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Perception of Depth by Michael Kalloniatis and Charles Luu

Michael Kalloniatis and Charles Luu


Stereopsis refers to our ability to appreciate depth, that is the ability to distinguish the relative distance of objects with an apparent physical displacement between the objects. It is possible to appreciate the relative location of objects using one eye (monocular cues). However, it is the lateral displacement of the eyes that provides two slightly different views of the same object (disparate images) and allow acute stereoscopic depth discrimination.


Monocular Cues

Several strong monocular cues allow relative distance and depth to be judged. These monocular cues include:

  1. Relative size
  2. Interposition
  3. Linear perspective
  4. Aerial perspective
  5. Light and shade
  6. Monocular movement parallax


Relative Size: Retinal image size allow us to judge distance based on our past and present experience and familiarity with similar objects. As the car drives away, the retinal image becomes smaller and smaller. We interpret this as the car getting further and further away. This is referred to as size constancy. A retinal image of a small car is also interpreted as a distant car (figure 1).

Figure 1. Relative size. A retinal image of a small car is considered to be distant


Interposition: Interposition cues occur when there is overlapping of objects. The overlapped object is considered further away (figure 2).

Figure 2. Interposition. The blue circle is reported to be closer since it overlaps the red circle



Linear Perspective: When objects of known distance subtend a smaller and smaller angle, it is interpreted as being further away. Parallel lines converge with increasing distance such as roads, railway lines, electric wires, etc (figure 3).

Figure 3. Linear perspective. Parallel lines such as railway lines converge with increasing distance


Aerial Perspective: Relative colour of objects give us some clues to their distance. Due to the scattering of blue light in the atmosphere, creating “wall” of blue light, distance objects appear more blue (figure 4). Thus distant mountains appear blue. Contrast of objects also provide clues to their distance. When the scattering of light blurs the outlines of objects, the object is perceived as distant. Mountains are perceived to be closer when the atmosphere is clear.

Figure 4. Aerial perspective. Mountains in the distance appear more blue


Light And Shade: Highlights and shadows can provide information about an object’s dimensions and depth (figure 5). Because our visual system assumes the light comes from above, a totally different perception is obtained if the image is viewed upside down.

Figure 5. Highlights and shadows provide information about depth


Monocular Movement Parallax: When our heads move from side to side, objects at different distances move at a different relative velocity. Closer objects move “against” the direction of head movement and farther objects move “with” the direction of head movement.


Binocular Cues

Stereopsis is an important binocular cue to depth perception. Stereopsis cannot occur monocularly and is due to binocular retinal disparity within Panum’s fusional space. Stereopsis is the perception of depth produced by binocular retinal disparity. Therefore, two objects stimulates disparate (non-corresponding) retinal points within Panum’s fusional area.

Fusion describes the neural process that brings the retinal images in the two eyes to form one single image. Fusion occurs to allow single binocular vision. Fusion takes place when the objects are the same. When the objects are different, suppression, superimposition or binocular (“retinal”) rivalry may occurs. Suppression occurs to eliminate one image to prevent confusion. Superimposition results in one image presented on top of the other image. Binocular rivalry describes alternating suppression of the two eyes resulting in alternating perception of the two images. This usually occurs when lines are presented to the two eyes differ in orientation, length or thickness. An example of binocular rivalry occurs when one eye is presented with a horizontal line and the other eye is presented with a vertical line. Binocular rivalry occurs at the intersection of the lines and some suppression also exists (figure 6)

Figure 6. (a) Binocular rivalry can be demonstrated by placing a pen between yourself and the screen. Keep you eye on the tip of the pen and notice the two bars merge. You may need to slowly move the pen from the screen towards you. (b) Result of (a)

Panum’s fusional area is the region of binocular single vision. Outside Panum’s fusional area, physiological diplopia occurs. Using the haplopic method of determining the horopter, Panum’s area can be determined (figure 7).

Figure 7. Haplopic method of determining the horopter involves locating the region of single binocular vision at a distance of 40cm. Panum’s fusional area lies between the outer and inner limits of the region of single binocular vision

Retinal disparity: Retinal disparate points are retinal points that give rise to different principal visual direction and diplopia. However, retinal disparity within Panum’s fusional area (zone of single binocular vision) can be fused resulting in single vision. Retinal disparity is essential for stereoscopic depth perception as stereoscopic depth perception results from fusion of slightly dissimilar images. Due to the lateral displacement of our eyes, slightly dissimilar retinal images result from the different perception of the same object from each eye.



Clinical Tests used to measure Stereopsis

There are two groups of clinical tests used to measure stereopsis. These are the contour stereotests and the random-dot stereotest. Random-dot stereograms were first used by Julesz (1960) to eliminate monocular cues. As there are no contours, depth perception (stereopsis) can only be appreciated when binocular fusion occurs. Two process of stereopsis are used and these are local and global stereopsis. Local stereopsis exists to evaluate the two horizontally disparate stimuli. This process is sufficient for contour stereotests. Global stereopsis is required in random-dot stereogram when the evaluation and correlation of corresponding points and disparate points are needed over a large retinal area.

An example of a contour stereotest used in the clinic is the Titmus Fly Stereotest. In the Titmus Fly Stereotest, horizontal disparity is presented via the vectographic technique (Fricke and Siderov, 1997). When tested a 40 cm the fly has a disparity of 3,600 sec of arc; the disparity of the animals range from 400 – 100 sec of arc and the disparity of the Wirt rings range from 800 – 40 sec of arc (figure 8).

Figure 8. Titmus Fly Stereotest


Examples of random-dot stereotests used in the clinic are the Frisby Stereotest, the Randot Stereotest, the Random-dot E Stereotest and the Lang Stereotest. The Frisby Stereotest (figure 9) uses real depth to determine stereoacuity. Three perspex of different thicknesses are used. Four squares of geometric shapes are painted on one side of the perspex. In one of the squares, a circle of these geometric shape is painted on the other side of the perspex. Both the Randot (figure 10) and the Random-dot E uses crossed polarised filters. Disparity is also constructed vectographically. The Randot Stereotest uses modified animals and ring designs with random dot backgrounds to eliminate monocular cues. The Lang Stereotest uses a panographic technique (Fricke and Siderov, 1997) to present disparity, therefore, no filters are required. Patients are required to identify pictures on the Lang Stereotest. The Lang II Stereotest has a monocularly visible shape on it (figure 11).


Figure 9. The Frisby Stereotest

Figure 10. The Randot Stereotest 


Figure 11. The Lang II


All the tests provides a measure of stereoacuity by asking the patient to identify the correct target that has stereoscoptic depth (target with disparity). The working distance and interpupillary distance will need to be taken into consideration when calculating stereoacuity. Patients with disturbed binocular vision or different refractive error in one eye, will perform poorly on depth discrimination tests.


We like to thank Tim Fricke for providing Figures 8-11.



Fricke TR and Siderov J (1997) Stereopsis, stereotest and their relation to vision screening and clinical practice. Clin Exp Optom. 80: 165-172.

Julesz B. Binocular depth perception of computer generated patterns. Bell Syst Tech J. 1960;39:1125–1162.2.

Moses RA and Hart WM (1987) Adler’s Physiology of the eye, Clinical Application, 8th ed. St. Louis: The C. V. Mosby Company.

Ogle KN (1950) Researches in Binocular Vision. London: Saunders. 1950

Schwartz SH (1999)Visual Perception, 2nd ed. Connecticut: Appleton and Lange.



Last Update: June 6, 2007.

The author

Michael Kalloniatis was born in Athens Greece in 1958. He received his optometry degree and Master’s degree from the University of Melbourne. His PhD was awarded from the University of Houston, College of Optometry, for studies investigating colour vision processing in the monkey visual system. Post-doctoral training continued at the University of Texas in Houston with Dr Robert Marc. It was during this period that he developed a keen interest in retinal neurochemistry, but he also maintains an active research laboratory in visual psychophysics focussing on colour vision and visual adaptation. He was a faculty member of the Department of Optometry and Vision Sciences at the University of Melbourne until his recent move to New Zealand. Dr. Kalloniatis is now the Robert G. Leitl Professor of Optometry, Department of Optometry and Vision Science, University of Auckland. e-mail:


The author

Charles Luu was born in Can Tho, Vietnam in 1974. He was educated in Melbourne and received his optometry degree from the University of Melbourne in 1996 and proceeded to undertake a clinical residency within the Victorian College of Optometry. During this period, he completed post-graduate training and was awarded the post-graduate diploma in clinical optometry. His areas of expertise include low vision and contact lenses. During his tenure as a staff optometrist, he undertook teaching of optometry students as well as putting together the “Cyclopean Eye”, in collaboration with Dr Michael Kalloniatis. The Cyclopean Eye is a Web based interactive unit used in undergraduate teaching of vision science to optometry students. He is currently in private optometric practice as well as a visiting clinician within the Department of Optometry and Vision Science, University of Melbourne.

Comment Feed

23 Responses

  1. Stereopticity/depth perception is well worth reading/understanding well. The virtual elimination of common problems (intermittent night blindness) depends on this.

  2. what is the simple military test for
    depth perception?
    much appreciation for your response.

    rebecca cooperJuly 14, 2011 @ 10:52 amReply
  3. how does this relate to dyslexia?

    • Co-incidence.
      Some one somewhere could persuade otherwise – no doubt for a payment.
      Stereopsis is locatable in the visual system; dyslexia is still lost in the complex unclear world of perception – starting with its very definition.

  4. How about a colour vision test that simultaniously measures stereoacuity, as stereoblindness is often comprared to colourblindness. And of the job that require good to perfect colour vision, I can think of one that also requries no less than average depth perception, that is commercial piloting.

    Fusion is analagous to combining sound signals from the left and right microphones to form mid and side channels, the mid channel is the sum and the side channel is the difference. Fusion involves inteploting a mid image from the left and right images as well as discering differences between them.

    MyrtonosJuly 28, 2011 @ 11:21 pmReply
    • Colours refract differently so give rise to colour stereopsis as compared with the above tests that are effectively in panchromatic (white) light.
      The two would conflict.
      Do you recall the cockerill on the Kelloggs’ packets? The red and green parts appear at different depths.

  5. Why not mention what is known regarding the places where the disparity information is made explicit in the visual areas?

    Alfredo RestrepoAugust 5, 2011 @ 9:38 amReply
  6. Please,i need a structural representation of stereopsis

    Wilfred justiceAugust 8, 2011 @ 5:51 pmReply

    VISHALL SRIVASTAVOctober 28, 2011 @ 10:36 pmReply
  8. I see a 3d image when I close one eye and watch a 2d movie when I uncover the other eye the movie looks 2d only when I cover one eye does the image look 3d this happens the best when I am watching a hq image such as watching a movie on my ipad

  9. i am a physical therapist looking for an easy depth perception test to be used in a screen for patients with balance issues

    jim stevensDecember 28, 2011 @ 3:51 pmReply
  10. I’ve got a doozy for you guys, not exactly on the lines of what is being discussed. I’ve been a commercial pilot for 38 years, last month I failed the depth perception test and lost my lic! I’ve flown helicopters in and out of just about any and every limited area you could imagine but still I failed the test. I have known since birth I have a color peception problem (red/green) but still I could get a pilots lic in 5 different countries. Color blindness is a load of hogwash to people who are affected, I can see everything a nornal person can, but I admit when you mix the colors it beocmes much more difficult.

    • I came across your post as I was doing some research concerning my son’s passion to become a commercial pilot and his lack of depth perception. He has always failed those those tests and a Dr. years ago said the only consequence would be if he wanted to be a pilot. Well that is all he dreams of doing. I haven’t broke that to him yet and am not sure he will be able to get a license at all; or would he be able to get a private license and not commercial? I know he needs to do the eye exam prior to solo but would it be a waste to start lessons – he’s now 15- in the hopes he may be able get the “private” just not “commercial” license. He really wants to fly in some capsity as a occupation. I would really appreciate an email if you have an info for me. Thanks

      • Donna
        Realize it has been a while but your son should continue to purue his pilot passion. I am a retired AF flight surgeon and senior AME (aviation medical examiner) and there are many ways to measure stereopsis. If all else fails he can obtain a statement of demonstrated ability (SODA). Good luck.

        Eric #574 647 1675October 8, 2012 @ 10:40 amReply
        • Eric,

          My name is “John.” That is an alias, for reasons that will immediately become obvious. I could not help but to notice your reply to Donna, regarding her Son. I am in the same spot as her Son. In fact, I retired from a career in the fire service to become a commercial pilot. I am in my early 40’s, and already have about 14 hours in a C-172, and am almost to the point of requiring an FAA Medical. I just discovered that I too, cannot pass the Randot Test, and I am in a near panic state. My desire is to fly commercially, and have very limited desire to fly privately. I can see, drive, and even competition rifle and pistol target shoot like the best of them. Still, that horrible Test has me petrified. I see nothing on the Net regarding SODAs pertaining to this issue, at least not for commercial aviation. Is there a way we can talk privately? I would really like to pick your brain, given your AME background? Here is my “anonymous” email. If we can talk, I can give you my real name.

          Most Respectfully,


  11. sir, which is gold standard test for stereopsis??For distance as well as near???

  12. Does anyone have more info on the SODA i would really like to become a pilot,but i was born with strabismus and i have had plenty of surgeries done and it is almost impossible to tell now but i have the depth perception problem. i just would like to know if spending all that money toward the license wont be a waste when i cant pass the vision test my vision is 20/20 but its the depth perception that worries me. after all the surgeries the ophthalmologist said i had no depth perception because i could never pass the random dot test.
    Thanks so much for the info and time


    AlejandroMarch 10, 2013 @ 12:10 pmReply
  13. How do to determine if the test is normal or

    April WoodJune 5, 2015 @ 1:28 pmReply
  14. Stereopsis is of no use beyond about 1 meter as we use monocular cues to depth. If tasks require you to judge depth within a meter or so, stereopsis would be of benefit. Monocular cues to depth can still be used but they would delay some responses.

    There are multiple clinical tests used to test stereopsis and pass/fail is simply set by regulatory bodies.

    Michael KalloniatisJune 6, 2015 @ 1:10 amReply

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Continuing the Discussion

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