Perception without guesswork

1. Gibson's Direct Theory of Perception (1966) claims that perception is direct, meaning you pick up information that is already present in the light reaching your eye.
2. Nothing has to be worked out, guessed at or remembered, because the input itself already specifies the world.
3. That makes it a bottom-up account, because perception is driven by the incoming input rather than by the brain.

Sensory input

1. The sensory input Gibson has in mind is the whole pattern of light reaching your eye from every surface around you, which he called the optic array.
2. That array is highly structured, carrying detailed information about edges, surfaces, gaps and distances.
3. It also changes constantly as you move, and Gibson argued the pattern of change carries as much information as the pattern itself.

Optic flow

1. As you move forwards, the whole optic array streams past you, flowing outwards from the point you are heading towards.
2. The faster that flow rushes past, the faster you are travelling, so your speed is specified directly by the input.
3. The single point from which the flow appears to expand stays still and marks exactly where you are aiming, so your direction is specified directly too.

Invariants

1. Invariants are features of the optic array that stay constant however you move around.
2. A texture gradient is one, because a pebble beach looks coarse near your feet and fine in the distance, and that gradient holds whichever way you walk across it.
3. The horizon ratio is another, because the proportion of an object sitting above and below the horizon stays the same however far off it is, so its real size is available straight from the array.

Affordances

1. An affordance is the action an object offers you, read straight off the object rather than worked out about it.
2. A chair affords sitting, a handle affords pulling and a gap in a hedge affords walking through, and Gibson argued you perceive that possibility as directly as you perceive the shape.
3. Affordances depend on the fit between the object and you, so the same flight of stairs affords an easy climb to an adult and a scramble to a toddler.
4. Because the action is available in the object itself, perception and behaviour are joined with no stage of thinking in between.

Why this is a nature account

1. In bottom-up processing, perception is built from the raw input upwards with no help from stored knowledge.
2. Gibson argued you are born ready to pick up this information, so perception does not have to be learned through experience.

Strengths of the theory

1. A strength is the theory's high ecological validity, because it was built from how people perceive while moving through natural settings rather than from still images in a laboratory.
2. A second strength is that it gives a strong account of how you perceive movement, which research using fixed images tends to ignore altogether.

Weaknesses of the theory

1. A weakness is that it struggles with visual illusions, because if the input were enough on its own you should never misjudge a figure whose lines are drawn accurately.
2. A second weakness comes from Haber and Levin (2001), who found that size estimates for familiar objects were about as accurate from memory as when the objects were in view, and that distance information did not improve them.
   1. Size perception is therefore leaning on what you already know about the object, which a purely bottom-up account leaves no room for.

Turning this into marks

1. Name the part of the account, such as optic flow or an affordance, and then say what it lets you perceive without any inference.
2. Give strengths and weaknesses together, because an answer that offers only one side stops short.
3. The mark most often lost is ending without a judgement, so say the account holds for an observer moving through a rich scene but falls short on a deliberately misleading figure.