Colour Vision and Observer Questions and Answers

Answer: A

The optics deliver the image and the retina transduces it. Colour discrimination depends almost entirely on the cones concentrated near the fovea. Optics and transduction are separate functions. The fovea carries the burden of colour vision.

Answer: B

Correct answer: rods for low light vision and cones for colour vision

The duplex retina divides the work. Rods are extremely sensitive but of a single type, so they cannot signal wavelength and give no colour at low light levels. Cones need far more light but come in three types, and comparing their responses is what makes colour vision possible.

Answer: C

Correct answer: 6 million

The retina holds roughly six million cones against something like 120 million rods, so cones are outnumbered twenty to one. They are heavily concentrated in the fovea, where the rods are absent altogether - which is why fine detail and colour both depend on looking directly at a thing.

Answer: D

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. Cone density is at its maximum in the fovea and falls steeply within a few degrees, so the periphery is served largely by rods. Peripheral vision is accordingly good at detecting movement and poor at judging colour, which is why colour assessment is done with the sample looked at directly.

Answer: A

Correct answer: A-2, B-4, C-1, D-3

The four receptors peak at different points. The L cone peaks around 565 nm in the yellow green, despite conventionally being called the red cone; the M cone around 535 nm in the green; the S cone around 440 nm in the blue violet; and the rod around 498 nm in the blue green, which is why the scotopic curve sits shifted towards the blue.

Answer: B

Correct answer: quite close together, within about thirty nanometres

The L and M peaks lie at roughly 565 and 535 nm, an overlap so extensive that neither cone alone distinguishes red from green - the discrimination comes entirely from the ratio of their responses. Their similarity also explains why the two photopigments are so nearly alike genetically, and why red green deficiency is common.

Answer: C

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. The L and M opsin genes sit adjacent on the X chromosome and are highly homologous, so unequal crossing over readily produces hybrid or missing genes. Being on the X, a single defective copy affects a man, which is why red green deficiency reaches about eight per cent of men and under one per cent of women.

Answer: D

They make up only a few per cent of the cone population and are absent from the very centre of the fovea. Fine blue detail is consequently resolved far less well than fine red or green detail. S cones are absent from the very centre of the fovea.

Answer: A

The first two are correct and follow from the retinal anatomy. The third reverses the position, since the L and M cones together account for the great majority of the cones. S cones number only a few per cent of the total.

Answer: B

It peaks near 555 nanometres and defines the photometric quantities. In the CIE system it corresponds exactly to the y bar colour matching function. Photometric quantities all derive from it. It peaks near 555 nanometres in the yellow green.