A quiz on the human visual system

Adrian F. Clark

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

   • retina

   • rod

   • fovea
   • cone

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

   • edge, motion

   • orientation, edge, motion

   • motion
   • orientation, edge

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

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

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

   • Sunlight is much more intense in the blue
   • The rods contribute to the blue response too
   • 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.

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

   • 1000--5000 nm

   • 400--700 nm

   • 100--1000 nm
   • 0--100 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.

5. Where does the "where" or "how" pathway run?

   • V2, V5

   • V2, V4
   • V2, V3, V4, V5
   • V2, V3

   This pathway is the dorsal stream, running from V2 to V5 (and thence to the posterior parietal cortex). It is associated with the representation of objects and control of the eyes and limbs.

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

   • a continuous stream of pixels
   • a derivative exists
   • an analogue signal

   • filling in missing boundaries

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

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

   • retina

   • cornea
   • fovea
   • optic nerve

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

8. What is the 'fovea?'

   • Where the optic nerve leaves the eye
   • 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 part of the eye that focuses light

   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?

   • laterate geniculate nucleus

   • visual striate cortex
   • V1
   • 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. Why are the decks of ships usually illuminated using red light at night?

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

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

    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.