“Rotating Snake” Illusion

First the orig­i­nal “Ro­tat­ing Snake” <http://​www.​ritsumei.​ac.​jp/​~ak­i­taoka/rotsnake.​gif> by Akiyoshi Ki­taoka, mod­i­fied just a lit­tle:

Back­ground – In the image above the strong (and beau­ti­ful) ro­ta­tion of the “wheels” oc­curs in re­la­tion to eye move­ments. On steady fix­a­tion the ef­fect van­ishes. For an ex­plana­tory hy­poth­e­sis, view the an­i­ma­tion on the right. You ma y wish to press the ‘Stop’ but­ton after a while … it gets on one’s nerves ;-). See also Backus & Oruc 2004 for their ex­pla­na­tion. What to ob­serve Even when fix­at­ing steadily, il­lu­sory mo­tion oc­curs on ap­pear­ance/ dis­ap­pear­ance Il­lu­sion does not de­pend on colour Ro­ta­tion di­rec­tion de­pends on the po­lar­ity of the lu­mi­nance steps (I arranged these to evoke a “gear­box ef­fect” here, by mir­ror­ing the im­ages ap­pro­pri­ately) Strength of il­lu­sion de­pends strongly on back­ground lu­mi­nance. The back­ground lu­mi­nance slowly ramps from 0% to 100% au­to­mat­i­cally, or you can use the slider. The il­lu­sion­ary ro­ta­tion is strongest for 50% gray (exact value will de­pend on your mon­i­tor ‘gamma’). Com­ment As Ki­taoka & Ashida (2003) de­scribe, asym­met­ric lu­mi­nance steps are re­quired. Pre­sum­ably ap­pear­ance of these trig­gers mo­tion de­tec­tors (as in the an­i­ma­tion on the right). I as­sume that the ac­tual mech­a­nism is quite sim­i­lar to the “Ro­tat­ing Spokes” il­lu­sion, where asym­met­ric lu­mi­nance steps occur as well. Gre­gory & Heard (1983) were the first to de­scribe that asym­met­ric lu­mi­nance steps cause il­lu­sory move­ment. A “Stress Test”? – No! Re­peat­edly, I was sent such pic­tures with the as­ser­tion that they com­prise a stress test (and some of the peo­ple send­ing me this were deeply wor­ried). And I just found a web page en­ti­tled “test on­line the level of stress” (I will not link to it) which con­tains these state­ments “For a nor­mal per­son, they should all move at a slow pace, barely ro­tat­ing. The slower the pic­tures ro­tate, the bet­ter your abil­ity of han­dling stress: Al­legedly, crim­i­nals see stress test im­ages mov­ing and spin­ning around madly, while se­niors and chil­dren see them still…” This is utter BS!   Don’t get alarmed.   For one, the ef­fect de­pends on eye move­ments, and these are known to dif­fer markedly be­tween sub­jects with­out re­lat­ing clearly to psy­cho­log­i­cal traits. Fur­ther, a few peo­ple do not see it at all (could be around 5%, among them a very renowned vi­sion sci­en­tist), in spite of ap­pro­pri­ate eye move­ments. There are no ac­tual data show­ing re­la­tions to stress (or age), so don't dis­tress your­self when you see it ro­tat­ing strongly or not at all. Sources Gre­gory RL, Heard PF (1983) Vi­sual dis­so­ci­a­tions of move­ment, po­si­tion, and stereo depth: Some phe­nom­e­nal phe­nom­ena. Quar­terly Jour­nal of Ex­per­i­men­tal Psy­chol­ogy 35A:217–237 [PDF] Ki­taoka A, Ashida H (2003) Phe­nom­e­nal char­ac­ter­is­tics of the pe­riph­eral drift il­lu­sion. VI­SION 15:261–262 Backus & Oruc 2004 VSS pre­sen­ta­tion ab­stract + back­ground ma­te­r­ial [seems no longer avail­able] Con­way BR, Ki­taoka A, Yaz­dan­bakhsh A, Pack CC, & Liv­ing­stone MS (2005) Neural basis for a pow­er­ful sta­tic mo­tion il­lu­sion. J Neu­rosci 25:5651–5656 Backus & Oruc (2005) Il­lu­sory mo­tion from change over time in the re­sponse to con­trast and lu­mi­nance. Jour­nal of Vi­sion 5:1055–1069