Can You Believe Your Eyes?

Visual Illusions:  Finding your Blind Spot

Today, the 3rd and 4th grade scientists at CHA had an opportunity to experiment on themselves! We introduced how vision works through the use of visual illusions. We first learned some basic structure and anatomy of the eye and then focused on function: why and how we see.  Students learned that the eye focuses light like a camera onto the retina, an array of light-responsive cells in the back of the eye. Specialized cells in the retina, called rods and cones, respond to light and send signals to the brain through the optic nerve. Students then did two activities that showed how the brain can be tricked into seeing images that are not really there.

In the first activity, the students looked at negative images projected onto a screen for 10 seconds. When they looked away, they saw a positive afterimage, like the spots in your eyes after flash photography.  Afterimages are the result of specific photoreceptors getting “tired” and needing time to recover before being able to respond again.

In the second activity, the students located the blind spot in their visual field by covering one eye and looking at a paper with strategically placed markings. The blind spot is the area of the retina where the axons of the optic nerve exit the eye on their way to the brain. It is devoid of photoreceptors. We were all impressed with how well our brains fill in the “missing” information with the information that is adjacent to it.

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