Il­lu­sions of grandeur

Popular Science - - CONTENTS -

WHICH OF THE TWO ORANGE cir­cles is big­ger? The one on the right, right? In re­al­ity, the pair are iden­ti­cal.

Brain re­searchers hy­poth­e­size that this ef­fect, known as the Eb­bing­haus il­lu­sion, plays its trick based on how our brains in­ter­pret depth. From a life­time of look­ing at things, we re­mem­ber that smaller items tend to be far­ther away, while larger ones are closer. In this im­age, the brain reads the left ring of large, black cir­cles as nearby and the right, smaller set as dis­tant. Si­mul­ta­ne­ously, the brain also groups the orange cir­cles with their black sur­round­ings. So the left one is also seen as nearby, and the right one far­ther away. When the brain com­pares the orange cir­cles, the only way for the right one to be more dis­tant is if it’s larger than the left cir­cle.

But not ev­ery­one is so eas­ily fooled by this il­lu­sion. Neu­ro­sci­en­tists and psy­chol­o­gists have two the­o­ries about why. First, they’ve found that in­di­vid­u­als whose pri­mary vis­ual cor­tex cov­ers a larger sur­face area—which varies among in­di­vid­u­als as much as three­fold—are less likely to fall for the il­lu­sion. The re­searchers sur­mise that’s be­cause the neu­ron con­nec­tions that com­pare the in­ner orange cir­cle to the outer black ones get weaker as the pri­mary vis­ual cor­tex gets larger.

A per­son’s daily sur­round­ings also af­fect their per­cep­tion. Stud­ies have found that peo­ple from ru­ral re­gions, who aren’t con­stantly bom­barded by vis­ual distractions such as mov­ing ve­hi­cles, traf­fic lights, and flash­ing street signs, are less sus­cep­ti­ble to the il­lu­sion than city dwellers are. Hav­ing a broader view does have an up­side, how­ever: It means peo­ple can spot speed­ing buses be­fore they hit them. The down­side is that they’re less at­tuned to finer de­tails, and more primed for Eb­bing­haus.

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