5 missions. Guess, learn, then check. Your place is saved.
Mission 1A photo is flat. How can you still see depth in it with one eye shut?
Mission 2Two equal lines; one looks longer even when you know. Why?
Mission 3A pilot lands by watching the ground rush past. How is that enough?
Mission 4Your eyes give your brain half a picture. What fills the gaps?
Mission 5One shape reads as H in THE and A in CAT. What decides which?
The specification points and sources are listed under the lesson: see Sources.
Mission 1
A photo is flat. How can you still see depth in it with one eye shut?
Make a guess: A photo is flat. How can you still see depth in it with one eye shut?
Perception · depth cues
Sensing versus perceiving
Sensation is your sense organs detecting physical energy, e.g. light falling on the retina at the back of each eye.
Perception is your brain organising and interpreting that sensory information so it means something.
The image on each retina is flat (2-D), yet you perceive a world with depth (3-D).
The brain works out distance from depth cues. Monocular cues need only one eye; binocular cues need both.
Quick check
?Mia glances at a dark shape on the floor. Light from it reaches her eyes, and a moment later she knows it is her cat. Which part is perception?
Perception · depth cues
Depth cues for one eye
Cue
What you see
What the brain concludes
Height in the plane
An object sits higher in the scene, nearer the horizon
It is further away
Relative size
Of two similar objects, one looks smaller
The smaller one is further away
Occlusion (superimposition)
One object partly hides another
The hidden one is further away
Linear perspective
Parallel lines seem to meet in the distance
Where they meet is far away
Monocular cues work with one eye and in flat pictures, which is why artists use them.
Quick check
?In a landscape painting, a range of hills is partly hidden behind a farmhouse, so the hills look further away. Which depth cue is the artist using?
Perception · depth cues
Depth cues for two eyes
Your eyes are a few centimetres apart, so each retina gets a slightly different image. This is retinal disparity. The closer the object, the bigger the difference.
The brain uses the difference to judge depth.
Convergence: your eyes turn inwards to look at a near object. The more they turn, the closer the brain judges it to be.
Binocular cues work best for objects close to you.
Sort it
?Is each cue monocular or binocular?
Tap a statement, then tap the group it belongs in. Then press Check.
Mission check
?A photo is flat. How can you still see depth in it with one eye shut?
A photo is flat, but it is full of monocular depth cues: height in the plane, relative size, occlusion and linear perspective. They need only one eye, so your brain reads depth from them even with one eye shut. Binocular cues only help with real 3-D scenes.
Mission 2
Two equal lines; one looks longer even when you know. Why?
Make a guess: Two equal lines; one looks longer even when you know. Why?
Perception · illusions and constancies
When perception gets it wrong
A visual illusion is when what you perceive does not match the physical stimulus.
Ambiguity (ambiguous figures): one image fits two interpretations, so perception flips. The Necker cube flips which face is at the front; Rubin's vase flips between a vase and two faces.
Fiction: you perceive something that isn't there. In the Kanizsa triangle you see a bright white triangle with edges, but no edges are drawn.
Perception · illusions and constancies
Size constancy and the Ames room
Size constancy: an object looks the same size even when its retinal image shrinks with distance.
The brain scales up anything it judges to be far away, so a friend walking off doesn't seem to shrink.
The Ames room looks rectangular through a peephole, but one back corner is much further away.
Assuming a normal room, the brain sees both people as equally far: one looks tiny, one huge.
Perception · illusions and constancies
Depth cues misread
Müller-Lyer: of two equal lines, the one with fins pointing outwards looks longer.
Gregory: fins out look like a room's inside corner (far); arrowheads, a building's outside corner (near).
Size constancy then scales up the "far" line.
Ponzo: converging lines act as linear perspective, so the upper of two equal bars seems further and bigger.
These are misinterpreted depth cues.
Quick check
?You look at four black discs, each with a quarter cut out, with the gaps facing inwards. You see a bright white square with sharp edges, though no edges are drawn. Which explanation fits?
Sort it
?Which explanation fits each illusion?
Tap a statement, then tap the group it belongs in. Then press Check.
Quick check
?An artist draws two identical cars on a road that narrows towards the horizon, one near the bottom of the page and one near the top. Why does the top car look bigger?
Mission check
?Two equal lines; one looks longer even when you know. Why?
The fins act as depth cues: one line looks like a far corner, the other like a near one. Your brain applies size constancy and scales up the "far" line. This happens automatically, so knowing the lines are equal doesn't switch it off.
Mission 3
A pilot lands by watching the ground rush past. How is that enough?
Make a guess: A pilot lands by watching the ground rush past. How is that enough?
Perception · Gibson's direct theory
Perception without guesswork
Gibson argued that perception is direct: the pattern of light reaching your eyes, the optic array, holds all the information you need.
No inference and no past experience are needed. Processing is bottom-up: it starts from the senses.
Influence of nature: we are born able to pick up this information, because it helped our ancestors survive.
Quick check
?Which statement would Gibson agree with?
Perception · Gibson's direct theory
Information in movement
Moving changes the optic array in a regular way, and the change is information.
Motion parallax: as you travel, near objects stream past fast and far objects slowly, so speed across your view shows distance.
Gibson's ideas grew from his wartime work on films for training pilots.
Quick check
?From a train window, the fence beside the track flashes past while distant hills barely seem to move. What is this cue, and what does it tell you?
Perception · Gibson's direct theory
Born ready to perceive?
Gibson's theory takes the nature side: picking up information from the optic array is innate, not learned.
In the visual cliff studies (Gibson and Walk, 1960), crawling babies would not cross a glass floor over an apparent drop, even when their mothers called them. Young animals of many species avoided it too.
But crawling babies are already several months old, so some learning can't be ruled out.
Sort it
?Strength or weakness of Gibson's direct theory?
Tap a statement, then tap the group it belongs in. Then press Check.
Mission check
?A pilot lands by watching the ground rush past. How is that enough?
For Gibson, the moving view is enough. As she descends, near ground streams past faster than far ground (motion parallax). That information is picked up directly from the light, with no inference.
Mission 4
Your eyes give your brain half a picture. What fills the gaps?
Make a guess: Your eyes give your brain half a picture. What fills the gaps?
Perception · Gregory's constructivist theory
Perception as a best guess
Gregory argued that sensory input is often incomplete or ambiguous.
The brain makes inferences: it goes beyond the input, using visual cues and past experience (prior knowledge).
It forms a perceptual hypothesis, a best guess about what is out there, and tests it against new input.
Processing is top-down: the brain constructs a model of reality from knowledge.
Influence of nurture: we learn to perceive through experience.
Perception · Gregory's constructivist theory
Illusions as evidence
If perception is a hypothesis, a wrong hypothesis should give an illusion.
Necker cube: the drawing fits two hypotheses equally well, so perception flips between them.
Müller-Lyer: the brain infers depth from the fins and applies size constancy, so one line looks longer.
Hollow face: a hollow mask is seen as a normal face sticking out, because every face you've seen does.
Quick check
?Walking home at dusk, Leo sees a dark shape on the path and jumps back, sure it's a dog. Closer up, it is a bin bag. How would Gregory explain the mistake?
Perception · Gregory's constructivist theory
Gibson versus Gregory
Gibson (direct)
Gregory (constructivist)
Direction
Bottom-up
Top-down
Is the input enough?
Yes: a rich optic array
No: often incomplete or ambiguous
Past experience
Not needed
Essential for inferences
Nature or nurture
Nature
Nurture
Best at explaining
Fast, accurate action in real settings
Illusions, perceptual set and errors
Each may be partly right: clear, rich viewing suits Gibson; brief, unclear or ambiguous input suits Gregory.
Sort it
?Strength or weakness of Gregory's constructivist theory?
Tap a statement, then tap the group it belongs in. Then press Check.
Quick check
?Which finding best supports Gregory's view that perception is shaped by nurture?
Mission check
?Your eyes give your brain half a picture. What fills the gaps?
Gregory's answer: your brain fills the gaps with what it already knows. It makes inferences from visual cues and past experience (prior knowledge) to form a perceptual hypothesis, a best guess it keeps testing against new input.
Mission 5
One shape reads as H in THE and A in CAT. What decides which?
Make a guess: One shape reads as H in THE and A in CAT. What decides which?
Perception · perceptual set
Ready to see
Perceptual set is a readiness to perceive certain things and not others. You notice some parts of the sensory input and ignore the rest.
It works top-down, as in Gregory's theory: what is already in your head shapes what you see.
So the same stimulus can be perceived differently by different people, or by you at different times.
Four factors affect it: expectation, motivation, emotion and culture.
Perception · perceptual set
Four factors that set you
Factor
What it does
Example
Expectation
You see what context or past experience leads you to expect
Skimming past a repeated "the the" in a sentence
Motivation
Needs and wants (hunger, thirst, interests) make related things stand out
When hungry, food signs jump out at you
Emotion
Feelings change what you notice, e.g. fear makes you alert to threat
Upsetting words were recognised more slowly than neutral ones (McGinnies, 1949)
Culture
Upbringing and surroundings shape how you interpret images
People with little experience of pictures often misread depth cues in drawings (Hudson, 1960)
Set usually helps you perceive quickly, but it can make you miss things or see what isn't there.
Key studyPerception · perceptual set
Gilchrist and Nesberg (1952)
Aim
To find out whether motivation (hunger) affects how food-related pictures are perceived.
Method
Lab experiment with student volunteers in two groups: one went without food for up to 20 hours, a control group ate normally. They saw pictures of meals, then adjusted the lighting to match each picture's brightness.
Results
The hungry group judged the food pictures as brighter than the control group did, and more so the longer they had gone without food.
Conclusion
Motivation (a physical need) creates a perceptual set: hunger makes food-related things more noticeable.
Strength
Controlled, standardised procedure, so it can be repeated and the cause (hunger) isolated.
Weakness
Pictures of food, not real food, and going without food causes discomfort, an ethical issue.
Key studyPerception · perceptual set
Bruner and Minturn (1955)
Aim
To test whether expectation, created by context, changes how an ambiguous figure is perceived.
Method
Lab experiment, independent groups of students. One group saw a series of letters, the other a series of numbers, flashed briefly. Both then saw the same ambiguous figure, which could be read as "B" or "13", and drew what they saw.
Results
Most of the letters group drew a "B"; most of the numbers group drew "13".
Conclusion
Expectation from context creates a perceptual set that shapes how the same stimulus is perceived.
Strength
Both groups saw an identical figure, so only the context differed: clear cause and effect.
Weakness
A brief, deliberately ambiguous figure is unlike everyday perception, so low ecological validity.
Quick check
?Omar has skipped lunch. Walking down a busy high street, he spots every café sign but walks straight past the bus stop he was looking for. Which factor best explains this?
Sort it
?Which factor is shaping perception?
Tap a statement, then tap the group it belongs in. Then press Check.
Quick check
?In Gilchrist and Nesberg's study, what was the independent variable?
Mission check
?One shape reads as H in THE and A in CAT. What decides which?
The shape is identical in both words. The letters around it set up an expectation, a perceptual set, so your brain reads it as H in THE and A in CAT. What you perceive depends on what you are ready to see, not only on the input.
Recap
Big ideas
Sensation is detecting energy with the sense organs; perception is the brain interpreting it.
Monocular cues (height in the plane, relative size, occlusion, linear perspective) need one eye and work in pictures.
Binocular cues need two eyes: retinal disparity and convergence.
Illusions happen when perception doesn't match the stimulus: ambiguity, fiction, misinterpreted depth cues and size constancy explain them.
Size constancy keeps a walking-away friend the same size; misread depth cues make it misfire (Müller-Lyer, Ponzo, Ames room).
Gibson: perception is direct and bottom-up; the optic array holds enough information, with no inference.
It reflects the influence of nature; motion parallax shows how movement gives depth information.
It explains fast, accurate real-world perception but not illusions or the effects of experience.
Gregory: perception is top-down; sensory input is incomplete, so the brain makes inferences from visual cues and past experience to construct a model of reality.
It reflects the influence of nurture; illusions are evidence of wrong inferences.
It explains illusions and perceptual set but struggles with fast real-world perception and newborn abilities.
Perceptual set is a readiness to perceive some things and not others; it works top-down.
Expectation, motivation, emotion and culture all shape it.
Gilchrist and Nesberg showed motivation (hunger) at work; Bruner and Minturn showed expectation from context.
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Mini-lesson complete!
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You've worked through Perception for AQA GCSE Psychology (8182). 🎉
Mission 1A photo is flat. How can you still see depth in it with one eye shut?
Mission 2Two equal lines; one looks longer even when you know. Why?
Mission 3A pilot lands by watching the ground rush past. How is that enough?
Mission 4Your eyes give your brain half a picture. What fills the gaps?
Mission 5One shape reads as H in THE and A in CAT. What decides which?