PSYC10003 · Mind, Brain and Behaviour 1
Motion and Colour Perception
Week 10 covers how the visual system extracts motion (real and apparent motion, motion aftereffects, the aperture problem) and colour. It teaches the two-stage account of colour vision — trichromatic cone theory at the receptor level and opponent-process theory at later stages — and how they jointly explain colour mixing, negative afterimages and impossible colours. The examinable skill, within the 25% Sensation & Perception block, is using both colour theories together and interpreting motion illusions as evidence about perceptual mechanisms.
What this chapter covers
- 01Functions of motion perception; akinetopsia (loss of motion perception, patient L.M.)
- 02Real vs illusory motion: apparent motion, motion aftereffects (waterfall illusion), induced motion
- 03Korte's third law and the aperture/barber-pole problem
- 04Physics of colour: visible light ~400-700 nm; additive (light) vs subtractive (paint) mixing
- 05Trichromatic theory: three cone types (S ~419 nm, M ~531 nm, L ~558 nm); colour from relative cone activity; metamers
- 06Colour deficiency: monochromatism; dichromatism (protanopia/deuteranopia/tritanopia)
- 07Opponent-process theory: red-green, blue-yellow, black-white channels; negative afterimages; impossible colours
- 08Colour constancy: chromatic adaptation and discounting the illuminant
Applying the two-stage theory of colour vision
- +1(a) Name the afterimage. Prolonged viewing of red fatigues the red response; on a white field the opponent partner dominates, so the negative afterimage is green. This is explained by opponent-process theory (the post-receptor stage), where colour is coded on a red-green channel.
- +1(b) Explain the impossible colour. In opponent-process coding, red and green are opposite ends of one channel that cannot signal both at once, so 'reddish-green' cannot be experienced (likewise bluish-yellow). Mixtures like purple (bluish-red) and orange (yellowish-red) pair colours from different channels and are seen.
- +1(c) Locate the three cone types. Having exactly three cone types (S, M, L) is the receptor (trichromatic) stage; colour is coded there by the relative activity across the three cones.
- +1Define a metamer. A metamer is a pair of physically different light spectra that look identical because they produce the same pattern of activation across the three cone types — the basis of colour matching. So the two stages are complementary: trichromacy at the cones, opponency in the cortex.
Key terms
- Apparent motion
- The perception of motion from stationary images shown in succession (the basis of film); governed by Korte's third law, which links the spatial separation and the timing needed to see smooth motion.
- Motion aftereffect
- The 'waterfall illusion': after prolonged viewing of motion in one direction, a stationary scene appears to drift in the opposite direction, revealing direction-selective adaptation.
- Trichromatic theory
- The receptor-level account: three cone types (short/medium/long, roughly blue/green/red) whose relative activity codes colour; explains colour matching and metamers.
- Opponent-process theory
- The post-receptor account: cone signals feed three opponent channels (red-green, blue-yellow, black-white), explaining negative afterimages and why reddish-green and bluish-yellow cannot be seen.
- Metamer
- Two physically different light spectra that appear identical because they produce the same pattern of activation across the three cone types.
- Colour constancy
- The tendency for an object's perceived colour to stay stable despite changes in illumination, achieved through chromatic adaptation and discounting the illuminant.
Motion and Colour Perception FAQ
Why are two theories of colour vision both correct?
Because they describe different stages. Trichromatic theory is right at the receptor level — there really are three cone types whose relative activity codes colour. Opponent-process theory is right at the post-receptor level — cone signals are combined into red-green, blue-yellow and black-white channels. The exam expects you to use both together, not to choose one.
How do I predict the colour of a negative afterimage?
The afterimage is the opponent partner of the colour you adapted to: red gives green, green gives red, blue gives yellow, yellow gives blue. Prolonged viewing fatigues one pole of an opponent channel, so a subsequent white field drives the opposite pole more strongly.
Why can't we see reddish-green or bluish-yellow?
Because red-green and blue-yellow are single opponent channels that cannot signal both poles at once — the two cancel. This is direct evidence for opponent-process theory, and it is why colours like purple (bluish-red) and orange (yellowish-red), which pair different channels, are perfectly visible.
What is the difference between additive and subtractive colour mixing?
Additive mixing combines lights, adding wavelengths — red plus green light gives yellow, and all three primaries give white. Subtractive mixing combines paints, each of which absorbs (subtracts) wavelengths — mixing blue and yellow paint leaves green because each absorbs what the other reflects.
What is the aperture problem?
When a moving line is viewed through an aperture so its endpoints are hidden, its true direction is ambiguous and its perceived motion is captured by the motion of the visible terminators. Seeing the actual line ends resolves it; the barber-pole illusion is the same principle.
Exam move
Treat colour vision as a two-stage machine and practise applying both stages to a single example: three cones and relative activity at the receptor level, three opponent channels at the cortical level. Drill the two reliable item types — predicting a negative afterimage (opponent partner) and explaining an impossible colour (opponent cancellation) — and keep additive vs subtractive mixing straight. For motion, hold a short list of illusions (apparent motion, motion aftereffect, aperture/barber-pole) each paired with the mechanism it demonstrates. This is a conceptual MCQ block, so rehearse predictions rather than definitions. Confirm exam details on Canvas.
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