Perception is direct and bottom-up
Gibson (1966): a moving observer in a rich, lit world perceives fast and accurately because perception is direct. Direct theory: the light at the eye already contains all the information needed, so perception is immediate, nothing added from memory or inference. It is bottom-up (data only), not top-down, closing the sensation–perception gap. Four components, in order: a. sensory input, b. optic flow, c. invariants, d. affordances.
Sensory input and optic flow
a. Sensory input = the optic array, the structured pattern of light into the eye. "Structured" matters: the light is already patterned (edges, shading, grain), so it is rich on its own. b. Optic flow — the array's outward movement as you move: the point you head for (focus of expansion) stays still; all else flows away. Its origin is where you are going; its rate, your speed. Describe the pattern, not the feeling.
Invariants and affordances
c. Invariants — array features that do not change as you move — constant, reliable information. The three: texture gradient (grain coarser near, finer far — fixed rate), relative size (size/distance fixed) and the horizon ratio. d. Affordances — the use an object offers, perceived directly: a chair affords sitting, a gap passing. Affordances are relative to the animal: a branch affords perching to a bird, not a horse.
Strengths and weaknesses of the theory
Pair strengths and weaknesses, each with a consequence. Strengths: high ecological validity (real settings); explains everyday speed and accuracy; applications in aviation/driving. Weaknesses: cannot explain visual illusions (unambiguous array, wrong perception — Gregory's strength) or perceptual set (expectation, emotion, culture); affordances depend on experience/culture; field evidence is low in control.
Drawn from real examiner reports.
Do not muddle Gibson with Gregory
Topic 8's top failure, both ways. Candidates justify Gibson with prior knowledge, inferences or a perceptual hypothesis — all Gregory's components; Gibson also gets described when Gregory was asked. Self-test: if your Gibson sentence says knowledge, memory, hypothesis, inference, guess or fill in, it is Gregory — Gibson needs none.
Candidates muddle the theories of Gregory and Gibson, justifying with mixed components (June 2019 P2 Q25); the reverse error, describing Gibson when asked about Gregory (June 2024 P2 Q27).
Optic flow is a pattern, not a feeling
Optic flow gets described by its effect, not its mechanism. Saying it feels realistic or enjoyable describes the trainee and earns nothing. Mechanism: the array flows outwards as you move; the still point is arrival (a pilot's touchdown); flow rate gives speed. Describe the pattern, not how it feels.
Optic flow misapplied to a flight simulator as realism or enjoyment rather than described as a flow pattern (June 2022 P2 Q23).
Texture gradient is not linear perspective
Texture gradient is confused with linear perspective. Texture gradient is the grain of a surface becoming finer and denser with distance; linear perspective is parallel lines converging to a point. A texture is a surface pattern, not a line, and never meets at a point. Describe it as the grain getting finer, not as lines converging.
Texture gradient confused with linear perspective (June 2022 P2 Q22).
Optic flow is not an invariant
Keep optic flow apart from invariants. Optic flow is a pattern of movement, only for a moving observer — the outward-streaming array. Texture gradient and relative size are invariants, present when standing still. Offering an invariant as optic flow mixes one theory's two parts: flow is motion, an invariant a constant relationship.
Improvement is not a weakness
If a question asks for an improvement to a study of optic flow, a weakness is not one. "It was not well controlled" is the weakness; "run the task on a simulator so approach speed is held the same for everyone" is the improvement — it says what you would change. Read the command word: a weakness is what is wrong, an improvement is the fix.
An improvement asked for and a weakness given is a recorded error (June 2019 P2 Q3c).
'Direct' does not mean fast
Three errors. "Direct" is not "quick" — speed is a consequence; direct means only the optic array, with nothing added. Gibson didn't say the brain plays no part — the claim is about what information is used, which needs no supplement. An affordance is not an object's purpose — it is what the object offers that observer's body.
Candidates muddle the theories of Gregory and Gibson (June 2019 P2 Q25); describing Gibson by mistake when asked about Gregory (June 2024 P2 Q27).
The 9-mark Assess: four components, balance
9-mark Assess: AO1 3 / AO2 3 / AO3 3; 7-9 needs AO3 and balance. Four components (AO1); scenario movement/surfaces (AO2); one strength, one weakness (illusions, perceptual set), Gregory as rival only (AO3); conclude. AO2 before AO3, prose.
Justify strengths and weaknesses
A strength/weakness must be justified, not named — bare labels are bottom-band. Use point→because→which means: "it can't explain illusions, because Muller-Lyer is unambiguous yet perception errs, which means the perceiver adds something Gibson denied".
Two unique points and a connective
A 2-mark describe needs two unique points — the same idea reworded scores once. "Describe the difference" needs a connective: "an invariant stays constant, whereas an affordance is the use an object offers". Side-by-side definitions score one.
Four components, in the order the specification lists them: a. sensory input · b. optic flow · c. invariants · d. affordances — plus strengths and weaknesses.
Direct theory of perception — the light reaching the eye already contains all the information needed, so perception happens immediately from that input, with nothing added from memory or guesswork.
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