object, whereas if an object is partially obscured or covered by an object is perceived as being behind that object (Eysenck & Keane, 2010).

The orientation of the lines that are perceived as parallel provide important monocular cues for depth (linear perspective). Lines that are parallel and converge is the approach the horizon are viewed as being further away, whereas lines that are believed to be and seem to diverge as they move away from the horizon are seen as being closer (Gillam & Borstingm, 1988).

Related to blur and linear perspective, images viewed as fuzzier or less clearly delineated are perceived as being in the distance, whereas crisper more clearly delineated images are seen as being closer. This cue can be picked up by one eye. For example, mountains on the horizon always look hazier slightly bluish due to dust and water particles in the air. The farther the mountains are way the hazier they appear (aerial perspective; Eysenck & Keane, 2010).

An important monocular cue for depth comes from the position/location of objects relative to the horizon. Objects will appear further away if they are above the horizon and lower in the picture plane or if they are below the horizon and higher in the picture plane. Objects appear closer when they are above the horizon and if they are higher in the picture plane or those below the horizon and lower in the picture plane (Gillam & Borstingm, 1988).

Shading or shadows can also provide monocular cues for depth. Objects that cast shadows on other objects are perceived as being closer to the source of light. Typically, illumination is directed downward and if there are ambiguities regarding where the light source is located this is the default choice. In addition, objects that are brighter appear to be closer than objects that are darker (Puerta, 1989).

Motion can provide important monocular cues for depth perception, a term called motion parallax. If one is moving the apparent motion of stationary objects against the background can provide depth cues. Objects that are approaching and getting larger at an ever-increasing speed will appear to be closer than those not changing their size. Likewise, objects moving away from the observer at an ever-increasing speed up your farther away than those not getting smaller at the same speed (Eysenck & Keane, 2010).

The apparent motion of the objects themselves can provide important monocular cues for depth perception. The kinetic depth effect is a visual phenomenon in which information projected onto a two-dimensional screen provides the illusion of three-dimensional structure when the projected image is moving or rotating. Wallach and O'Connell (1953) presented a projection of a shadow of a wireframe that was rotating on a screen. The rotating shadow was perceived as being a three-dimensional representation of the wireframe. The effect can also occur if the rotating object is solid if the shadow that has lines with definite endpoints and these endpoints change during the rotation in both their orientation and length (Schwartz, 2010).

All of these cues can be used in isolation or in combination to provide the perception of depth. Thus, depth perception is a cognitive process that takes a reconstructed two dimensional retinal image and provides the perception of depth.

References

Blake, R. & Sekuler, R. (2006). Perception (5th ed.). Boston: McGraw-Hill.

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Gillam, B. & Borsting, E. (1988). The role of monocular regions in stereoscopic displays.

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Puerta, A.M. (1989). The power of shadows: Shadow stereopsis. Journal of the Optical Society

of America 6(2), 309 -- 311.

Schwartz, S.H. (2010). Visual perception: A clinical orientation (4th ed.). New York: McGraw-

Hill.

Sternberg, R.J. & Sternberg, K. (2011). Cognitive Psychology (6th ed.). Belmont,

CA: Wadsworth Publishing Company.

Wallach, H. & O'connell, D.N. (1953). The kinetic depth effect.…