Processing approaches
Perception means organising and giving meaning to sensations.
- Bottom-up processing: start with small features (lines, edges, colours) and build up the whole object.
- Top-down processing: start with knowledge and expectation, then check the details. This is why we read a word even with a letter missing.
In real life both run together. The steps are: noticing, recognising, then understanding the object.
The perceiver
We are not cameras. What we perceive depends on us too:
- Motives and needs: a hungry person spots food quickly.
- Expectations (set): we see what we expect to see.
- Cognitive style and past experience: some people notice details, others the whole picture.
- Culture and learning: what we grow up with trains our eyes.
- Mood and emotion: a scared person sees a rope as a snake.
Principles of perceptual organisation
Gestalt psychologists said: the whole is different from the sum of its parts. The brain groups parts using these rules:
- Figure and ground: one part stands out as the object (figure) and the rest becomes background.
- Proximity: things close together are seen as a group.
- Similarity: things alike in colour or shape group together.
- Continuity: we follow smooth lines rather than sharp breaks.
- Closure: we fill gaps to see a complete shape.
- Common region and simplicity: things inside one boundary group; we prefer the simplest reading.
Depth perception
The eye's image is flat, yet we see a 3D world using depth cues.
Monocular cues (one eye is enough)
- Relative size: smaller looks farther.
- Overlap (interposition): the thing in front hides the one behind.
- Linear perspective: parallel lines seem to meet far away.
- Texture gradient: texture looks finer with distance.
- Aerial perspective: far hills look hazy and bluish.
- Light and shadow; motion parallax: near things move fast past a train window, far ones slowly.
Binocular cues (need two eyes)
- Retinal disparity: each eye gets a slightly different picture.
- Convergence: eyes turn inward for near objects.
- Accommodation: the lens changes shape to focus (a helper cue).
Perceptual constancies
Constancy means we see objects as stable even when the image on the eye changes.
- Size constancy: a friend walking away does not seem to shrink.
- Shape constancy: an opening door still looks rectangular.
- Brightness constancy: white paper looks white in sun and shade.
Illusions and culture
An illusion is a perception that does not match the real stimulus, and it happens to most people every time.
- Müller-Lyer illusion: two equal lines, one with fins pointing out, one with fins pointing in; the first looks longer.
- Apparent movement (phi phenomenon): lights blinking one after another seem to move, as in cinema and festival lights.
Culture: people raised among straight walls and corners fall for the Müller-Lyer illusion more than people raised in round huts and open land. Ecology and learning shape how we see depth, pictures and colour words.
Key formulas and definitions
- Perception: organising and interpreting sensations
- Bottom-up: parts → whole; Top-down: idea → details
- Gestalt: figure-ground, proximity, similarity, continuity, closure
- Monocular cues: size, overlap, linear perspective, texture, aerial, motion parallax
- Binocular cues: retinal disparity, convergence
- Constancy: size, shape, brightness
- Illusion: perception that does not match reality
Worked examples
1. You read "Th_ c_t sat on the m_t" easily. Which processing is this?
Top-down processing. Your knowledge of language fills in the missing letters.
2. From a moving bus, trees near the road rush past but far hills move slowly. Which depth cue?
Motion parallax, a monocular cue: near things seem to move faster than far things.
3. A plate on a table looks round though the eye sees an oval. Name the idea.
Shape constancy. The brain keeps the known shape stable.
4. In a painting, a hill behind a hut is partly hidden by the hut. What cue makes the hill look farther?
Overlap (interposition), a monocular cue.
Common mistakes
- Thinking perception is just like a camera photo. The perceiver's needs, set and culture shape it.
- Mixing up monocular and binocular cues. Linear perspective and relative size work with one eye.
- Calling an illusion a hallucination. An illusion misreads a real stimulus; a hallucination has no stimulus.
- Thinking Gestalt means only closure. It includes figure-ground, proximity, similarity and continuity too.