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Roots of “free will” seen in flies

May 15, 2007
Courtesy Public Library of Science
and World Science staff

Low­er an­i­mals are of­ten seen as robotic and pro­grammed in their ac­tions and reac­tions. In­sects, with their blank eyes and stiff lit­tle frames, seem to epit­o­mize these drea­ry qual­i­ties.

But sci­en­tists stu­dy­ing fruit flies say they have dis­cov­ered that the lit­tle bugs have been slan­dered: they in fact dis­play con­si­der­able spon­ta­ne­i­ty, and this may be at the ev­o­lu­tion­ary root of what we sense as “free will.”

The fruit fly Dro­soph­i­la me­la­no­gas­ter (Cour­te­sy NASA)


The re­search could shed light on a cen­tur­ies-old de­bate over free will—wheth­er an­y­one real­ly has it or wheth­er, in­stead, mind­less move­ments of atoms in our brains con­trol all our de­ci­sions.

The study ap­pears in the May 16 is­sue of the re­search jour­nal PLoS One

“An­i­mals and es­pe­cial­ly in­sects are usu­al­ly seen as com­plex robots which on­ly re­spond to ex­ter­nal stim­uli,” said Björn Brembs of the Free Uni­ver­si­ty Ber­lin, one of the re­search­ers. When sci­en­tists see an­i­mals re­sponding dif­fer­ent­ly to the same stim­u­li, they typ­i­cal­ly as­cribe this “to ran­dom er­rors in a com­plex brain.”

Us­ing be­hav­ior re­cord­ings and math­e­mat­i­cal anal­y­ses, the re­search­ers found that such var­i­a­bil­ity can’t be due to pure chance but is gen­er­at­ed spon­ta­ne­ous­ly and non-ran­domly by the brain.

The re­sults caught com­put­er sci­ent­ist and co-in­vest­i­ga­tor Al­ex­an­der Maye of the Uni­ver­si­ty of Ham­burg, Ger­ma­ny by sur­prise: “I would have nev­er guessed that sim­ple flies who oth­er­wise keep bounc­ing off the same win­dow have the ca­pac­i­ty for nonran­dom spon­ta­ne­ity if giv­en the chance.” 

The re­search­ers teth­ered fruit flies, Dro­soph­i­la melanogaster, in uni­form white sur­round­ings and recorded their turn­ing be­hav­ior. The flies in this set­up re­ceive no vis­u­al cues from the en­vi­ron­ment and since they are fixed in space, their turn­ing at­tempts have no ef­fect. Thus lack­ing any in­put, their be­hav­ior should re­sem­ble ran­dom noise, si­m­i­lar to a ra­di­o tuned be­tween sta­tions, the in­ves­ti­ga­tors rea­soned.

How­ev­er, the anal­y­sis found that fly be­hav­ior is very dif­fer­ent. Var­i­ous in­creas­ing­ly com­plex ran­dom com­put­er mod­els failed to ad­e­quate­ly ac­count for fly be­hav­ior, the re­search­ers said.

They found the source of the spon­ta­ne­i­ty, they added, af­ter an­a­lyz­ing the be­hav­ior with meth­ods de­vel­oped by co-au­thors George Sug­i­hara and Chih-hao Hsieh of the Scripps In­sti­tu­tion of Ocean­og­ra­phy at the Uni­ver­si­ty of Cal­i­for­nia San Die­go. 

“We found that there must be an evolved func­tion in the fly brain which leads to spon­ta­ne­ous vari­a­tions in fly be­hav­ior,” Sug­i­hara said. “The re­sults of our anal­y­sis in­di­cate a mech­an­ism which might be com­mon to many oth­er an­i­mals and could form the bi­o­log­i­cal founda­tion for what we ex­pe­ri­ence as free will.” 

Our sub­jec­tive no­tion of “free will” is an ox­y­mo­ron, Brembs re­marked: the term ‘will’ would not ap­ply if our ac­tions were com­plete­ly ran­dom and it would not be ‘free’ if they were en­tire­ly de­ter­mined. So if there is free will, it must be some­where be­tween chance and ne­ces­si­ty—just where fly be­hav­ior lies. “The ques­tion of wheth­er or not we have free will ap­pears to be posed the wrong way,” said Brembs. “In­stead, if we ask ‘how close to free will are we’ one finds that this is pre­cise­ly where hu­mans and an­i­mals dif­fer.” 

The next step will be to use ge­net­ics to locate and un­der­stand the brain cir­cuits re­spon­si­ble for the spon­ta­ne­ous be­hav­ior, re­search­ers added. This could lead to the de­vel­op­ment of robots with the ca­pac­i­ty for spon­ta­ne­ous nonran­dom be­hav­ior, they said. It may also help, they predicted, in treat­ing dis­or­ders lead­ing to com­pro­mised spon­ta­ne­ous be­hav­ioral var­i­a­bil­ity in hu­mans—such as de­pres­sion, schiz­o­phre­nia or ob­ses­sive com­pul­sive dis­or­der.


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Lower animals are often seen as robotic and programmed in their actions and reactions to the world. Insects, with their stiff little frames and apathetic eyes, seem to epitomize such qualities. But scientists studying fruit flies say they have discovered that the little bugs have been slandered: they in fact display con siderable spontaneity, which may be the evolution ary root of what we sense as “free will.” The research could help shed light on a centuries-old debate about what free will is—and whether anyone really has it, or whether, instead, a mindless movement of atoms in our brains ultimately controls all decisions. The study appears in the May 16 issue of the research journal PLoS One. “Animals and especially insects are usually seen as complex robots which only respond to external stimuli,” said senior author Björn Brembs from the Free University Berlin. When scientists see animals responding differently even to the same stimuli, they typically “attribute this variability to random errors in a complex brain.” Using a combination of automated behavior recording and mathematical analyses, the team of researchers found that such variability cannot be due to simple random events but is generated spontaneously and non-randomly by the brain. These results caught computer scientist and lead author Alexander Maye from the University of Hamburg, Germany by surprise: “I would have never guessed that simple flies who otherwise keep bouncing off the same window have the capacity for nonrandom spontaneity if given the chance.” The researchers tethered fruit flies, Drosophila melanogaster, in uniform white surroundings and recorded their turning behavior. The flies in this setup receive no visual cues from the environment and since they are fixed in space, their turning attempts have no effect. Thus lacking any input, their behavior should resemble random noise, similar to a radio tuned between stations, the invest igators reasoned. However, the analysis found that fly behavior is very different. Various increasingly complex random computer models failed to adequately account fly behavior, the researchers said. They found the source of the spontaneity, they added, after analyzing the behavior with methods developed by co-authors George Sugihara and Chih-hao Hsieh from the Scripps Institution of Oceanography at the University of California San Diego. “We found that there must be an evolved function in the fly brain which leads to spontaneous variations in fly behavior” Sugihara said. “The results of our analysis indicate a mechanism which might be common to many other animals and could form the biological foundation for what we experience as free will.” Our subjective notion of “Free Will” is an oxymoron: the term ‘will’ would not apply if our actions were completely random and it would not be ‘free’ if they were entirely determined, Brembs remarked. So if there is free will, it must be somewhere between chance and necessity—just where fly behavior lies. “The question of whether or not we have free will appears to be posed the wrong way,” said Brembs. “Instead, if we ask ‘how close to free will are we’ one finds that this is precisely where humans and animals differ.” The next step will be to use genetics to localize and understand the brain circuits responsible for the spontaneous behavior, researchers added. This could lead to the development of robots with the capacity for spontaneous nonrandom behavior and may help combating disorders leading to compromised spontaneous behavioral variability in humans such as depression, schizophrenia or obsessive compulsive disorder.