optical illusion

An optical illusion. Square A is exactly the same shade of grey as square B

The science of optical illusions – what tricking the brain reveals about how our minds work. Sight, hearing, taste, touch, smell. We believe what our senses tell us but most of all we trust our eyes. Without us realizing it, our brains are instantly processing the information they receive to make sense of the world around us. And that has been crucial to our evolution.

Illusions – Illusions are useful as a research tool because they tell us how the brain works, that the brain evolved NOT to see the retinal image (which is made up of meaningless, or ambiguous, patterns of light) – i.e. not the world ‘as it is’ – but to see the world in a way that proved useful in the past. It constructs what it knows by searching for useful patterns in sensory information and then associating those patterns with a past record of their behavioral relevance, and then using that information to guide behavior.

  • visual illusions – cases when the inference process goes wrong
  • brightness
  • color
  • form

octopus camouflage – 3 main types of camouflage:

  • Uniform camouflage, when the octopus’ body is one color, with nearly no contrast in pattern
  • Mottled camouflage, when the octopus’ skin is covered with small scale spots with large contrast (i.e., light and dark)
  • Disruptive camouflage, when the octopus uses large areas of high contrast to visually disrupt their form, making their body difficult to recognize

Benham’s disc – this spinning black and white pattern makes you see imaginary colors
. If the disk is spinning, turn the knob to make it stop. Notice that the pattern on the disk is black and white.
. Increase the disk speed and notice that you see colors on the disk. Experiment with the speed and direction controls to see how they affect the colors
. Ask other people what colors they see.

What’ is going on
Different people see different amounts of yellow, red, green, purple and blue on this spinning disk. Just why some people see color is not fully understood, but the illusion must involve the color vision cells in your eye. The sells come in three varieties. Some are most sensitive to red light, some green light, and some blue light. These different types of color sensors responds at different rates.

When you gaze at one place on the spinning disk, you are looking at alternating flashes of black and white. You see white only when all three color sensors respond to a flash of light. If one type of color sensor responds at a different rate than the others, you see the illusion of color. This spinning disk would appear colored even on a black-and-white television set.
Do It
Challenges for you to work on…