PSYC10006 Chap.6 Depth and Size Perception
Depth and Size Perception
The retina is a two-dimensional surface, so distance is never given directly and has to be inferred. Because it is inferred, size has to be inferred too: the image an object casts shrinks as it recedes, so image size alone says nothing about how big something is. The module's master claim follows from that pair of facts, and almost every demonstration in it is an instance of the claim: most size illusions are depth illusions.
It works through eleven separate sources of distance information grouped into three families, shows why the redundancy is coverage rather than duplication, and then presents an experiment that removes the cues one at a time and watches the size judgement fail in the direction the account predicts.
What this chapter covers
- 01
Why distance is inferred, and why size inherits every error in the inference
- 02
The direction rule: looks nearer means looks smaller, looks farther means looks larger
- 03
Three families of cue, divided by where the information comes from
- 04
The seven pictorial cues, and what each one alone can and cannot establish
- 05
Motion parallax, and the second movement-based cue the module describes without naming
- 06
Convergence and accommodation as readings taken from the eyes themselves
- 07
Binocular disparity, and a demonstration you can run with two fingers
- 08
Why the eye-based cues run out with distance while pictorial cues do not
- 09
Angular size against perceived size, and what size constancy achieves
- 10
The cue-removal experiment, and why angular size was held constant in it
- 11
The result: matching fails once parallax and shadows are gone, in the predicted direction
- 12
Three illusions read as controlled cue conflict rather than as curiosities
Take a cue inventory for a described situation
- 1Take the eye-based cues first. At that distance the eyes are effectively parallel and the lens is at rest, so convergence and accommodation are both at their far limits and cannot separate one far distance from another.
- 1Take the binocular family. The difference between the two eyes' views shrinks with distance and is negligible here, so binocular disparity contributes nothing.
- 1Work the pictorial cues one at a time. Darkness removes most of them: no texture gradient without a visible surface, no relative height without a visible ground plane, no occlusion without other visible objects, no perspective convergence without visible parallel edges.
- 1Identify what survives. Familiar size, and only if the pilot knows what kind of light it is; motion parallax is unavailable because head movement is prevented, even though the aircraft itself is moving.
- 1State the exposure. With distance resting on familiar size alone, the estimate inherits any error in the assumption about what the light is, and by the direction rule a light judged nearer than it is will also be judged smaller and less significant than it is.
Key terms
- Angular size
- The visual angle an object subtends at the eye, which halves as its distance doubles. It is the raw input to a size judgement and is not what is perceived, which is the distinction size constancy rests on.
- Motion parallax
- The difference in apparent speed of stationary objects as the observer moves, with nearer objects sweeping across the view faster. It works because a nearer object's image is displaced further for the same movement of the head.
- Binocular disparity
- The difference between the two eyes' views of the same scene, arising because the eyes are horizontally offset. Its size indexes relative depth, and it shrinks to nothing at long distances.
- Perspective convergence
- A pictorial cue in which parallel edges appear to draw together with distance. It shares a word with the oculomotor cue and nothing else: one is in the image, the other is in the eye muscles.
- Size constancy
- Perceiving an object as a stable size although its angular size changes with distance. It is achieved by scaling the retinal image by perceived distance, which is what makes it break in a predictable direction when distance is misjudged.
- Texture gradient
- A pictorial cue in which the elements of a repeating surface pattern appear finer and smoother with distance. It works best over exactly the ranges where the eye-based cues have stopped discriminating.
Depth and Size Perception FAQ
Why are there so many cues for one quantity?
Because each one fails somewhere. Binocular cues run out at distance, familiar size fails on an unfamiliar object, parallax needs you to be moving, atmospheric perspective needs an atmosphere. A system with one cue would be blind to depth whenever that cue was unavailable, so the redundancy is coverage.
What is the fastest way to answer an illusion question?
Do not start from the size. Ask what made the object's distance look wrong, which is always a cue that has been removed, faked or set against another, and then apply the direction rule. Answers that go straight for the size get the phenomenon right and leave out the mechanism the objective asks for.
What made the cue-removal experiment interpretable?
Two design choices. The test objects differed in physical size but were placed so their angular size stayed constant, so the retinal image was not varying; and each viewing condition differed from the one before it by a single cue, so the point where performance broke down localises which cues had been doing the work.
Can I say what the size-matching errors were numerically?
No. The subject reports the result qualitatively: matching was accurate with the fuller cue sets and, once parallax and then shadows were removed, observers judged the object nearer and therefore smaller than it was. No matching values are given, so describe the pattern and its direction instead.
Assessment move
Learn the direction rule as a sentence you can recite, because it converts almost every illusion in this module into a one-line explanation and it is the part most answers omit. Then build the eleven cues into three families rather than one list, since the family a cue belongs to is what a classification item asks for and the two cues that share the word convergence sit in different families.
Practise cue inventories on situations you invent: a foggy street, a photograph, a one-eyed view, a stationary observer. Each takes two minutes and rehearses the exact skill the cue-removal study is testing. When you revise that study, be able to say why angular size was held constant, because the mark there is for recognising what the design controlled rather than for reciting the four conditions.
Working through Depth and Size Perception in PSYC10006? Sia is AskSia’s AI Psychology tutor — ask any PSYC10006 Depth and Size Perception question and get a clear, step-by-step explanation grounded in how PSYC10006 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
