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88 Vi­su­al Phe­nom­e­na & Op­ti­cal Il­lu­sions  (Vi­su­al Il­lu­sion · Op­tis­che Täuschung)

by Michael Bach

These pages demon­strate vi­su­al phe­nom­e­na, and »op­ti­cal« or »vi­su­al il­lu­sions«. The lat­ter is more ap­pro­pri­ate, be­cause most ef­fects have their basis in the vi­su­al path­way, not in the op­tics of the eye. When I find the time I will ex­pand the ex­pla­na­tions, to the de­gree that these phe­nom­e­na are re­al­ly un­der­stood; any nice and thought­ful com­ment wel­come.

You may be glad to know: There are no taste­less sur­pris­es here; you know, these in­fan­tile pranks where you scru­ti­nize a pic­ture and sud­den­ly an ugly face (or the op­po­site end) screams at you…

Don’t dis­tress your­self if you don’t see the ef­fect de­scribed, even if try­ing care­ful­ly. For many il­lu­sions, there is a small per­cent­age of peo­ple with per­fect­ly nor­mal vi­sion who just don’t see it, for rea­sons cur­rent­ly un­known.

Most vis­i­tors of this site are not vi­sion sci­en­tists, so you might find the ex­plana­to­ry at­tempts too high­brow. That is not on pur­pose, but, like any sci­ence, vi­sion re­search just is not triv­ial. So, if the ex­pla­na­tion seems gib­ber­ish, sim­ply enjoy the phe­nom­e­non ;–). Oth­er­wise, there is a short paper: Bach & Poloschek (2006) Op­ti­cal Il­lu­sions primer.

»Op­ti­cal il­lu­sion« sounds pe­jo­ra­tive, as if ex­pos­ing a mal­func­tion of the vi­su­al sys­tem. Rather, I view these phe­nom­e­na as bring­ing out par­tic­u­lar good adap­ta­tions of our vi­su­al sys­tem to stan­dard view­ing sit­u­a­tions. These adap­ta­tions are »hard-wired« in our brains, and thus under some ar­ti­fi­cial ma­nip­u­la­tions can cause in­ap­pro­pri­ate in­ter­pre­ta­tions of the vi­su­al scene. As Purk­in­je put it: »Il­lu­sions of the sens­es tell us the truth about per­cep­tion« (cited by Teu­ber 1960).

Be­fore we delve in, I’d like to ex­press my thanks to the many vis­i­tors who gave over­whelm­ing­ly pos­i­tive @feed­back, adding con­struc­tive ad­vice on con­tent and style ; please do keep it com­ing! [Or use the guest­book.] I have also been able to im­prove ex­pla­na­tions when con­vinc­ing sci­en­tif­ic ev­i­dence was sent ;-). Yes, it does take siz­able re­sources to set up and main­tain this, but it is also very re­ward­ing, bring­ing me into con­tact with so many charm­ing peo­ple. Should you want to give back, sim­ply link to this page or use the do­na­tion link ;-). Any ad­vice is ap­pre­ci­at­ed – and, yes, for a non-na­tive speak­er, Eng­lish does rep­re­sent siz­able chal­lenges.

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