From Photons to Spikes: the Retina · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Your eye reports edges, not light

Science · The Human Body · ages 19-20
Name ______________________   Date ____________

  1. Which order carries the signal out of the eye?

    • Photoreceptor to bipolar to ganglion to optic nerve
    • Ganglion to bipolar to receptor
    • Optic nerve to retina to brain

  2. A flash of light hits a dark adapted photoreceptor. What happens to its membrane?

    • It fires a spike
    • It hyperpolarises, turning more negative
    • Nothing changes

  3. Which receptors handle a dim moonlit path?

    • Neither, eyes switch off
    • Cones
    • Rods

  4. Why does a faint star appear when you look slightly aside?

    • Rods crowd the periphery and catch dim light
    • The fovea sees best at night
    • The blind spot amplifies faint light

  5. A bipolar cell views an evenly lit white wall. How strongly does it reply?

    • Maximally
    • Barely at all, with no contrast to report
    • Only in colour

  6. What job do horizontal cells do?

    • They carry output to the brain
    • They detect photons
    • They wire the surround to cancel the centre

  7. Two grey patches share one shade, yet the bordered one looks darker against bright rings. What explains the illusion?

    • The retina reports centre surround contrast, not absolute brightness
    • The cones change shade
    • The optic nerve delays signals

  8. Photoreceptors sit silent in the dark and fire when lit, like most sensory cells.

    Circle one:   True   False

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Answer key

For grown-ups. Fold this page away before handing over the rest.

Your eye reports edges, not light W1-mt_kviN7WskmP-s1

  1. Photoreceptor to bipolar to ganglion to optic nerve · Receptors feed bipolars, bipolars feed ganglion cells, and their axons form the optic nerve.
  2. It hyperpolarises, turning more negative · Light closes channels and drives the cell more negative instead of firing it.
  3. Rods · Rods are exquisitely sensitive and own dim night vision.
  4. Rods crowd the periphery and catch dim light · The rod rich periphery outsees the cone packed fovea in dim light.
  5. Barely at all, with no contrast to report · Centre matched by surround cancels out, so uniform light earns no reply.
  6. They wire the surround to cancel the centre · Horizontal cells bridge receptors to bipolars, building the cancelling surround.
  7. The retina reports centre surround contrast, not absolute brightness · Centre surround cells report differences, so the ring shifts the patch you see.
  8. False · They stay active in darkness and quiet down, hyperpolarising in light.
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