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Seeing Spots

In this demonstration, students see imaginary spots and relate this illusion to the structure of the retina.

The Hermann Grid is an illusion based on the way our eyes work. Most people "see" grey spots where lines of white space intersect, even though there is no grey there at all.

This illusion is an example of lateral inhibition–a feature of our visual system. Light sensitive cells, or rods, are arranged in rows on the retina. It is possible that light hits just one rod, Rod X, which sends a signal to the brain. But if light also hits Rod X's neighbours, Rod X's signal won't be as strong. It's as though the sensitivity of Rod X is 'turned down' when its neighbours also receive light.

In Hermann's Grid, the places where the white lines cross have white surroundings in four different directions. The rods getting light from the middle of the intersection have neighbour cells in every direction that also get lots of light. So the middle rods are 'turned down' and the intersection looks darker.

These spots don't generally appear directly where you're looking but rather a little off to the side. This is due to the fact that there are no rods cells directly in the centre of the retina. It also explains why dim stars tend to 'disappear' when we look straight at them and look brighter when we look slightly beside them.

Objectives

  • Describe the cause and effect of an optical illusion.

Materials

  • Per Class:
    Hermann Grid template
    , printed on either an overhead sheet or on a large sheet of paper
    overhead projector (optional)

Key Questions

  • What do you see?
  • Are the gray dots really there?
  • Why do you think they appear? Or disappear, when focus is shifted or distance changed?

What To Do

  1. Place the large Herman Grid template at eye level in a visible central point at the front of the room. More than one grid station is ideal, if space permits.
  2. Have the students stand facing the grid, centered,  0.5m from the grid.
  3. Have students stare centrally at the grid and report what they see. Do they report grey smudges in any areas of the grid? Where?
  4. If students reported seeing grey smudges, then instruct them to rate the intensity level of the smudges from 1 (barely visible) to 10 (strong intensity/visibility)
  5. Have students focus on the left, then right sides of the grid. Do the effects they see change?
  6. Have students record differences in what they saw when focused on different areas of the grid.
  7. Have students vary the distance from the Herman Grid and repeat the experiment (1m, 1.5m, 2m). Do the effects they see change?
  8. Have students record differences in what they saw with differing distances.
  9. Report the difference in “grey snudge” intensity level that was rated by each student on the scale from 1 (barely visible) to 10 (definitely visible).

Extensions

  • Try varying the distance from the grid. Do the effects change?
  • Calculater the Mean intensity level for each distance using student data.
  • Visually display the results by creating bar graphs for each condition to display the mean intensity ratings for the condition levels.