An­swer to a baf­fling au­dio post is all in the mind’s ear

The West Australian - - OPINION - Sarah Knap­ton

Not since the dress colour illusion have we called into ques­tion our own san­ity and judg­ment to such a de­gree. A sim­ple au­dio en­try for “lau­rel” on Vo­cab­u­ left mil­lions be­wil­dered this week be­cause half of lis­ten­ers in­sisted they could only hear the sound “yanny”.

The global baf­fle­ment was sim­i­lar to that sparked by the Ro­man Orig­i­nals dress posted on Twit­ter in 2015, which many swore was white and gold but the rest were sure it was black and blue.

But, un­like the dress illusion, sci­en­tists say the four-sec­ond au­dio clip may reveal far more about how peo­ple per­ceive the world than they re­alise.

It might even sig­nal a gen­er­a­tional di­vide.

“Stuff go­ing on at a high-fre­quency range you would get young peo­ple hear­ing, and be­ing in­flu­enced by that, but not oldies,” Charles Spence, pro­fes­sor of ex­per­i­men­tal psy­chol­ogy at Ox­ford Univer­sity, said.

Dr Han­nah Critchlow, a neu­ro­sci­en­tist from Cam­bridge Univer­sity, said: “The brain is try­ing to make sense of the world all the time and ev­ery­one has a unique per­cep­tion of what is go­ing on around them, and what they see and hear.

“I have just been sent flow­ers for my birth­day and I hear ‘lau­rel’ be­cause my mind is fo­cused on those flow­ers. Younger peo­ple can also hear higher fre­quen­cies so there could be some­thing in that, too. There are prob­a­bly sev­eral things go­ing on.”

Sci­en­tif­i­cally, it is not ac­tu­ally an illusion at all, but rather an “am­bigu­ous fig­ure”, in which the mind is forced to choose be­tween two dif­fer­ent states.

It is the au­di­tory equiv­a­lent of Joseph Jas­trow’s well-known rab­bit/duck illustration, or Ru­bin’s vase, where the brain in­ter­prets ei­ther two vases or two faces.

In the word “lau­rel”, the noises made by the throat and mouth to pro­duce the sound are at two dif­fer­ent fre­quen­cies, cre­at­ing the am­bi­gu­ity.

A high fre­quency is needed for “l” but a low fre­quency is re­quired for “r”.

A spec­tro­gram of the clip shows that both the sounds “lau­rel” and “yanny” are present, but at dif­fer­ent ends of the sound spec­trum.

Young peo­ple find higher fre­quen­cies eas­ier to hear, while peo­ple suf­fer­ing age-re­lated hear­ing loss start to lose the abil­ity to hear sounds around 4000HZ, ex­actly the fre­quency of the “lau­rel” noise.

So if you can’t hear “lau­rel”, it could be a sign of in­creas­ing years or even dam­aged hear­ing.

Like­wise, be­cause the orig­i­nal au­dio clip is slightly muf­fled it leaves room for in­di­vid­ual in­ter­pre­ta­tion.

The way peo­ple make sense of sound is in­flu­enced by what they hear reg­u­larly, so peo­ple who have friends called Danny or An­nie would likely pick up “yanny”.

Trevor Cox, pro­fes­sor of acous­tic engi­neer­ing at Sal­ford Univer­sity, said: “If you look at the spec­tro­gram, you can see both sounds are there, on top of each other.

“So the sound that an in­di­vid­ual picks up could be based on sounds they hear of­ten, or how words are pro­nounced in their lan­guage or di­alect.

“Also if you have noise-in­duced hear­ing loss, you will strug­gle to hear sounds in the mid­dle of that range, so would only hear ‘lau­rel’. So if you strug­gle to hear ‘yanny’, maybe you are get­ting into that re­gion of hear­ing loss.”

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