A quiz on the human visual system

Adrian F. Clark

1. 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

   • The rods contribute to the blue response too
   • 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.

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

   • rod

   • fovea
   • retina
   • 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.

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

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

   • filling in missing boundaries

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

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

   • retina

   • cone

   • rod
   • fovea

   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.

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

   • orientation, edge, motion

   • motion
   • edge, motion
   • orientation, edge

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

6. 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
   • To reduce the amount of light pollution from the ship
   • Because ships may not show any other colour lights

   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.

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

   • 400--700 nm

   • 0--100 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.

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

   • cornea
   • optic nerve
   • fovea

   • retina

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

9. What causes the illusion of the Hermann grid?

   • The excitatory centre and inhibitory surround of the ganglion cells

   • V4
   • V5
   • The inhibitory centre and excitatory surround of the ganglion cells

   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.)

10. 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.