How Much of What You See Is Actually a Hallucination?: An Animated TED-Ed Lesson

All of us have, at one time or anoth­er, been accused of not see­ing what’s right in front of us. But as a close exam­i­na­tion of our bio­log­i­cal visu­al sys­tem reveals, none of us can see what’s right in front of us. “Our eyes have blind spots where the optic nerve blocks part of the reti­na,” says the nar­ra­tor of the new ani­mat­ed TED-Ed video above. “When the visu­al cor­tex process­es light into coher­ent images, it fills in these blind spots with infor­ma­tion from the sur­round­ing area. Occa­sion­al­ly we might notice a glitch, but most of the time, we’re none the wis­er.” This absence of gen­uine infor­ma­tion in the very cen­ter of our vision has long cir­cu­lat­ed in the stan­dard set of fas­ci­nat­ing facts.

What’s less well known is that these same neu­ro­log­i­cal process­es have made the blind see — or rather, they’ve induced in the blind an expe­ri­ence sub­jec­tive­ly indis­tin­guish­able from see­ing. It’s just that the things they “see” don’t exist in real­i­ty.

Take the case of an elder­ly woman named Ros­alie, with which the video opens. On one oth­er­wise nor­mal day at the nurs­ing home, “her room sud­den­ly burst to life with twirling fab­rics. Through the elab­o­rate drap­ings, she could make out ani­mals, chil­dren, and cos­tumed char­ac­ters,” even though she’d lost her sight long before. “Ros­alie had devel­oped a con­di­tion known as Charles Bon­net Syn­drome, in which patients with either impaired vision or total blind­ness sud­den­ly hal­lu­ci­nate whole scenes in vivid col­or.”

This leads us to the coun­ter­in­tu­itive find­ing that you don’t need sight to expe­ri­ence visu­al hal­lu­ci­na­tions. (You do need to have once had sight, which gives the brain visu­al mem­o­ries on which to draw lat­er.) But “even in peo­ple with com­plete­ly unim­paired sens­es, the brain con­structs the world we per­ceive from incom­plete infor­ma­tion.” Take that gap in the mid­dle of our visu­al field, which the brain fills with, in effect, a hal­lu­ci­na­tion, albeit not one of the elab­o­rate, some­times over­whelm­ing kinds induced by “recre­ation­al and ther­a­peu­tic drugs, con­di­tions like epilep­sy and nar­colep­sy, and psy­chi­atric dis­or­ders like schiz­o­phre­nia.” At the end of the les­son, the nar­ra­tor sug­gests that inter­est­ed view­ers seek out the work of neu­rol­o­gist-writer Oliv­er Sacks, which deals exten­sive­ly with what opens gaps between real­i­ty and our per­cep­tions — and which we here at Open Cul­ture are always pre­pared to rec­om­mend.

Relat­ed con­tent:

Oliv­er Sacks Explains the Biol­o­gy of Hal­lu­ci­na­tions: “We See with the Eyes, But with the Brain as Well”

Real­i­ty Is Noth­ing But a Hal­lu­ci­na­tion: A Mind-Bend­ing Crash Course on the Neu­ro­science of Con­scious­ness

A Beau­ti­ful 1870 Visu­al­iza­tion of the Hal­lu­ci­na­tions That Come Before a Migraine

Alice in Won­der­land Syn­drome: The Real Per­cep­tu­al Dis­or­der That May Have Shaped Lewis Carroll’s Cre­ative World

This is What Oliv­er Sacks Learned on LSD and Amphet­a­mines

Based in Seoul, Col­in Marshall writes and broad­casts on cities, lan­guage, and cul­ture. His projects include the Sub­stack newslet­ter Books on Cities, the book The State­less City: a Walk through 21st-Cen­tu­ry Los Ange­les and the video series The City in Cin­e­ma. Fol­low him on Twit­ter at @colinmarshall or on Face­book.