A quiz on the human visual system

Adrian F. Clark

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

   • motion

   • orientation, edge, motion

   • orientation, edge
   • edge, motion

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

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

   • 100--1000 nm

   • 400--700 nm

   • 1000--5000 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.

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

   • laterate geniculate nucleus

   • V1
   • 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.

4. What causes the illusion of the Hermann grid?

   • The inhibitory centre and excitatory surround of the ganglion cells

   • The excitatory centre and inhibitory surround of the ganglion cells

   • V5
   • V4

   Galglion cells have an excitatory centre and inhibitory suround, and this appears to be the root cause of the Hermann grid. (This is similar to a Laplacean mask in convolution.)

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

   • cone
   • retina

   • rod

   • fovea

   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.

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

   • There are more blue-sensitive cones in the fovea
   • Sunlight is much more intense in the blue
   • The rods contribute to the blue response too

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

   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.

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

   • Because ships may not show any other colour lights
   • 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

   • 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.

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
   • The part of the eye that focuses light

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

   • fovea

   • retina

   • cornea
   • optic nerve

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

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

    • V2, V4

    • V2, V5

    • 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.