Nature and Perception of Colour Questions and Answers

Answer: A

Correct answer: the visual system adjusts its sensitivity to the prevailing light

Chromatic adaptation rescales the responses of the three cone types according to the illumination, so that a surface keeps roughly its appearance as the light changes colour. It is the mechanism underlying colour constancy, and it works within the observer - nothing about the object or the light itself changes.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. Tungsten light is markedly yellower than daylight, and on first entering the room the difference is obvious; within a minute or two the cone responses have been rescaled and the room looks normal. The adaptation is in the observer, which is why a camera, having none, records the yellow that was there all along.

Answer: C

Correct answer: True

The camera records the physical stimulus, which under tungsten light really is yellow-biased; the eye had adapted and did not see it that way. This is the everyday demonstration that chromatic adaptation is a property of the observer and not of the scene, and it is why cameras provide a white balance control to imitate what the visual system does automatically.

Answer: D

Correct answer: a colour appears to shift depending on the colour surrounding it

A grey patch on a red ground looks greenish and the same patch on a green ground looks reddish, the induced shift being towards the complement of the surround. It follows from opponent coding, and it is why a colour must always be judged against a neutral surround if the judgement is to mean anything.

Answer: A

Both are true and the reason gives the reason for the practice, and the standard grey is specified. A coloured surround can shift a judgement by a visually significant amount. Contrast effects are large enough to matter commercially.

Answer: B

Staring at red fatigues the long wavelength response and a cyan afterimage follows. The complementary relationship follows directly from the opponent organisation of the visual system. Fatigue of one channel reveals its opposite.

Answer: C

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: the afterimage is approximately complementary and arises from local adaptation of the retinal response to the stimulus. Statement 3 is a misconception - the retina emits no light, and the afterimage is a change in the sensitivity of the receptors and the channels above them, which is why it moves with the eyes.

Answer: D

Correct answer: red against green, yellow against blue, and light against dark

The three cone signals are recombined in the retina into one achromatic and two chromatic opponent channels. The arrangement explains a great deal that trichromacy alone does not: why there are four unique hues rather than three, why no colour is reddish green, and why afterimages and simultaneous contrast take the complementary form they do.

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the cause. Red and green are the two poles of one channel, so a signal on it can be positive or negative but not both at once, and there is no way to represent reddishness and greenness simultaneously. Reddish yellow, by contrast, is orange and is described readily, because red and yellow lie on different channels.

Answer: B

Correct answer: 1 and 2 only

Statements 1 and 2 are correct, and statement 3 is the misconception the pair resolves. The two theories describe different stages: trichromacy the three cone types at the receptors, opponency the recoding of their outputs further along the pathway. They were long thought to be rivals, and the discovery that both are true at their own level settled the argument.