The human sense of smell, long den­i­grated, can fi­nally hold its head (and nose) high

Newsweek International - - NEWS - BY JES­SICA WAPNER @jes­si­cawap­ner

I Stink, There­fore I Am

AN EX­TRA­OR­DI­NARY human su­per­power has long been hid­den in plain sight, a se­cret weapon as easy to spot as the nose on your face…be­cause it is the nose on your face. Con­trary to pop­u­lar be­lief, hu­mans have an ex­cel­lent sense of smell. And the story be­hind why we ever thought dif­fer­ently is an in­cred­i­ble illustration of how facts can be buried by bias.

In a newly pub­lished pa­per in Science, neu­ro­sci­en­tist John Mc­gann, who stud­ies sen­sory per­cep­tion at Rut­gers Univer­sity, ex­plains how re­li­gious pol­i­tics in 19th-cen­tury France spurred the mis­con­cep­tion that hu­mans have a poor sense of smell. The Catholic Church ob­jected to what it deemed the teach­ing of athe­ism and ma­te­ri­al­ism, par­tic­u­larly the cour­ses taught at the Univer­sity of Paris Med­i­cal School by an anatomist named Paul Broca whose work fo­cused on the brain.

The crit­i­cism, writes Mc­gann, wor­ried Broca, who needed to nd ev­i­dence to sup­port his view that the mys­ter­ies of life could be re­duced to sim­ple sci­enti c facts. For ex­am­ple, Mc­gann ex­plains that Broca, who sub­scribed to Charles Dar­win’s the­o­ries about evo­lu­tion, rst pub­lished in 1859, did not think the human soul ex­isted sep­a­rately from the human body. Based on his ob­ser­va­tions that hu­mans have larger frontal lobes—the por­tion of the brain be­hind our fore­head—than other mam­mals and that dam­age to this re­gion could im­pair speech and cog­ni­tion, Broca con­cluded that this mass must be where the soul resided. That con­clu­sion went against the be­liefs em­braced by strong and pow­er­ful re­li­gious lead­ers at the time, who held that the soul was cre­ated by God, was the seat of con­scious­ness and freedom, was not con ned to any sin­gle part of the body and did not die with the body.

En­ter our sense of smell. In hu­mans, the ol­fac­tory bulb—the por­tion of the brain that reg­is­ters odors—ap­pears com­par­a­tively smaller than that of some other an­i­mals. But, ex­plains Mc­gann, that di er­ence is just the re­sult of the bulb’s lo­ca­tion. In ro­dents, the ol­fac­tory bulb is rel­a­tively big­ger and sits right at the front of the brain. Human ol­fac­tory bulbs are smaller, squashed at and tucked un­der the frontal lobe. Broca in­ferred that the smaller size meant a less pow­er­ful ol­fac­tory sys­tem.

He then leapt to the con­clu­sion that the human sense of smell was di­min­ished in ex­change for its pow­er­ful in­tel­lect. “It is no longer the sense of smell that guides the an­i­mal: It’s in­tel­li­gence guided by all the senses,” Broca wrote in 1879. He be­lieved that free will stemmed from the frontal lobe and that the phys­i­cal space re­quired for this de­vel­op­ment meant the ol­fac­tory bulb had to shrink. This ex­pla­na­tion pro­vided Broca with the sci­enti c justi cation he needed for views that the church, pow­er­ful both cul­tur­ally and po­lit­i­cally, did not sup­port.

Mc­gann ex­plains that mak­ing this case had the un­ex­pected con­se­quence of shap­ing how we think about our abil­ity to de­tect and iden­tify scents. The shrunken human ol­fac­tory bulb and the ob­ser­va­tion that hu­mans weren’t as preoc-

cu­pied with odors as, say, ro­dents led Broca and oth­ers—through what Mc­gann de­scribes as “a chain of mis­un­der­stand­ings and ex­ag­ger­a­tions”—to con­clude that hu­mans have a poor sense of smell.

That mis­take had wide­spread im­pact. Mc­gann cites the ex­am­ple of Sig­mund Freud, who the­o­rized that our loss of smell led to sex­ual re­pres­sion. Fur­ther­more, Freud said, a per­son who was par­tic­u­larly de­lighted by smell prob­a­bly had a men­tal dis­or­der. All of this, Mc­gann ex­plains, was tied to the view that a bet­ter sense of smell was some­how anath­ema to our dis­tin­guished role as hu­mans in the world of an­i­mals. To err may be human, but to smell was be­neath us.

Richard Doty, who di­rects the Smell and Taste


Cen­ter, part of the Perel­man School of Medicine at the Univer­sity of Penn­syl­va­nia, says Dar­win also dis­missed the power of our ol­fac­tory sys­tem. In 1871, in a pro­nounce­ment in­fused with racism, Dar­win wrote in The De­scent of Man that smell “is of ex­tremely slight ser­vice, if any, even to sav­ages, in whom it is gen­er­ally more de­vel­oped than in the civ­i­lized races. It does not warn them of dan­ger, nor guide them to their food; nor does it pre­vent the Esquimaux from sleep­ing in

the most fetid at­mos­phere, nor many sav­ages from eat­ing half-pu­trid meat.”

Mc­gann trans­forms that view. First, it turns out the human ol­fac­tory bulb has about the same num­ber of neu­rons as that of species tra­di­tion­ally thought to have a strong sense of smell. In fact, human males have more ol­fac­tory neu­rons than mice, ham­sters and guinea pigs. Human fe­males have some­what fewer neu­rons but still beat out rats. And it’s the neu­rons that mat­ter when it comes to de­tect­ing and re­spond­ing to odors.

It is true that human smell di ers from that of other species. In par­tic­u­lar, hu­mans can smell only odors that waft through the air. Other species do not have this lim­i­ta­tion. “Dogs can smell odors that stay in liq­uid form,” says Mc­gann. “That’s why they put their nose on things.” And while we can de­tect all man­ner of odors, we can’t iden­tify the in­di­vid­ual chem­i­cals cre­at­ing them. “Co ee is about 150 chem­i­cal com­pounds,” says Mc­gann. “But you don’t smell 150 things, you smell co ee.” Un­like our sense of hear­ing, which can iso­late the ute in an orchestra, our sense of smell does not zero in that way, he says.

Still, Mc­gann em­pha­sizes that our sense of smell is much bet­ter than we usu­ally ac­knowl­edge. The human ol­fac­tory sys­tem can iden­tify an odor from just an atom or two of a fra­grant chem­i­cal. One re­cent study es­ti­mated that hu­mans can de­tect more than 1 tril­lion dis­tinct com­pounds. “It’s widely, and in­cor­rectly, be­lieved that hu­mans are pri­mar­ily visual, that we lost our ol­fac­tory prow­ess when we be­came a bipedal species with our noses far from the ground,” says Leslie Vosshall, who stud­ies sen­sory per­cep­tion at Rock­e­feller Univer­sity in New York. This in­ves­ti­ga­tion by Mc­gann, says Vosshall, “shows con­vinc­ingly that the human sense of smell is in­cred­i­bly pow­er­ful.”

The in uence that odors have on us also hints at the im­por­tance of this sense. Fra­grances waft­ing through our noses and reach­ing our ol­fac­tory bulbs can change thoughts and feel­ings in an in­stant. We re­spond to one an­other’s “body odor cock­tail,” as Mc­gann puts it, and de­cide whether a per­son, place or thing is safe or dan­ger­ous partly based on smell. Ac­cord­ing to one re­cent study, we un­con­sciously smell our palms after shak­ing hands with strangers.

The myth that hu­mans have a poor sense of smell has had con­se­quences. About 1 to 2 per­cent of peo­ple in the U.S. su er from an ol­fac­tory dis­or­der, in­clud­ing com­plete loss of the sense or smell hal­lu­ci­na­tions. Chemo­ther­apy can also di­min­ish or al­ter smell as a side e ect. The change, which can be per­ma­nent, leaves many pa­tients de­pressed and can also pose nu­tri­tion prob­lems be­cause taste is closely tied to fra­grance. A change in ol­fac­tory abil­i­ties can also in­di­cate an un­der­ly­ing neu­rode­gen­er­a­tive dis­ease. But re­search to un­der­stand why these con­di­tions oc­cur and how to treat them has been slow to progress, partly be­cause of how smell has been min­i­mized by this history. No reme­dies ex­ist for these con­di­tions. “Be­cause for 100 years we’ve had this idea that our sense of smell is an af­ter­thought,” says Mc­gann, “loss of smell as a med­i­cal prob­lem has got­ten the short shrift.”

Doty cau­tions against over­es­ti­mat­ing the pro­ciency of our ol­fac­tory sys­tem. “This sys­tem is over­looked and taken for granted,” he ac­knowl­edges. “How­ever, any­one who has owned a dog


knows that hu­mans clearly do not have the same ca­pa­bil­i­ties.” He notes that hu­mans don’t rely on this sense in the same way as many other mam­mals do and are able to sur­vive with­out it. The same is not true for many other species: a ham­ster’s odor, for ex­am­ple, is a pri­or­ity for mat­ing.

Mc­gann is con­tin­u­ing to study smell, par­tic­u­larly how our ol­fac­tory sys­tem changes as our brains ac­crue in­for­ma­tion about odors. He of­ten rec­om­mends to his stu­dents that they blind­fold them­selves and crawl around in the back­yard as a way to be­gin understanding the power of their ol­fac­tory sys­tem, an ex­pe­ri­ence he found rev­e­la­tory. “Par­tially, I never paid at­ten­tion,” he says, “and par­tially, I never put my nose where the good stu was.”

+ WHIFF OFTRUTH: Re­li­gious pol­i­tics led to the mis­con­cep­tion that hu­mans have a poor sense of smell. The human ol­fac­tory sys­tem is ac­tu­ally ex­traor­di­nar­ily sensitive.

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