A quiz on the human visual system

Adrian F. Clark

1. What type of cell is responsible for vision in low-light conditions?

   • cone
   • fovea

   • rod

   • retina

   Rods are responsible for low-light vision as they are much more responsive to light than cones. The fovea is a region of the retina, the surface that coats the inside of the eyeball.

2. What is the name of the rear surface of the eye?

   • cornea
   • fovea
   • optic nerve

   • retina

   The rear surface of the eye, which is coated in light-sensitive cells, is known as the retina.

3. What type of cell is responsible for colour vision?

   • cone

   • fovea
   • rod
   • retina

   There are three types of cone, with peak responses in the red, green and blue regions of the spectrum; hence, they are the cells that allow humans to sense colour. The fovea is a region of the retina, the surface that coats the inside of the eyeball.

4. What is "continuity" in the context of visual processing?

   • a derivative exists

   • filling in missing boundaries

   • a continuous stream of pixels
   • an analogue signal

   Continuity in this context means making the boundary of a region complete.

5. Roughly, what is the range of wavelengths of visible light?

   • 0--100 nm

   • 400--700 nm

   • 1000--5000 nm
   • 100--1000 nm

   The visible wavelengths lie roughly in the range 400 nm (blue) to 700 nm (red). The red light from a He-Ne laser is about 650 nm, for example.

6. To which of the following do the cells in V1 respond to?

   • orientation, edge, motion

   • edge, motion
   • motion
   • orientation, edge

   V1 cells respond to all three stimuli; finding this out won the Nobel prize in 1950.

7. Why are the decks of ships usually illuminated using red light at night?

   • So that the dark-adaption of the rods is affected as little as possible

   • So that only one type of cone is used, leaving the others for looking out
   • Because ships may not show any other colour lights
   • To reduce the amount of light pollution from the ship

   The aim is to keep the rods as sensitive as possible; as rods are more sensitive towards the blue, using red light affects them least.

8. What is the 'fovea?'

   • Where the optic nerve leaves the eye
   • The part of the eye that focuses light
   • The part of the eye that controls how much light is admitted into the eye

   • A depression in the retina which has many more cones than elsewhere

   The fovea is the depression or hollow in the retina where the concentration of cones is highest; this is what humans naturally turn their heads to focus the light onto.

9. Signals from the eye travel via the optic nerve to which of the following?

   • V1

   • laterate geniculate nucleus

   • visual striate cortex
   • V4

   The optic nerve travels to the laterate geniculate nucleus, though about 10% also go to the superior colliculus, where the signal from the two eyes are mixed.

10. Blue cones are much less responsive than red- and green-sensitive ones. How does the brain handle this?

    • The rods contribute to the blue response too

    • The brain amplifies the blue signal much more than the red and green ones

    • Sunlight is much more intense in the blue
    • There are more blue-sensitive cones in the fovea

    The evidence that we have suggests that the signal from the blue-sensitive cones is amplified much more than those from the other types of cone.