Perception — AQA GCSE Psychology Revision Notes

What you'll learn

• The difference between sensation and perception, and why it matters
• Monocular and binocular depth cues, and when each type does the work
• Why visual illusions happen, and what they reveal about how the visual system operates
• Gibson's direct theory and Gregory's constructivist theory, and what separates them
• Perceptual set: how expectation, motivation, emotion and culture change what we see
• The key studies — Gilchrist and Nesberg, Bruner and Minturn — and what each demonstrates
• How to evaluate two theories that are each strongest where the other is weakest

Key terms and definitions

Sensation — the raw information received by the sense organs.

Perception — the process of interpreting sensory information to make sense of it.

Monocular depth cue — a depth cue requiring only one eye.

Binocular depth cue — a depth cue requiring both eyes working together.

Linear perspective — parallel lines appearing to converge with distance.

Relative size — more distant objects casting smaller images.

Superimposition — an object partly covering another being perceived as nearer.

Height in plane — objects higher in the visual field being perceived as further away.

Texture gradient — surface detail becoming less distinct and more densely packed with distance.

Size constancy — perceiving an object as unchanged in size although its retinal image shrinks with distance.

Visual illusion — a stimulus that produces a perception contradicting the physical input.

Ambiguous figure — a figure supporting more than one interpretation, with perception switching between them.

Bottom-up processing — perception built directly from sensory information.

Top-down processing — perception guided by stored knowledge and expectation.

Perceptual set — a readiness to perceive in a particular way, created by expectation, motivation, emotion or culture.

Core concepts

Sensation and perception

The eye receives a two-dimensional pattern of light. What we experience is a stable three-dimensional world of objects at known distances. Everything between those two things is perception.

This gap is the whole subject. The retinal image is incomplete, constantly changing and two-dimensional, yet perception is rich, stable and three-dimensional. Perception must therefore be doing work — and the question that divides the theories is how much.

Depth cues

Monocular cues need only one eye, which is why depth can be perceived in a flat photograph.

• Linear perspective: parallel lines such as railway tracks appear to converge as they recede.
• Relative size: two objects known to be the same size, one casting a smaller image, are judged to be at different distances.
• Superimposition: an object partly blocking another must be in front of it.
• Height in plane: objects higher in the visual field are read as further away.
• Texture gradient: on a pebble beach, individual stones are visible nearby and merge into an even grey further off.

Binocular cues require two eyes. The two eyes view a scene from slightly different positions, and the brain uses the difference between the two images. This works well for nearby objects, where the two views differ noticeably, but for distant objects the views are almost identical and the difference becomes too small to be informative. At longer distances monocular cues carry the work.

Size constancy

An object's retinal image shrinks as it moves away, yet we perceive the object as staying the same size. A person walking away is understood to be the same person, not a shrinking one.

This is useful — it keeps the world stable — but it comes at a cost. Size constancy operates by combining the retinal image with an estimate of distance. Give the system misleading distance information and it will produce the wrong answer, confidently. That is where illusions come from.

Visual illusions

Two well-known illusions both exploit misapplied size constancy.

In the first, two lines of identical length appear different: one has outward-pointing arrowheads at each end, the other inward-pointing ones. The arrowheads resemble the corners of buildings — the inside corner of a room, the outside corner of a building — so depth cues are applied. One line is judged to be further away, and size constancy enlarges it accordingly.

In the second, two identical bars are placed between converging lines, one nearer the point of convergence. The converging lines act as a linear perspective cue suggesting distance, so the bar apparently further away is enlarged. The mechanism is the same; only the cue being exploited differs.

Ambiguous figures work differently. A drawing may be seen either as a vase or as two facing profiles, but never both simultaneously. The sensory information stays constant while the interpretation flips. Perception must settle on one reading in order to be useful for action, so the system commits — and the switching is direct evidence that a decision is being made beyond the sensory input.

Two theories of perception

Gibson's direct theory is bottom-up. Gibson argued that the information available to a moving observer in a real environment is far richer than laboratory studies suggest, and is sufficient on its own. Perception is direct: we pick up the information rather than inferring anything from it. Stored knowledge is not required.

Gregory's constructivist theory is top-down. Gregory argued the retinal image is genuinely incomplete, so the brain makes inferences using stored knowledge and expectation, effectively forming a best guess about what is out there. Perception is a construction. Illusions are what happens when a normally sensible inference misfires.

The evidence divides. Gibson explains how perception can be fast and accurate enough to guide action, and why newborn animals with no relevant experience perceive adequately. Gregory explains illusions, ambiguous figures, perceptual set and cultural differences — none of which a purely direct account handles well.

The better conclusion is that each is strongest where the other is weakest. Direct processing accounts for rapid perception in rich, well-lit natural conditions; constructive inference accounts for degraded, ambiguous or artificial input where the information really is insufficient. They may describe different circumstances rather than compete for the same explanation.

Perceptual set

Perceptual set is a readiness to perceive in a particular way. Four factors produce it.

Expectation. Bruner and Minturn showed participants an ambiguous figure after presenting either a series of letters or a series of numbers. The same figure was reported as a letter or as a number depending on which context had preceded it. The input was identical; the expectation differed.

Motivation. Gilchrist and Nesberg studied participants who had gone without food and then viewed pictures of food. The food-deprived participants judged the pictures as brighter, suggesting an internal state had biased perception.

Emotion. Threatening or distressing material may be recognised more slowly or avoided, while material matching a current emotional state is noticed more readily.

Culture. People raised in environments with few straight lines and right angles appear less susceptible to illusions that exploit angled corners. They have not built up the experience that makes those depth cues misleading, so the misapplication does not occur.

Perceptual set also explains expertise. A radiographer spots an abnormality an untrained viewer misses entirely — expectations about which features matter direct attention, so the same image yields more information. The same mechanism can also make an expert see what they expect rather than what is there.

Worked examples

Example 1: Identifying a depth cue

In a photograph, a tree partly hides a house, and the tree is judged to be nearer. Name the depth cue and state whether it is monocular or binocular. (2 marks)

The cue is superimposition (1). It is a monocular cue, since it requires only one eye (1).

Example 2: Explaining an illusion

Explain why two lines of equal length appear different when one has outward-pointing and the other inward-pointing arrowheads. (3 marks)

The arrowheads resemble the corners of buildings, so they act as depth cues (1). This leads one line to be judged as further away than the other (1). Size constancy then enlarges the apparently more distant line, so it is perceived as longer even though the two are physically identical (1).

The term to include is misapplied size constancy — it is the mechanism, and answers omitting it rarely reach full marks.

Example 3: Applying perceptual set

Sam is told he is about to see pictures of animals, and reports seeing a rabbit in an ambiguous drawing that could also be a duck. Explain his perception. (3 marks)

Sam has a perceptual set created by expectation (1). Being told to expect animals made him ready to interpret the ambiguous figure in a particular way (1). Because the drawing supports more than one interpretation, the expectation determined which one he perceived, even though the sensory information was unchanged (1).

Example 4: Distinguishing the theories

Explain one difference between Gibson's and Gregory's theories of perception. (3 marks)

Gibson's theory is bottom-up: it claims the sensory information available is rich enough for perception to be built directly from it, without drawing on stored knowledge (1). Gregory's theory is top-down: it claims the sensory information is incomplete, so the brain uses past experience and expectation to infer what is being perceived (1). The theories therefore disagree about whether stored knowledge is necessary for perception at all (1).

Common mistakes and how to avoid them

Mixing up monocular and binocular cues. Monocular needs one eye, and these are the cues present in photographs. If a cue works in a flat picture, it is monocular.

Saying an illusion means "the eyes are tricked". The eyes report accurately; the interpretation is what goes wrong. Locate the error in the brain's processing.

Getting top-down and bottom-up the wrong way round. Top-down starts from knowledge in the head; bottom-up starts from the sensory data. Gregory is top-down, Gibson is bottom-up.

Describing a study without saying what it shows. Bruner and Minturn's finding matters because expectation changed perception of identical input. State the implication.

Concluding that perception is generally unreliable. Illusions are artificial figures constructed specifically to defeat the system. The shortcuts that produce those errors are what make perception fast and stable the rest of the time.

Claiming one theory has been disproved. Both have supporting evidence. A good evaluation says which conditions each handles well.

Forgetting that cultural evidence cuts both ways. It supports a learned component, but the groups compared differ in many respects besides architecture.

Exam technique for Perception

Name the cue or the process precisely. "Depth cue" is not enough when the question wants linear perspective. Precise terminology is where marks are.

For illusion questions, give the mechanism in steps. Cue present → distance misjudged → size constancy applied → wrong perception. Working through the chain earns the marks.

Use the named studies as evidence, briefly. One sentence of procedure and one of finding is usually enough. The marks are for what the study shows, not for how much you can recall about it.

In evaluation, pair each strength with what it cannot explain. Gibson explains rapid natural perception but not illusions; Gregory explains illusions but relies heavily on artificial two-dimensional figures.

Apply to the scenario. Application questions give a person and a context. Name them and refer to what they actually perceived.

Watch for research methods questions in disguise. Studies of perception invite questions about ecological validity, samples and demand characteristics. Judgements of brightness, for instance, are subjective and open to demand characteristics — a useful evaluation point.

Quick revision summary

• Sensation is the raw input; perception is the interpretation of it
• Monocular cues (one eye): linear perspective, relative size, superimposition, height in plane, texture gradient
• Binocular cues (two eyes) rely on the difference between the two views, and work best up close
• Size constancy keeps the world stable but produces illusions when distance is misjudged
• Misapplied size constancy explains both the arrowhead and the converging-lines illusions
• Ambiguous figures flip between interpretations while the input stays constant — evidence of active interpretation
• Gibson: bottom-up, direct, information-rich environment, explains fast natural perception
• Gregory: top-down, constructive, incomplete retinal image, explains illusions and perceptual set
• The theories may describe different conditions rather than genuinely compete
• Perceptual set arises from expectation, motivation, emotion and culture
• Bruner and Minturn: expectation determined whether an ambiguous figure was read as letter or number
• Gilchrist and Nesberg: food-deprived participants judged food pictures as brighter
• Cultural differences in illusion susceptibility support a learned component to perception
• Illusions reveal the rules the visual system uses — they do not show that perception is generally unreliable