6 Perception · 6.1 Perception
6.1.2 Perceptual organisation, constancy, depth, colour and influences on perception
Perception turns raw sensations into organised, meaningful, stable experience: we group parts into wholes (Gestalt laws, figure-ground), judge distance from depth cues, see colour through three cone types and opponent channels, keep objects the same size, shape and colour as conditions change (constancy), are sometimes fooled (illusions), and see what our needs, expectations, learning and culture prepare us to see (perceptual set).
- APPSC Paper I
- Standard level
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Core facts
- Perceptual organisation: the mind groups sensory elements into wholes (form) before we notice the parts. Studied first by the Gestalt school (Wertheimer, Köhler, Koffka); the laws were set out by Max Wertheimer (1923). Gestalt background (meaning of the word, founders, insight) is in EDU-LR-04.
- Gestalt laws of grouping: proximity, similarity, closure, continuity (good continuation), common fate (things moving together are grouped), symmetry, and the master law Prägnanz (simplicity, "good figure"). Classroom examples are in EDU-LR-04.
- Figure-ground: we see a figure (shaped, in front, "thing-like") standing out from a formless ground (background). Danish psychologist Edgar Rubin (1915) studied it; his vase-faces drawing is a reversible figure. Another reversible figure: the Necker cube (1832).
- Depth perception = seeing the world in three dimensions and judging distance, though the retinal image is two-dimensional.
- Binocular cues (need both eyes): retinal disparity (each eye gets a slightly different image; the brain fuses them = stereopsis) and convergence (eyes turn inwards for near objects).
- Monocular cues (one eye is enough):
- Pictorial: relative size, interposition (overlap), linear perspective (parallel lines meet in the distance), texture gradient, aerial (atmospheric) perspective (far things hazy and bluish), height in the plane (relative height), light and shadow;
- Movement: motion parallax (near objects seem to rush past, far ones move slowly);
- Physiological: accommodation (the lens changes shape to focus).
- Visual cliff: Eleanor Gibson and Richard Walk (1960); most infants of about 6–14 months refused to crawl over the "deep" side, so depth perception is present early.
- Retina: rods (about 120 million; dim light, black-white, night vision) and cones (about 6 million; bright light, colour and fine detail; packed in the fovea).
- Colour vision: normal-sighted people have three types of cones: S (short wavelength, "blue"), M (medium, "green"), L (long, "red"). Visible light is about 400–700 nm.
- Three dimensions of colour: hue (set by wavelength), saturation (purity), brightness (intensity).
- Colour theories:
- Trichromatic (Young-Helmholtz): Thomas Young (1802), Hermann von Helmholtz (1850s); three receptor types, every colour = a mix of their outputs;
- Opponent-process: Ewald Hering (1870s); three opposing pairs: red–green, blue–yellow, black–white; explains negative afterimages;
- Dual-process (zone) theory: Hurvich and Jameson (1957); trichromatic at the cones, opponent-process beyond them (ganglion cells, thalamus).
- Colour blindness: mostly red-green, inherited on the X chromosome, so far commoner in men (roughly 8%) than women (under 1%); tested with Ishihara plates (1917). One cone type missing = dichromat; none working = monochromat.
- Time perception: no sense organ for time; it is judged from mental events and an internal "clock". Vierordt's law (1868): short intervals are overestimated, long ones underestimated.
- Perceptual constancy: perceiving objects as stable (same size, shape, brightness, colour) though the retinal image changes. Types: size, shape, brightness (lightness), colour, and location (position) constancy.
- Illusion: a wrong perception of a real stimulus, shared by most people (unlike a hallucination, which has no stimulus). Well-known: Müller-Lyer (1889), Ponzo (1911), Ames room (1946), moon illusion, phi phenomenon (Wertheimer, 1912, apparent motion), autokinetic effect.
- Perceptual set: a readiness to perceive in a particular way, created by expectation, motivation, emotion, past experience, values and culture.
- Who said what: J.J. Gibson: direct (ecological) perception, texture gradients, affordances (asked as "Perception theory" in 2023). Richard Gregory: perception as hypothesis (top-down, constructive).
Explained
1. Form perception.
- Form needs a contour (a sharp change in brightness or colour) to separate figure from ground. Camouflage works by breaking up contours so the figure merges with the ground.
- Figure vs ground: the figure looks nearer, has a shape and is remembered; the ground looks behind, shapeless and continues behind the figure. In a reversible figure the two swap, but you cannot see both at once.
- Grouping laws decide which parts form one figure: dots close together (proximity) or alike (similarity) form rows; a broken circle is seen whole (closure); crossing curves are seen as two smooth lines (continuity); birds flying together are one flock (common fate).
- Subjective (illusory) contours: in the Kanizsa triangle (1955) we "see" a white triangle that has no drawn edges: closure at work.
- Brain basis: Hubel and Wiesel (Nobel Prize 1981) found feature detector cells in the visual cortex of cats that fire to lines of a particular orientation, edges and movement: the building blocks of form.
2. Depth cues at a glance.
| Cue | Type | How it works |
|---|---|---|
| Retinal disparity | Binocular | Bigger difference between the two images = nearer |
| Convergence | Binocular | Strain of eye muscles turning inwards; near objects only |
| Accommodation | Monocular, physiological | Lens bulges for near objects; works within a couple of metres |
| Motion parallax | Monocular, movement | Near things seem to move fast and opposite; far ones slow |
| Relative size | Monocular, pictorial | Of two like objects, the smaller image looks farther |
| Interposition | Monocular, pictorial | The object that covers another is nearer |
| Linear perspective | Monocular, pictorial | Parallel lines (railway tracks) meet in the distance |
| Texture gradient | Monocular, pictorial | Texture grows finer and denser with distance |
| Aerial perspective | Monocular, pictorial | Distant hills look hazy and bluish |
| Height in plane | Monocular, pictorial | Below the horizon, higher in the field = farther |
| Light and shadow | Monocular, pictorial | Shading shows bumps and hollows |
Painters use only the pictorial cues; that is why they are also called artistic or secondary cues. A stereoscope (Wheatstone, 1838) and 3-D films create depth by giving each eye a different picture (disparity).
3. Colour vision explained.
- Trichromatic theory explains colour mixing and most colour blindness (a missing cone type).
- It cannot explain why we never see a "reddish green" or a "bluish yellow", or why staring at a green square leaves a red afterimage (yellow leaves blue). Opponent-process theory explains these: tiring one member of a pair lets the other dominate.
- Both are right at different levels: three cone types in the eye, opponent channels in the cells beyond them (dual-process / zone theory).
| Type of colour defect | What is missing | Result |
|---|---|---|
| Protanopia | L ("red") cones | Red-green confusion, reds look dark |
| Deuteranopia | M ("green") cones | Red-green confusion; green (deutan) defects are the commonest, mostly in milder form |
| Tritanopia | S ("blue") cones | Blue-yellow confusion; rare |
| Monochromacy | All but one or no cones | Sees only shades of grey |
4. Time perception.
- Felt time depends on what fills it and on attention to time: a boring wait (attention on the clock) feels long; an absorbing task makes "time fly".
- Looking back, the rule reverses: an eventful holiday seems long in memory, an empty week short (more memories = longer remembered duration).
- Arousal and body temperature speed the inner clock, so time seems to drag (fever, fear). Drugs such as stimulants have a similar effect; depressants the opposite.
- William James called the brief stretch we experience as "now" the specious present.
5. Perceptual constancies.
| Constancy | The retinal image changes... | ...but we perceive |
|---|---|---|
| Size | A friend walking away casts a smaller image | Same-sized friend |
| Shape | An opening door projects a trapezium | A rectangular door |
| Brightness | White paper in shade reflects less light than coal in sunlight | Paper still white, coal still black |
| Colour | A red apple under yellow or blue light | Still red |
- Size constancy depends on distance cues. Take the cues away (looking through a tube, or across a featureless plain) and size is judged from the retinal image alone, so constancy breaks down.
- Brightness constancy depends on the ratio of light from the object and its surroundings, not on the absolute amount.
- Location (position) constancy: the world seems still when we move our head or eyes, though the image sweeps across the retina.
6. Illusions.
| Illusion | What is seen | Usual explanation |
|---|---|---|
| Müller-Lyer | Line with outward fins looks longer than an equal line with inward arrowheads | Fins read as the inner and outer corners of a room or building (depth cues), so constancy scaling misfires (Gregory) |
| Ponzo (railway) | Of two equal bars between converging lines, the upper one looks longer | Converging lines suggest distance; the "farther" bar is scaled up |
| Ames room | A person in one corner looks a giant, in the other a dwarf | The room is trapezoid but looks rectangular through a peephole; size constancy is fooled |
| Moon illusion | The moon looks larger near the horizon | The horizon moon seems farther (terrain cues), so it is perceived larger |
- Motion illusions: phi phenomenon (two lights flashed one after the other look like one light moving; the basis of neon signs and films; Wertheimer 1912 started Gestalt psychology with it); autokinetic effect (a still point of light in a dark room seems to move; Sherif used it to study group norms); induced motion (the moon seems to race behind moving clouds).
- Illusion vs hallucination vs delusion: illusion = misperceiving a real stimulus; hallucination = perceiving without any stimulus; delusion = a fixed false belief.
7. Influences on perception (perceptual set).
| Influence | Classic study | Finding |
|---|---|---|
| Expectation (context) | Bruner and Minturn (1955) | An ambiguous figure was read as "B" among letters and as "13" among numbers |
| Prior exposure | Bugelski and Alampay (1961) | After animal pictures, the ambiguous "rat-man" was seen as a rat |
| Need (motivation) | McClelland and Atkinson (1948) | Hungry sailors "saw" more food objects in dim, blank slides |
| Value | Bruner and Goodman (1947) | Poor children overestimated coin sizes more than rich children |
| Emotion | McGinnies (1949) | Taboo words needed longer exposure to be recognised (perceptual defence) |
- Selective perception: among many stimuli we notice those linked to our needs and interests (the hungry person notices restaurants).
- Learning: perception is partly learnt. Adults who gained sight after cataract operations (Senden's cases, 1932) could see colour and figure-ground at once but needed weeks to recognise shapes by sight. Kittens moved passively in Held and Hein's kitten carousel (1963) did not develop normal visually guided behaviour; active movement was needed. People wearing inverting goggles (Stratton, 1890s) slowly adapted.
- Culture: Segall, Campbell and Herskovits (1966) found people from "carpentered" worlds (rectangular rooms, straight edges) more prone to the Müller-Lyer illusion than people living in round huts and open country. Turnbull's (1961) forest-dwelling BaMbuti guide, Kenge, seeing distant buffalo across open plains for the first time, took them for insects: size constancy at long distance needs experience. Hudson (1960) found that some African groups without exposure to pictures did not read pictorial depth cues.
- Personality and attitudes: a prejudiced person notices behaviour that fits the stereotype (link: person perception in the social-behaviour notes).
Common traps
- ✗ "The visual cliff is J.J. Gibson's." ✓ Eleanor J. Gibson (with Richard Walk, 1960); J.J. Gibson is the direct (ecological) perception theorist.
- ✗ "Trichromatic theory explains afterimages." ✓ Opponent-process (Hering) explains afterimages; trichromatic explains colour mixing.
- ✗ "Three cone types are called depth vision (or space vision)." ✓ They give colour vision.
- ✗ "Rods see colour." ✓ Cones see colour and detail; rods work in dim light, in black and white.
- ✗ "Convergence and accommodation are both binocular cues." ✓ Convergence is binocular; accommodation works with one eye.
- ✗ "Motion parallax is a binocular cue." ✓ It is monocular (a movement cue).
- ✗ "Perceptual constancy means the retinal image stays the same." ✓ The image changes; the perceived size, shape or colour stays stable.
- ✗ "An illusion is perceiving something that is not there." ✓ That is a hallucination; an illusion misperceives a real stimulus.
- ✗ "In Müller-Lyer, the line with arrowheads pointing in looks longer." ✓ The line with fins pointing outwards looks longer.
- ✗ "Phi phenomenon is real movement." ✓ It is apparent movement (Wertheimer, 1912).
Quick revision
- Gestalt laws (Wertheimer 1923): proximity, similarity, closure, continuity, common fate, Prägnanz; figure-ground (Rubin's vase).
- Binocular cues: retinal disparity, convergence. Monocular: pictorial cues, motion parallax, accommodation.
- Visual cliff: Eleanor Gibson and Walk, 1960. Direct perception, affordances: J.J. Gibson.
- Rods = dim light; cones = colour (S, M, L); fovea is all cones.
- Young-Helmholtz = three receptors; Hering = opponent pairs, afterimages; zone theory joins them.
- Red-green colour blindness, X-linked, commoner in men; Ishihara plates.
- Constancies: size, shape, brightness, colour, location; size constancy needs distance cues.
- Illusions: Müller-Lyer, Ponzo, Ames room, moon; phi = apparent motion; autokinetic = still light seems to move.
- Perceptual set: expectation (B/13), need (hungry sailors), value (coin sizes), emotion (perceptual defence).
- Culture: carpentered world → more Müller-Lyer; Kenge's "insect" buffalo; Vierordt: short times overestimated.
Go deeper: Deeper (beyond what APPSC Paper I usually asks)
Deeper Advanced
1. Bottom-up and top-down together. Gibson argued that the light reaching the eye carries all the information needed (optic flow, texture gradients, affordances: what an object lets us do), so perception is direct, bottom-up. Gregory argued the input is often incomplete, so the brain tests hypotheses from knowledge (top-down); illusions and perceptual set support him. Modern accounts combine both: features flow up, expectations flow down.
2. Recognising objects. Template matching fails because objects appear in endless sizes and angles. Feature theories (Selfridge's Pandemonium, 1959) and recognition by components (Biederman, 1987: about 36 basic 3-D shapes, geons) explain recognition from parts and their relations, which echoes the Gestalt point that relations matter.
3. Infant studies refined. Later work (Campos and colleagues) found that infants of about 2 months, too young to crawl, show slower heart rate (interest, not fear) on the deep side of the visual cliff; fear of the drop appears with crawling experience. So depth perception is early, but the fear is learnt through movement.
4. Critical periods. Blakemore and Cooper (1970) reared kittens in cylinders of only vertical stripes; as adults they behaved as if blind to horizontal lines, and their cortical cells responded mainly to vertical lines. Early visual experience tunes the feature detectors.
5. Limits of the culture findings. The carpentered-world effect is real but modest, and later studies linked part of it to differences in retinal pigmentation and to education with pictures. Exams accept the classic conclusion: experience and environment shape susceptibility to illusions.
Practise this topic. Questions on Perceptual organisation, constancy, depth and colour; factors affecting perception, with answers explained.
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