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How Many Dots Can You See at Once?

sue_frantz
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This extinction illusion (Hermann grid variation) has been making the rounds on social media courtesy of a Facebook post by psychological scientist Akiyoshi Kitaoka. There are 12 black dots in this grid. Most people cannot see them all at once.

The illusion itself first made an appearance in a 2000 journal article by Ninio & Stevens. Check out their paper for other illusions.

Ninio Extinction Illusion

The larger lesson for your students: Our senses, including vision, allow our brains to create a representation of the world around us. Our senses do not allow our brains to generate a perfect replication of the world.

For students who want to know why the illusion works, well, that’s a little more challenging. The short answer is that our retinas are hard-wired to send the clearest, sharpest signals to the brain. Receptor cells that get the strongest signal block out the weaker signals. Those dots in the periphery get blocked by the grey that surround them thanks to the sparser rods in the periphery.

For the longer answer, read up on lateral inhibition. Wikipedia provides a nice summary. (Yes, lateral inhibition is also used to explain Mach bands.)

If you want to wade into this even deeper with your students, Wesley Jordan (St. Mary’s College of Maryland) has created a class activity that should help students understand lateral inhibition.

 

References

Ninio, J. & Stevens, K.A. (2000). Variations on the Hermann grid: An extinction illusion. Perception, 29, 1209-1217.

About the Author
Sue Frantz has taught psychology in community colleges since 1992, and has been at Highline College in the Seattle area since 2001. She has served on several APA boards and committees, and was proud to serve the members of the Society for the Teaching of Psychology as their 2018 president. In 2013, she was the inaugural recipient of the APA award for Excellence in the Scholarship of Teaching and Learning at a Two-Year College or Campus. She received in 2016 the highest award for the teaching of psychology--the Charles L. Brewer Distinguished Teaching of Psychology Award . She presents nationally and internationally on the topics of educational technology and the pedagogy of psychology. She is co-author with Doug Bernstein and Steve Chew of Teaching Psychology: A Step-by-Step Guide, 3rd ed.