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The division of labor by the two cerebral hemispheres—once thought to be uniquely human—predates us by half a billion years. Speech, right-handedness, facial recognition and the processing of spatial relations can be traced to brain asymmetries in early vertebrates
photoilluStrAtion by twiSt creAtive; MedicAlrf.coM Corbi s (brain); Medio iMAgeS Get ty Im ages (calculator);
Joerg SteffenS Corbis (fa ces); weStend 61 Corbis ( woman smiling); dougAl wAter S Getty Imag es ( ballerina); MiKe KeMp Get ty I mage s (rattlesnake);
c SQuAred Stud ioS Gett y Ima ges ( palette); vlAdi Mir godniK Get ty I mage s (paintbrushes); cA rrie boretZ Corbi s (gi rls whispering); robert lle wellyn Corbis (calipers)
The left hemisphere of the human brain con-
tro ls l an guage , a rguab ly o ur gre atest m en-
tal attribute. It also controls the remark-
able dexterity of the human right hand. The right
hemisphere is dominant in the control of, among
other things, our sense of how objects interrelate
i n s pa ce . F or ty ye ar s ago t he bro ad sc ient i c co n-
sensus held that, in addition to language, right-
handedness and the specialization of just one side
of the brain for processing spatial relations occur
in humans alone. Other animals, it was thought,
have no hemispheric specializations of any kind.
Those beliefs  t well w ith the view that peo-
ple have a special evolutionary status. Biologists
and behavioral scientists generally agreed that
right-handedness evolved in our hominid ances-
tors as they learned to build and use tools, about
2.5 million years ago. Right-handedness was
also thought to underlie speech. Perhaps, as the
story went, the left hemisphere simply added
sign language to its repertoire of skilled manual
actions and then converted it to speech. Or per-
haps the left brain’s capacity for controlling
manual action extended to controlling the vocal
apparatus for speech. In either case, speech and
la ng uage evo lve d fro m a re lat ivel y re cent m anu-
al talent for toolmaking. The right hemisphere,
meanwhile, was thought to have evolved by de-
fault into a center for processing spatial rela-
tions, after the left hemisphere became special-
ized for handedness.
In the past few decades, however, studies of
many other animals have shown that their two
brain hemispheres also have distinctive roles. De-
spite those ndings, prevailing wisdom continues
to ho ld th at pe opl e a r e d if fe re nt . M a ny in ve st ig a-
tors still think the recently discovered specializa-
tions of the two brain hemispheres in nonhumans
are unrelated to the human ones; the hemispheric
spec ial izations of huma ns beg an w it h huma ns.
Here we present evidence for a radically dif-
ferent hy po the si s t ha t i s ga in in g s up po rt , p ar t ic -
ularly among biologists. The specialization of
each hemisphere in the human brain, we argue,
wa s a lr ea dy p re sent in its b as ic f or m when verte-
brates emerged about 500 million years ago. We
suggest that the more recent specializations of
the brain hemispheres, including those of hu-
m an s, ev ol ve d f ro m t he or ig in al on es by th e D a r-
winian process of descent with modi cation. (In
that process, capabilities relevant to ancient traits
a re ch an ge d or c o- op te d i n th e s er v ic e o f o th er de -
veloping traits.) Our hypothesis holds that the left
hemisphere of the vertebrate brain was original-
ly specialized for the control of well-established
patterns of behavior under ordinary and famil-
iar circumstances. In contrast, the right hemi-
sphere, the primary seat of emotional arousal,
The au thors have prop os ed that
the specialization of the brain’s
two hemispheres was already
in place when vertebrates arose
500 mill ion years ago.
The left hemisphere originally
seems to have focused in
general on controlling well-
established pa tterns of be hav-
ior; the right specialized in
detecting and responding to
unexpected stimuli.
Bot h speech and r ight- handed-
ness m ay have evolved from a
specialization for the control of
routine behavior.
Face recognition and th e pro -
cessing of spatial relations may
trace their heritage to a need to
sense predators quickly.
The Editors
The division of labor by the two cerebral hemispheresonce thought to be uniquely human
predates us by half a billion years. Speech, right-handedness, facial recognition and the
processing of spatial relations can be traced to brain asymmetries in early vertebrates
By Peter F. MacNeilage, Lesley J. Rogers and Giorgio Vallortigara
IN THE HUMAN BR AIN th e lef t hem i-
sphere controls language, the
dexterity of the righ t hand,
the ability to classify, and routine
behavior in general. The right
hemisphere specializes in react-
ing to emergencies, organizing
item s spa tially, recog nizing fa ce s
and processing emotions. SCIENTIFIC AMERICAN 61
was at rst specialized for detecting and respond-
ing to u ne xpe cte d stimuli in the envi ronm ent.
In early vertebrates such a division of labor
probably got its start when one or the other
hemisphere developed a tendency to take control
in particular circumstances. From that simple
beginning, we propose, the right hemisphere
took primary control in potentially dangerous
circumstances that called for a rapid reaction
from the animaldetecting a predator nearby,
for instance. Otherwise, control passed to the
left hemisphere. In other words, the left hemi-
sphere became the seat of self-motivated behav-
ior, sometimes called top-down control. (We
stress that self-motivated behavior need not be
innate; in fact, it is often learned.) The right
hemisphere became the seat of environmentally
motivated behavior, or bottom-up control. The
processing that directs more specialized behav-
iorslanguage, toolmaking, spatial interrela-
tions, facial recognition, and the likeevolved
from thos e two basic cont rols.
The Left Hemisphere
Most of the evidence that supports our hypoth-
esis does not come from direct observation of the
brain but rather from observations of behavior
that favors one or the other side of the body. In
the vertebrate nervous system the connections
cross between body and braint o a la rg e d eg r ee ,
nerves to and from one side of the body are linked
to the opposite-side hemisphere of the brain.
Ev ide nce fo r the r st p ar t of our h ypot he sis
that the vertebrate left hemisphere specializes in
controlling routine, internally directed behav-
iorsh as been bui ldi ng for some ti me. One rou-
tine behavior with a rightward bias across many
vertebrates is feeding. Fishes, reptiles and toads,
for instance, tend to strike at prey on their right
side under the guidance of their right eye and left
hemisphere [see box on page 64]. In a variety of
bird specieschickens, pigeons, quails and
stiltsth e r ig ht e ye is t he pr im ar y g uide fo r va ri -
ous kinds of food pecking and prey capture. In
one instance, such a lateralized feeding prefer-
ence has apparently led to a lateralized bias in the
a ni ma l’ s e xt er na l a na to my. Th e b ea k of t he N ew
Zealand wry-billed plover slopes to the right;
that way, the plover’s right eye can guide the beak
as the bird seeks food under small river stones.
As for mammals, the feeding behavior of
humpback whales is a spectacular example of a
lateral feeding preference. Phillip J. Clapham,
no w a t t he A la sk a Fi sh er ie s S ci en ce Ce nter i n S e -
attle, and his colleagues discovered that 60 out of
And rew Swif t
75 whales had abrasions only on the right jaw;
the other 15 whales had abrasions only on the left
jaw. The ndings were clear evidence that whales
favor one side of the jaw for food gathering and
that “right-jawedness” is by far the norm.
In short, in all vertebrate classesshes, rep-
tiles, amphibians, birds and mammalsanimals
tend to retain what was probably an ancestral
bias toward the use of the right side in the rou-
ti ne ac tivity of feedin g.
Origins of Right-Handedness
What do these ndings say about the alleged
uniqueness of human right-handedness? Evidence
for a right-side bias in birds and whales is intrigu-
ing, but it hardly makes a convincing argument
against the old belief that right-handedness in
humans had no evolutionary precursors. Yet
more than a dozen recent studies have now dem-
onstrated a right-handed bias among other pri-
mat es , o ur cl ose st e vo lut io na ry re lat ive sclearly
suggesting that human right-handedness descend-
ed from that of earlier primates. The right-hand
preference shows itself in monkeys (baboons,
Cebus monkeys and rhesus macaques) as well as
in apes , par ticularly in chimpa nzees .
Many of the studies of apes have been done
by William D. Hopkins of the Yerkes National
Primate Research Center in Atlanta and his col-
leagues. Hopkins’s group observed right-hand
preferenc es pa rt ic u la rl y i n t as ks th at inv ol ve d e i-
ther coordinating both hands or reaching for
food too high to grab without standing upright.
For example , experimenters placed honey (a fa-
vorite food) inside a short length of plastic pipe
and gave the pipe to one of the apes. To get the
honey, the ape had to pick up the pipe in one
hand and scrape out the honey with one nger
of the opposite hand. By a ratio of 2 to 1, the apes
preferred to scrape honey out with a nger of the
right hand. Similarly, in the reaching experi-
ments, the apes usually grabbed the food they
wanted with the right hand.
The Yerkes ndings also suggest to us that as
early primates evolved to undertake harder and
more elaborate tasks for finding food, their
handedness preferences became stronger, too.
The reason, we suspect, is that performing ever
more complex tasks made it increasingly neces-
IN PEOPLE an d oth er ve rtebrates, ner ves t o
and from one side of the body are linked to
the opposite-side hemisphere of the brain. As
a result, each hemisphere generally controls
the opp osite side of the bo dy.
In feeding,
animals from all
ve vertebrate
classes retain
an ancestral
bias for
the right side. SCIENTIFIC AMERICAN 63
Andre w Swif t; Source: “heMiS pheric SpeciAliZ Ation of MeMory for viSuAl hie rArchicAl Sti Muli,” by d. c. deliS et Al., in NEUROPSYCHOLOGIA, vol . 24, n o. 2; 19 86
takes control in highly emotional vocalizing; the
left brain st icks to t he rout ine.
Nonvocal communication in humans has
evolutionary antecedents as well. Not only do
chimpanzees tend to be right-handed when they
manipulate objects, but they also favor the right
hand for communicative gestures. Gorillas, too,
tend to incorporate the right hand into complex
communications that also involve the head and
the mouth. Adrien Meguerditchian and Jacques
Vauclair, both at the University of Provence in
France, have even observed a right-handed bias
for one manual communication (pat ting the
ground) in baboons.
The evolutionary signi cance of all this be-
comes clear as soon as one notes that humans
also tend to make communicative gestures with
the right hand. The lateralized behavior we share
with baboons suggests that right-handed com-
munications arose with the  rst appearance of
the monkeylike ancestor we share with baboons.
That creature emerged perhaps 40 million years
agowell before hominids began to evolve.
In a classic experiment , Dean C. Delis of the Uni versi-
ty of California, San Diego, and his colleagues asked
brain-damaged patients to study a picture of a large
capital H made up of little A’s (left) and then redraw it
from memory. The patients with damage to the right
hemisphere (thus dependent solely on the left hemi-
sphere) often simply scattered A‘s over the page
(below left ). Patients with da mage to the le ft he mi-
sphere often just drew a large capital H with no A‘s
(below right). Thus, the human left brain characterizes
stimuli according to one or a few details, whereas the
right brain specializes in synthesizing global patterns.
Division of Labor in the Hemispheres
sary for the control signals from the brain to pass
as directly as possible to the more skilled hand.
Since the most direct route from the left hemi-
spherethe hemisphere specialized for routine
tasksto the body follows the body-crossing
pathways of the peripheral nerves, the right hand
increasingly became the preferred hand among
nonhuman primates for performing elaborate,
albeit routine, t asks.
Communication and the Left Brain
The evolutionary descent of human right-handed
dexterity via the modi cation of ancient feeding
behavior in ancestral higher primates now seems
very likely. B ut could fe edi ng behav ior al so have
given ris e to t he left-brai n specialization for lan-
guage? Actually we do not mean to suggest that
this development was direct. Rather we argue
t ha t t he “l an gu a ge br ai n” e me rg ed fr om an in te r-
mediate and somewhat less primitive specializa-
tion of the left hemispherenamely, its special-
ization for routine communication, both vocal
and nonvocal. But contrary to long-held beliefs
among students of human prehistory, neither of
those communicative capabilities rst arose with
humans; they, too, are descended from hemi-
spheric specializations that  rst appeared in ani-
mals that l ived long befo re ou r spe cies emerg ed.
In birds, for instance, studies have shown that
the left hemisphere controls singing. In sea lions,
dogs and monkeys, the left hemisphere controls
the perception of calls by other members of the
same species. One of us (Rogers), in collabora-
tion with Michelle A. Hook-Costigan, now at
Texas A&M University, observed that common
marmosets open the right side of their mouths
wider than the left side when making friendly
calls to other marmosets. People also generally
open the right side of their mouths to a greater
ex tent than the lef t when the y sp eakthe result
of greater activation of the right side of the face
by the left hemisphere.
Little is universal in nature, though, and in
some animals a vocal response to highly emo-
tional c ircumst ances h as a lso b een l inked to t he
left brain, notas one might expectto the
right. When a male frog is clasped from behind
and held by a rival male, for instance, the left
hemisphere seems to control the vocal responses
of the  rst frog. The left hemisphere in mice con-
trols the reception of distress calls from infant
mice, and in gerbils it controls the production of
calls during copulation. But those animals may
be exceptions. In humans and monkeysand
perhaps in most other animalsthe right brain
brain-damaged patients to study a picture of a large
from memory. The patients with damage to the right
hemisphere (thus dependent solely on the left hemi-
right brain specializes in synthesizing global patterns.
Patients with
damage to the rig ht
hemisphere could
remember details of
the original but not
the overall pattern.
Original picture
Peter F. MacNeilage is a pr ofes -
sor of psych ol ogy a t the Un iv er sity
of Tex as at Aust in . He has w rit te n
more than 120 articles on the
evolution of complex action
systems; his book Th e Origi n of
Speech was published by Oxford
Univer sity Pr ess l as t y ear.
Lesley J. Rogers is an emerit a
professo r of neuroscience and
anim al behavi or at th e Uni vers it y
of New England in Australia. She
discovered lateralization in the
chick forebrain when lateralization
was still t hought to be a uni qu e
feat ur e of the h uman brain.
Giorgio Vallortigara is a profe s-
sor of co gnitive n euro scie nce a t
the Center for Mind/Brain Sciences
and in the departm ent of cognitive
sciences at the Unive rsity of Tren to
in Italy. With Roger s, he discovered
the  rs t evidenc e o f fun ctio nal
brain asym metry in  shes and
Patients with damage
to the left hemisphere
could reproduce the
global pattern but not
it s detail s.
organizational unit underlying a stream of speech
in time. The typical syllable is a rhythmic alter-
nation between consonants and vowels. (Conso-
nants are the sounds created when the vocal tract
is momentarily closed or almost closed; vowels
are the sounds created by resonance with the
sh ap e o f t he vo ca l t ra c t a s a ir  o ws re lat ive ly fre e -
ly out through the open mouth.) The syllable may
h ave ev ol ve d a s a by -p ro du ct of th e a lt er nat e r a is -
ing (consonant) and lowering (vowel) of the man-
dible, a behavior already well established for
chewing, sucking and licking. A series of these
mouth c ycles, produce d as l ip sm acks , may have
begun to serve a mong early humans as com mu-
nication signals, just as they do to this day among
ma ny ot her prim ates.
Somewhat later the vocalizing capabilities of
the larynx could have paired with the commu-
nicative lip smacks to form spoken syllables.
Syllables were perhaps rst used to symbolize
individual concepts, thus forming words. Sub-
sequently, the ability to form sentences (lan-
g ua ge) pr es um ab ly e vo lved wh en ea rl y hu ma ns
combined the two kinds of words that carry the
main meaning of sentences: those for objects
(nouns) and those for actions (verbs).
The R ight Hemisphere
What about the second half of our hypothesis?
How strong is the evidence that, early in verte-
brate evolution, the right hemisphere specialized
in detecting and responding to unexpected stim-
uli? In what ways has that underlying specializa-
tion evolved and been transformed?
One set of  ndings that lend strong support
to our hypothesis comes from studies of the re-
actions to predators by various animals. After
all, few events in ancient vertebrate environ-
ments could have been more unexpected and
emotion-laden than the surprise appearance of
a deadly predator. Sure enough,  shes, amphib-
ians, birds and mammals all react with greater
avoidance to predators seen in the left side of
their visual  eld (right side of the brain) than in
thei r right v isual  eld [see box on page 66].
Evidence that the same hemispheric specializa-
tion for reactions holds for humans comes from
brain-imaging studies. In a summary of those
studies, Michael D. Fox and his colleagues at
Washington University in St. Louis conclude that
humans possess an “attentional system” in the
right hemisphere that is particularly sensitive to
unexpected and “behaviorally relevant stim uli”
or in other words, the kind of stimuli that say, in
effect, Danger ahead! The existence of such an
Andre w Swif t; SourceS: “coMpl eMentAry right And left heM ifield uSe for predAtory And AgoniS tic behAvior,” by g. vAllortigA rA et Al., in NEUROREPORT, vol. 9, no. 14; 1998, And “lAte rAliZed prey cAtchin g reSpo nSeS in the cAne toAd, BUFO MARINUS:
AnAlySiS of coMpl eX viSuAl StiMul i,by A. robinS And l. J. roge rS, in ANIMAL BEHAVIOUR, vol . 68, no. 4; 200 4; Adr ien M egue rdit chiA n And JAcQ ueS vA uclA ir Un iver sity of Pro venc e (baboon); Source : “bAboo nS coMMunicAte with the ir right hAnd,”
by A. Meguerditc hiAn And J. vAuclAir, in BEHAVIOURAL BRAIN RESEARCH, vol. 171, no.1; 2006 ; prov inc etow n cen ter fo r coA StAl Stud ieS ( whale)
Evolution of Speech
A fundamental question remains: Just how could
any of the behaviors already controlled by the
left brainfeeding, vocalizing, communicating
with the right handhave been modified to
become speechone of the most momentous
steps i n the h istor y of l ife on ea r th?
One of us (MacNeilage) has hypothesized that
it re qui red t he evo lutio n of the syl lable, the basic
A r i g h t - s i d e hence left-brainbias for controlling behavior patterns under ordinary
circumstances has been found in nearly every class of vertebrate animal tested so far.
Catching prey is a typical routine behavior. In the experiment diagrammed below, a
simulated grasshopper was glued to a turntable and rotated into one or the other visual
eld o f a toa d. Wh en the gr asshoppe r was placed to th e toad’s left and rotated clo ck-
wise, the toad struck at the insect only when it crossed the midline into the toad’s right
visual  eld. When the prey was rotated counterclockwise, the toad struck at it less often
ove rall a nd with a bout the same frequency in eac h visual e ld (not shown).
[ L E F T B R A I N ]
chewing, sucking and licking. A series of these
mouth c ycles, produce d as l ip sm acks , may have
begun to serve a mong early humans as com mu-
nication signals, just as they do to this day among
ma ny ot her prim ates.
Somewhat later the vocalizing capabilities of
the larynx could have paired with the commu-
nicative lip smacks to form spoken syllables.
Syllables were perhaps rst used to symbolize
individual concepts, thus forming words. Sub-
sequently, the ability to form sentences (lan-
g ua ge) pr es um ab ly e vo lved wh en ea rl y hu ma ns
combined the two kinds of words that carry the
main meaning of sentences: those for objects
(nouns) and those for actions (verbs).
The R ight Hemisphere
What about the second half of our hypothesis?
How strong is the evidence that, early in verte-
1 Toad ignores
grasshopper entering
toa d’s l eft v isua l  e ld . . .
2 ... but strikes when
prey rotates clockwise into
toad’s right visual  eld.
Mid line of to ad’s v isua l  e ld
Among the many other animals that also display a preference for the right side in cer tain
behav iors are b aboo ns and whal es, indicati ng co ntro l by th e lef t side of the brai n. Ad rien M eguer-
dit chia n and J acqu es Vau clair, bo th at the Un iver sity of Pr ovenc e in Fr ance , have repo rted that
babo ons se em to commu nica te by p att ing th e rig ht han d on t he groun d. Ph illip J. Cla pham , now a t
the Alaska Fisheries Science Center in Seat tle, found that whales suffered abrasions primarily on
the ri ght side of the ja w (arrow), ind icat ing t hat th ey st rong ly fav ored that s ide i n gath erin g foo d.
Left brain
Vocal cords
face recognition. Prosopagnosia, a neurological
disorder that impairs that ability, is more often
a result of damage to the right hemisphere than
to the left. Extending face recognition to what
seems another level, both monkeys and humans
interpret emotional facial expressions more ac-
curately with the right hemisphere than with the
left. We think that this ability is part of an an-
cient evolutionary capacity of the right hemi-
sphere for determining identity or familiarity
for judging whether a present stimulus, for in-
stance, has be en seen or enc ou ntered before .
Globa l and Loca l
We have argued for a basic distinction between
the role of the left hemisphere in normal action
and the role of the right hemisphere in unusual
circumstances. But i nvestigators have h igh light-
ed additional dichotomies of hemispheric func-
tion as well. In humans the right hemisphere
“takes in the whole scene,” attending to the glob-
al aspects of its environment rather than focus-
in g on a l imit ed numb er of f eat ure s. T hat capac -
ity gives it substantial advantages in analyzing
spatial relations. Memories stored by the right
hemisphere tend to be organized and recalled as
According to one of the authors (MacNeilage), the origin of human speech may
be t raceable to t he evolu tion of t he syllable typically an alternation between
consonant and vowel. In the word “mama,” for instance, each syllable begins with
the consonant sound [m] and ends with the vowel sound [a]. As the cutaway diagrams
show, the [m] sound is made by temporarily raising the jaw, or lower mandible, and
stopping t he ow of air from the lun gs by clo sing the li ps (b elow lef t). To make the
following vowel sound [a], the jaw drops and air ows freely through the vocal tract
(below right). MacNeilage has thus proposed that the making of syllabic utterances
is an evolutionary modication of routine chewing behavior, which rst evolved in
mam mals 2 00 milli on yea rs ag o.
Andre w Swif t; Source: SuSA nnA douglAS Un ive rsit y of Te xas at Aust in
at t en ti on al sy st em he lp s t o m ak e s en se of an ot h-
er wi se i ne xp li ca bl e h um a n pro p en si ty : i n t h e l ab -
oratory, even right-handed people respond more
quickly to unexpected stimuli with their left hand
(right hemisphere) than with their right hand.
Even in nonthreatening circumstances, many
vertebrates keep a watchful left eye on any visi-
ble predators. This early right-hemisphere spe-
cialization for wariness in the presence of preda-
tors also extends in many animals to aggressive
behavior. Toads, chameleons, chicks and ba-
boons are more likely to attack members of their
own spe cies to t he ir lef t th an to their rig ht .
In humans the relatively primitive avoidance
and wariness behaviors that manifest right-hemi-
sphere attentiveness in nonhuman animals have
morphed into a variety of negative emotions.
Nineteenth-century physicians noticed that pa-
tients complained more often of hysterical limb
paralyses on the left side than on the right. There
is some evidence for right-hemisphere control of
emotional cries and shouts in humansin strik-
ing contrast with the emotionally neutral vocal-
izations controlled by the left hemisphere. People
are more likely to become depressed after dam-
age to the left hemisphere than to the right. And
in states of chronic depression the right hemi-
sphere is more active than the left.
Recognizing Others
Along with the sudden appearance of a predator,
the most salient environmental changes to which
early vertebrates had to react quickly were en -
counters with others of their own species. In sh-
es and birds the right hemisphere recognizes
social companions and monitors social behavior
that might require an immediate reaction. Hence,
the role of the right hemisphere in face perception
must have descended from abilities of relatively
early vertebrates to recognize the visual appear-
ance of other individuals of their species.
For example, only some species of shes
among the earliest evolving vertebratesmay be
able to recognize individual sh, but birds in gen-
eral do show a right-hemisphere capacity to rec-
ognize individual birds. Keith M. Kendrick of the
Babraham Institute in Cambridge, England, has
shown that sheep can recognize the faces of oth-
er sheep (and of people) from memory and that
the right hemisphere is preferentially involved.
Charles R. Hamilton and Betty A. Vermeire,
both at Texas A&M, have observed similar be-
hav ior in mon ke ys.
In humans neu roscientists have recently rec-
ognized that the right hemisphere specializes in
Did the Syllable Evolve from Chewing?
Photogenic Left
A 1999 study of the pictures in
London’s National Portrait Gallery
analyzed the directions that the
portrait sitters turned their heads.
Overall, the sitters turned
their heads slightly to the right,
showing the left side of the face.
The investigators argued that
sitters wanted to show their
left side because it is controlled
by the emotive, right hemisphere
of t he brain.
Portraits of males, however, show
a reduced leftward bias, perhaps
out of a desi re to conce al emotion.
Portraits of scientists from
the Royal Society show no
leftward bias.
overall patterns rather than as a series of single
items. In contrast, the left hemisphere tends to
focu s on loca l a spects of its environm ent.
St ri ki ng evid en ce for t he gl oba l-lo ca l d ic ho to -
my in humans has been brought to light by a task
invented by David Navon of the University of
Haifa in Israel. Brain-damaged patients are asked
to cop y a pi ct ur e i n w hi ch 2 0 or so s m al l c op ie s o f
the uppercase letter A have all been arranged to
form the shape of a large capital H [see box on
page 63]. Pat ients with damage to the left hem i-
Andre w Swif t; Source: “lAte rAliSAtion of predAto r AvoidAn ce reSpo nSeS in three SpecieS of toAdS,” by g. lippol iS et Al.,
in LATERALITY, vol. 7, no. 2; 2002 ; Joh n giu Stin A Getty Ima ges (sheep); Kevin MorriS Ge tty Imag es (blue-footed booby)
sphere often make a simple line drawing of the H
with no small A letters included; patients with
damage to the right hemisphere scatter small A
letters unsys tematic ally all over the page.
A similar dichotomy has been detected in
ch ic ke ns , s ug ge s ti ng it s r el at iv el y e ar ly ev olu t ion .
Richard J. Andrew of the University of Sussex in
E ng la nd a nd on e of u s ( Val l or ti ga ra) ha ve di sc ov-
ered that, as in humans, the domestic chick pays
special attention to broad spatial relations with
its right hemisphere. Moreover, chicks with the
right eye covered, hence receiving input only to
the right hemisphere, show interest in a wide
range of stimuli, suggesting they are attending to
their global environment. Chicks that can attend
only with the left hemisphere (left eye covered)
focus only on specic, local landmark features.
Why Do Hemispheres Specialize?
Why have vertebrates favored the segregation of
certain functions in one or the other half of the
bra in ? To as se ss a n i nc om ing s t im ul us , a n or ga n-
ism must carry out two kinds of analyses simul-
taneously. It must estimate the overall novelty of
the stimulus and take decisive emergency action
if needed (right hemisphere). And it must deter-
m in e w he th er th e s ti mu lu s  ts so me fa mi li a r c at -
egory, so as to make whatever well-established
response, if any, is called for (left hemisphere).
To detect novelty, the organism must attend
to features that mark an experience as unique.
Spatial perception calls for virtually that same
kind of “nose for novelty,” because almost any
standpoint an animal adopts results in a new
con guration of s timuli. T hat is the fun ction of
the right hemisphere. In contrast, to categorize
an experience, the organism must recognize
which of its features are recurring, while ignor-
ing or discarding its unique or idiosyncratic
ones. The result is selective attention, one of the
brain’s most important capabilities. That is the
function of the left hemisphere.
Perhaps, then, those hemispheric specializa-
tions initially evolved because collectively they
do a more efcient job of processing both kinds
of information at the same time than a brain
without such specialized systems. To test this
idea , we had to compa re the abil ities of ani mal s
having lateralized brains with animals of the
same species having nonlateralized brains. If our
idea was correct, those with lateralized brains
would be able to perform parallel functions of
the left and right hemisphere more efciently
than those with nonlateralized brains.
Fortunately, one of us (Rogers) had already
The su dden ap pearance o f a pre datoror of another member of one’s own species
calls for instant, appropriate action, and the right brain has evolved to handle such
events. In another experiment with toads, the rubber head of a model snake
atta ched to the en d of a b lack p lastic b ar was pushed tow ard the toad f rom
the right or left, then quickly withdrawn. When the “snake”
appeared to the toad’s right, the toad ignored it. Yet
when the simulated predator appeared to the toad’s
left, it triggered a response from the toad’s right
brain, and the toad jumped away.
Many ver tebrates recognize individuals of their own species. Keith
M. Kendrick of the Babraham Institute in Cambridge, England, has
shown that sheep can recognize other individual sheep from memory,
primarily with the right hemisphere. The right hemisphere in birds such
as t he blue -foot ed bo oby also en able s them to recognize one an othe r.
Right brain
1 Toad ignores snake
approaching from right.
2 Toad jumps away from
snake approaching from left.
versities of Stockholm in Sweden and of Bologna
in Italy, Vallortigara recently showed mathe-
matically that populations dominated by left-
type or by right-type individuals can indeed
arise spontaneously if such a population has
frequency-dependent costs and benets. The
mathematical theory of games often shows that
the best course of action for an individual may
depend on what most other members of its own
group decide to do. Applying game theory, Ghir-
landa and Vallortigara demonstrated that left-
or right-type behavior can evolve in a population
under social selection pressuresthat is, when
asymmetrical individuals must coordinate with
others of their species. For example, one would
ex pe ct sc ho ol i ng  sh to h ave ev ol ve d m os tl y u ni -
form turning preferences, the better to remain
together as a school. Solitary sh, in contrast,
would probably vary randomly in their turning
preferences, because they have little need to
swim toget her. Thi s i s in fact the ca se.
With the realization that the asymmetrical
brain is not specic to humans, new questions
about a number of higher human functions arise:
What are the relative roles of the left and right
hemispheres in having self-awareness, con-
sciousness, empathy or the capacity to have
 as he s o f i ns ig ht ? L it tl e i s k no wn ab ou t t ho se is -
sues. But the ndings we have detailed suggest
that th es e func tio ns like the other human phe-
no me n a d is cu ss e d here will be best understood
i n ter ms of t he de sc en t w it h m od i cat io n o f p re -
human capabilities.
One of the authors (Rogers) discovered that if she exposed chick embryos to light or to
dark before they hatched, she could control whether the two halves of the chick
brains developed their specializations for visual processingthat is, whether the chicks
hatched with weakly or strongly lateralized brains. Rogers and another one of the au-
thors (Vallortigara), with Paolo Zucca of the University of Teramo in Italy, then compared
normal, strongly lateralized chicks with weakly lateralized chicks on two tasks. One
task was to sort food grains from small pebbles (usually a job for the left
hemisphere); the other task was to respond to a model of a
predator (a cutout in the shape of a hawk) that was passed
over the chicks (usually a task for the right hemisphere).
The weakly lateralized chicks had no trouble learning to
tell grains from pebbles when no model hawk was
present. But when the hawk “ew” overhead, they
frequently failed to detect it, and they were much
slower than normal chicks in learning to peck at grains
instead of pebbles. In short, without the lateral
specializations of their brain, the chicks
could not attend to two tasks
StocKby te Gett y Ima ges (chick)
shown that by exposing the embryo of a domes-
tic chick to light or to dark before hatching, she
could manipulate the development of hemi-
spheric specialization for certain functions. Just
befo re hat chi ng, the ch ick e mbr yo’s head i s nat -
urally turned so that the left eye is covered by the
body and only the right eye can be stimulated by
light passing through the egg shell. The light
triggers some of the hemispheric specializations
for visual processing to develop. By incubating
eggs in the dark, Rogers could prevent the spe-
cializations from developing. In particular, she
found, the dark treatment prevents the left hemi-
sphere from developing its normal superior abil-
ity to sort food grains from small pebbles, and it
also prevents the right hemisphere from being
more responsive th an t he left to predato rs.
Rogers and Vallortigara, in collaboration with
Pa olo Z uc ca of the U ni ve rsit y o f Te ra mo i n I ta ly,
tested both kinds of chicks on a dual task: the
chicks had to nd food grains scattered among
pebbles while they monitored for the appearance
of a model predator overhead. The chicks incu-
bated in light could perform both tasks simulta-
neously; those incubated in the dark could not
thereby conrming that a lateralized brain is a
more efcient processor.
Social “Symmetry Brea k ing
Enabling separate and parallel processing to take
place in the two hemispheres may increase brain
efciency, but it does not explain why, within a
species, one or the other specialization tends to
predominate. Why, in most animals, is the left
eye (and the right hemisphere) better suited than
the right eye (and the left hemisphere) for vigi-
lance against predation? W hat makes the pre-
dominance of one kind of handedness more likely
than a symme tric, 5050 m ixt ure of both?
From an evolutionary standpoint a “broken
symmetry, in which populations are made up
mainly of left types or mainly of right types,
could be disadvantageous because the behavior
of i nd iv id ua ls wo ul d b e m or e p re d ic ta bl e t o p re d-
ators. Predators could learn to approach on the
prey’s less vigilant side, thereby reducing the
chance of being detected. The uneven proportion
of left- and right-type individuals in many popu-
lations thus indicates that the imbalance must be
so valuable that it persists despite the increased
vulnerability to predators. Rogers and Vallorti-
gara have suggested that, among social animals,
the advantage of conformity may lie in knowing
what to expe ct from others of one’s own sp ecie s.
Together with Stefano Ghirlanda of the Uni-
Mo r e to
Comparative Vertebrate Lateral-
ization. Edited by Lesley J. Rogers
and Richard J. Andrew. Cambridge
Universit y Press, 2002.
Adva ntag es of Ha ving a L ateral -
ized Brain. Lesley J. Rogers, Paolo
Zucca and Giorgio Vallortigara in
Proceedings of the Royal Society B,
Vol. 271, Suppl. 6, pages S420–S422;
December 7, 2004.
Survival with an Asymmetrical
Brain: Advantages and Disadvan-
tages of Cerebral Lateralization.
Giorgio Vallortigara and Lesley J.
Rogers in Behavioral and Brain
Sciences, Vol. 28, No. 4, pages
575–633; August 2005.
The Origin of Speech. Peter F.
MacNeilage. Oxford University
Press, 2008.
Mechanisms and Functions
of Brain and Behavi oura l
Asymmetries. Luca Tommasi
in Philosophical Transactions of
the Royal Society B, Vol. 364,
pages 855– 859; April 12, 2009.
A Lateralized Brain Is More Efcient
... Finally, the spontaneous paw preference of mice that completed the training was assessed one last time to test for its stability (PND 98). Throughout the experiment, individual body weights of mice were monitored on a weekly basis on PND's 34,41,48,55,62,69,76,83, 90 and 97. ...
... Notably, the unsuccessful retraining of right, but not left pawed mice, might be due to functional hemispheric differences. Indeed, the left hemisphere controls routine behaviours [62,63] and the right hemisphere is involved in the detection and analysis of novelty [61,62]. Furthermore, the retrieval of short-and long term memory seems to be a lateralized process [64]. ...
... Notably, the unsuccessful retraining of right, but not left pawed mice, might be due to functional hemispheric differences. Indeed, the left hemisphere controls routine behaviours [62,63] and the right hemisphere is involved in the detection and analysis of novelty [61,62]. Furthermore, the retrieval of short-and long term memory seems to be a lateralized process [64]. ...
Spontaneous limb preferences exist in numerous species. To investigate the underlying mechanisms of these preferences, different methods, such as training, have been developed to shift preferences artificially. However, studies that systematically examine the effects of shifting preferences on behaviour and physiology are largely missing. Therefore, the aim of this study was to assess the impact of shifting paw preferences via training on spontaneous home cage behaviour, as well as anxiety-like behaviour and exploratory locomotion (Elevated plus maze test, Dark light test, Open field test, Free exploration test), learning performance (Labyrinth-maze) and stress hormones (fecal corticosterone metabolites) in laboratory mice (Mus musculus f. domestica). For this, we assessed spontaneous paw preferences of C57BL/6 J females (Nambilateral = 23, Nleft = 23, Nright = 25). Subsequently, half of the individuals from each category were trained once a week for four weeks in a food-reaching task to use either their left or right paw, respectively, resulting in six groups: AL, AR, LL, LR, RL, RR. After training, a battery of behavioural tests was performed and spontaneous preferences were assessed again. Our results indicate that most mice were successfully trained and the effect of training was present days after training. However, a significant difference of preferences between RL and LL mice during training suggests a rather low training success of RL mice. Additionally, preferences of L mice differed from those of A and R mice after training, indicating differential long-term effects of training in these groups. Furthermore, left paw training led to higher levels of self-grooming, possibly as a displacement behaviour, and more time spent in the light compartment of the Dark light test. However, overall, there was no systematic influence of training on behavioural measures and stress hormones. Different explanations for this lack of influence, such as the link between training and hemispheric functioning or the intensity and ecological relevance of the training, are discussed.
... Prieur et al. 2017b), en raison notamment de leur socialité ou de leur écologie. La théorie d'une origine sociale de la latéralité des comportements propose qu'une latéralisation à échelle populationnelle favorise la coordination et la coopération entre les individus d'un même groupe (Ghirlanda et Vallortigara 2004;Vallortigara et Rogers 2005;MacNeilage et al. 2009). Suivant cette théorie, il est ainsi attendu qu'un plus haut niveau de coopération au sein d'une espèce favorise une plus forte latéralité gestuelle à échelle populationnelle, et que celle-ci soit soumise à des contraintes sociales. ...
... Tester l'hypothèse d'une latéralité gestuelle à échelle populationnelle chez les mangabeys, avec un échantillon plus important, pourra permettre des comparaisons avec d'autres espèces. Cette approche comparative est un moyen de déterminer les facteurs écologiques et sociaux impactant la latéralité gestuelle, comme le degré d'arboricolisme ou la complexité sociale (MacNeilage 2007;MacNeilage et al. 2009;Schaafsma et al. 2009;Hopkins et al. 2011b;. ...
... Les vocalisations d'alarmes sont plus conservées et moins flexibles au sein d'une espèce que des vocalisations associées à des contextes moins urgents (Bouchet et al. 2016). De même, les gestes produits dans des situations d'agressions pourraient être plus latéralisés car moins soumis à l'influence d'autres facteurs proximaux, et plus déterminés par des influences sociales au moment de leur acquisition (théorie de l'origine sociale de la latéralité : Ghirlanda et Vallortigara 2004;Vallortigara et Rogers 2005;MacNeilage et al. 2009). Inclure l'effet du type de geste dans l'analyse chez les mangabeys permettrait de tester cette hypothèse, en s'intéressant en particulier à la latéralité de mêmes gestes produits en contextes négatifs ou positifs. ...
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Ce travail s’inscrit dans l’étude des origines évolutives du langage, par la recherche de propriétés langagières dans la communication gestuelle et multimodale de primates cercopithécidés en captivité, les mangabeys à collier. Par une double approche observationnelle et expérimentale, nous avons montré que les gestes des mangabeys remplissent les critères de définition d’une communication intentionnelle, et peuvent être produits de manière flexible dans différents contextes. Nos observations fournissent également de premiers éléments en faveur d’une intentionnalité des expressions faciales des cercopithécidés, souvent considérées comme de simples indices d’état émotionnel. Cette propriété sociocognitive langagière pourrait ainsi être plus ancienne que ce que nous pensions dans l’histoire évolutive des primates, et être héritée de la communication gestuelle des ancêtres des catarrhiniens, il y a environ 29 millions d’années. De plus, nous avons mis en évidence un effet significatif du contexte interactionnel sur la latéralité gestuelle des mangabeys, suggérant une importance particulière de facteurs sociaux dans l’émergence d’une spécialisation hémisphérique pour la communication intentionnelle, dont le langage humain. Enfin, par une méthode originale, reposant sur des analyses de séquences et de réseau, nous avons décrit la communication multimodale et multicomposante des mangabeys à collier, et montré qu’ils combinent de manière flexible différents types et modalités de signaux en fonction du contexte et de facteurs sociodémographiques. Nos résultats soulignent l’importance d’une approche multimodale pour comprendre la complexité de la communication des primates, et apportent de premier éléments de compréhension sur la fonction des combinaisons de signaux. De futures comparaisons à d’autres espèces et dans différents environnements pourraient permettre d’affiner nos connaissances quant aux possibles contraintes évolutives ayant favorisé une telle complexité de la communication des primates humains et non-humains.
... Although the Kong™ Test is widely used, its interpretation and validity as a measure of paw preference is unclear. First, given that feeding behavior is associated with a general specialization of left-brain networks, which indirectly manifests itself in different lateralized behaviors in various species [55][56][57], dogs' motor response in the Kong™ Test could be biased by food-related lateralization patterns. Therefore, the Kong™ Test may be less indicative of a general paw preference, but rather it may measure a highly task-specific motor laterality pattern that reflects food-related specialized hemispherical functioning. ...
... Both locomotion tests required the dogs to take up a stationary start position on command and finally to walk to their owner after being called. It is therefore possible that some dogs were expecting some form of reward for the completion of these tasks and that this expectation was associated with food-related and/or positive emotion-related hemispherically lateralized brain activity that could have influenced dogs' motor response [31][32][33][34][35][36][37][54][55][56]. This is unlikely to account for our results, as they are not consistent between the Kong™ Test and the locomotion tests. ...
Full-text available
Research with humans and other animals has suggested that preferential limb use is linked to emotionality. A better understanding of this still under-explored area has the potential to establish limb preference as a marker of emotional vulnerability and risk for affective disorders. This study explored the potential relationship between paw preference and emotionality in pet dogs. We examined which paw the dogs preferentially used to hold a Kong™ and to perform two different locomotion tests. Dogs’ emotionality was assessed using a validated psychometric test (the Positive and Negative Activation Scale—PANAS). Significant positive correlations were found for dogs’ paw use between the different locomotion tasks, suggesting that dogs may show a more general paw preference that is stable across different types of locomotion. In comparison, the correlations between the Kong™ Test and locomotion tests were only partially significant, likely due to potential limitations of the Kong™ Test and/or test-specific biomechanical requirements. No significant correlations were identified between paw preference tests and PANAS scores. These results are in contrast to previous reports of an association between dog paw preference and emotionality; animal limb preference might be task-specific and have variable task-consistency, which raises methodological questions about the use of paw preference as a marker for emotional functioning.
... The left and right hemispheres are neurologically connected, mostly to the contralateral side of the body. Therefore, information received through the right eye, ear and tactile senses will largely be processed in the left hemisphere, which mostly controls the movements of the contralateral, i.e., the right, limbs [39], while information perceived by the left eye, ear and tactile senses is processed in the right hemisphere and given back to the left limbs [39]. However, information gathered by the nostrils is processed by the ipsilateral hemisphere, i.e., the same side of the brain [8,12,36]. ...
... The left and right hemispheres are neurologically connected, mostly to the contralateral side of the body. Therefore, information received through the right eye, ear and tactile senses will largely be processed in the left hemisphere, which mostly controls the movements of the contralateral, i.e., the right, limbs [39], while information perceived by the left eye, ear and tactile senses is processed in the right hemisphere and given back to the left limbs [39]. However, information gathered by the nostrils is processed by the ipsilateral hemisphere, i.e., the same side of the brain [8,12,36]. ...
Full-text available
For centuries, a goal of training in many equestrian disciplines has been to straighten the horse, which is considered a key element in achieving its responsiveness and suppleness. However, laterality is a naturally occurring phenomenon in horses and encompasses body asymmetry, motor laterality and sensory laterality. Furthermore, forcibly counterbalancing motor laterality has been considered a cause of psychological imbalance in humans. Perhaps asymmetry and laterality should rather be accepted, with a focus on training psychological and physical balance, coordination and equal strength on both sides instead of enforcing “straightness”. To explore this, we conducted a review of the literature on the function and causes of motor and sensory laterality in horses, especially in horses when trained on the ground or under a rider. The literature reveals that body asymmetry is innate but does not prevent the horse from performing at a high level under a rider. Motor laterality is equally distributed in feral horses, while in domestic horses, age, breed, training and carrying a rider may cause left leg preferences. Most horses initially observe novel persons and potentially threatening objects or situations with their left sensory organs. Pronounced preferences for the use of left sensory organs or limbs indicate that the horse is experiencing increased emotionality or stress, and long-term insufficiencies in welfare, housing or training may result in left shifts in motor and sensory laterality and pessimistic mentalities. Therefore, increasing laterality can be regarded as an indicator for insufficiencies in housing, handling and training. We propose that laterality be recognized as a welfare indicator and that straightening the horse should be achieved by conducting training focused on balance, coordination and equal strength on both sides.
... Studies in comparative psychology have suggested that brain asymmetry was in place before the appearance of vertebrates [186,187]. A primary functional differentiation between the two hemispheres would have been already set at this point in evolution. ...
Experimental and theoretical studies have tried to gain insights into the involvement of the Temporal Parietal Junction (TPJ) in a broad range of cognitive functions like memory, attention, language, self-agency and theory of mind. Recent investigations have demonstrated the partition of the TPJ in discrete subsectors. Nonetheless, whether these subsectors play different roles or implement an overarching function remains debated. Here, based on a review of available evidence, we propose that the left TPJ codes both matches and mismatches between expected and actual sensory, motor, or cognitive events while the right TPJ codes mismatches. These operations help keeping track of statistical contingencies in personal, environmental, and conceptual space. We show that this hypothesis can account for the participation of the TPJ in disparate cognitive functions, including "humour", and explain: a) the higher incidence of spatial neglect in right brain damage; b) the different emotional reactions that follow left and right brain damage; c) the hemispheric lateralisation of optimistic bias mechanisms; d) the lateralisation of mechanisms that regulate routine and novelty behaviours. We propose that match and mismatch operations are aimed at approximating "free energy", in terms of the free energy principle of decision-making. By approximating "free energy", the match/mismatch TPJ system supports both information seeking to update one's beliefs and the "cultural" pleasure of being right in one's own' current choices and beliefs. This renewed view of the TPJ has relevant clinical implications because the misfunctioning of TPJ-related "match" and "mismatch" circuits in unilateral brain damage can produce low-dimensional deficits of predictive coding that can be associated with different neuropsychological disorders.
... In general, such so-called hemispheric asymmetries have been observed in the brains of all major vertebrate classes (Güntürkün et al., 2020;MacNeilage et al., 2009;Manns et al., 2021;Ocklenburg et al., 2013b;Rogers and Vallortigara, 2021;Ströckens et al., 2013;Vallortigara and Rogers, 2020). Therefore, they are considered a general organizational principle in vertebrate brain architecture (Ocklenburg and Güntürkün, 2018). ...
The amygdala is a core structure in the neuronal network underlying emotion processing in the vertebrate brain. Its structure and function have been extensively studied in both neuroimaging studies in human volunteers and comparative studies in animal models. Across different studies and research questions regarding the amygdala, one often-encountered finding is that the left and the right amygdala are not equivalent in terms of function and structure. Hemispheric asymmetries in the amygdala have been reported on many different levels, yet a systematic integration of these findings has been missing from the literature. Researchers in both cognitive and clinical neurosciences are often puzzled why they find a specific effect or association for the left but not the right amygdala, or vice versa. In this review article, we provide an integrated overview of existing basic and clinical findings regarding amygdala asymmetries in structure, connections, and functions. Importantly, the literature suggests that functional amygdala lateralization is determined by temporal characteristics, emotional valence, and perceptual properties. Furthermore, we highlight alterations of amygdala asymmetries reported in different patient groups, thereby allowing for a deeper understanding of atypical amygdala asymmetries. Lastly, we aim to provide guidelines and approaches concerning the interpretation of results for researchers investigating amygdala asymmetries.
... Cerebral laterality comprises motor and sensory laterality and is the result of different specialisations in the brain hemispheres [4,5]. Depending on the type of information and the situation, processing is either predominantly in the left or the right hemisphere as summarised by [6], with information transferring from the sensory organs and limbs on one side of the body to the brain hemisphere on the opposite side [7]. One proposed distinction between the functions of the hemispheres in vertebrates is to describe the left hemisphere as instruction driven and the right hemisphere as stimulus driven [6]. ...
Full-text available
Laterality in horses has been studied in recent decades. Although most horses are kept for riding purposes, there has been almost no research on how laterality may be affected by carrying a rider. In this study, 23 horses were tested for lateral preferences, both with and without a rider, in three different experiments. The rider gave minimal aids and rode on a long rein to allow the horse free choice. Firstly, motor laterality was assessed by observing forelimb preference when stepping over a pole. Secondly, sensory laterality was assessed by observing perceptual side preferences when the horse was confronted with (a) an unfamiliar person or (b) a novel object. After applying a generalised linear model, this preliminary study found that a rider increased the strength of motor laterality (p = 0.01) but did not affect sensory laterality (p = 0.8). This suggests that carrying a rider who is as passive as possible does not have an adverse effect on a horse’s stress levels and mental state.
... A preferential use of the left eye (mainly feeding the right brain structures) is associated with response to novelty in birds with laterally placed eyes such as domestic chicks (Rogers et al., 2013). Note that the selective involvement of structures in the right hemisphere when attending to novel stimuli is widely documented among vertebrates (review in Rogers et al., 2013), being likely a general feature inherited by early chordates (MacNeilage et al., 2009). In animals with laterally placed eyes and lack of callosum, such as birds, fish, reptiles, and amphibians the brain asymmetry can be easily documented without any invasive procedure by simply measuring preferences in eye use (Vallortigara, 2000;Vallortigara and Versace, 2017;Vallortigara and Rogers, 2020). ...
Full-text available
Absence is a notion that is usually captured by language-related concepts like zero or negation. Whether nonlinguistic creatures encode similar thoughts is an open question, as everyday behavior marked by absence (of food, of social partners) can be explained solely by expecting presence somewhere else. We investigated 8-day-old chicks’ looking behavior in response to events violating expectations about the presence or absence of an object. We found different behavioral responses to violations of presence and absence, suggesting distinct underlying mechanisms. Importantly, chicks displayed an avian signature of novelty detection to violations of absence, namely a sex-dependent left-eye-bias. Follow-up experiments excluded accounts that would explain this bias by perceptual mismatch or by representing the object at different locations. These results suggest that the ability to spontaneously form representations about the absence of objects likely belongs to the initial cognitive repertoire of vertebrate species.
Full-text available
The aim of the study was to assess the relationship between the motor lateralization in dogs and the concentration of cortisol and tyrosine in their plasma during a visit to a veterinary clinic. The research group consisted of 56 dogs. Motor lateralization was tested by of an adhesive tape test. The stress intensity was assessed basing on the levels of cortisol and ft4 in the serum blood. The statistical analysis revealed that in the group of 56 dogs there were 14 left-pawed dogs, 36 right-pawed dogs, and 6 dogs did not show particular paw preference. The average cortisol levels in particular groups were as follows: 7.94 ug/dl, 3.92 ug/dl and 3.7 ug/dl, whereas the level of tyrosine in the subjects that demonstrated left-sided lateralization (mean ± SE) (1.95 ± 0.46 ng/dl) for those with right-side lateralization (1.56 ± 0.23 ng/dl) and ambilateral dogs (1.01 ± 0.22 ng/dl). The statistical calculation of Pearson's χ2 showed a significant relationship between the sex and the lateralization (χ2 = 6.238, df = 2, p = 0.0442).
The current research focuses on color preference between red and green stimuli and manual laterality in the emperor tamarin (Saguinus imperator). Trichromacy in primates has been related to a foraging advantage allowing frugivore primates to distinguish ripe from unripe fruits as well to socio-sexual communication, as trichromats would be advantaged in recognizing social and sexual signals. As warm colors can affect the emotive state of the subjects, leading to the activation of one hemisphere over the other (e.g., right hemisphere), this could lead to behavioral lateralization. Thus, studying of hand preference may be relevant when testing color preference. Nine adult zoo emperor tamarins were involved and the study aimed to investigate the preference between red, green, and white cones as well as manual laterality. Tamarins were provided with pairs of red-green, red-white, and green-white combinations of cones. Ten 30-min sessions per combination were carried out and data on the interaction with one of the two cones of each apparatus were collected to assess subjects' color preference. We also recorded the hand used by each subject during the interaction with cones of different colors and the position of the apparatus in respect to the tamarin. We found no preferences for colored versus white cones. Similarly, we reported no group-level preferences within different color combinations, whereas individual-level preferences were found when considering all choices. Finally, we found that red cones elicited a left-hand preference, suggesting a right-hemisphere involvement in the presence of red cones. Although we do not have genetic data on trichromat and dichromat females, the tendency to use the left hand when interacting with red stimuli provides further evidence that warm colors can influence the emotive state of the perceiver, affecting their manual lateralization.
ResearchGate has not been able to resolve any references for this publication.