INSIGHTS | PERSPECTIVES
478 30 JANUARY 2015 • VOL 347 ISSUE 6221 sciencemag.org SCIENCE
contrast, after training with a numerosity
of 20, eight elements initiated a leftward
search (see the figure, panel A). These findings suggest the existence of a mental number line in the chick’s mind. Small numbers
are represented to the left of larger numbers,
but whether a quantity is deemed small or
large depends on the situation.
The work casts doubt on the importance of
language and symbolic thought for the ability
to represent discrete quantities larger than 3
and to develop a sense of numerical order
and counting routines. Field studies of avian
behavior have previously documented this
ability in adult birds. A particularly revealing
model system is that of brood parasites such
as cowbirds. Females of this species must
locate, observe, and remember the exact location of host nests (4). They must also synchronize their own egg laying with that of the
host by keeping track of the daily increase in
host clutch sizes (5). The cowbirds’ reproductive success thus depends on their ability to
accurately judge magnitudes in space, time,
and number. Whereas the observations in
cowbirds involved adult individuals, Rugani
et al.’s experiments with 3-day-old domestic
chicks support the view that a general magnitude system may be functional at birth. Recent experiments with newborn humans are
compatible with this notion; de Hevia et al.
have shown that infants aged 8 hours to 3
days systematically related increases in numerical magnitude with increases in spatial
extent and temporal duration (6).
A more specific insight from Rugani et
al.’s study is that a chick’s sense of numerical order is tightly coupled with its sense
of space: “More than” is equivalent to “to
the right of.” This leads to a left-to-right directionality in the mapping of numbers to
space—a finding that puts several previous
proposals for the origin of mental number
lines into perspective. One reason why researchers have assumed that this kind of
numerical mapping is an invention of the
human mind is its cultural modification.
In cultures with a left-to-right reading and
writing direction, the number line expands
from left to right, but cultures with an opposite directional handling of script align
numbers from right to left (2). Obviously,
reading/writing direction cannot be the
ultimate cause of directionality, nor can
finger-counting habits (7). Presumably, the
predominant role of the right hemisphere
for numerical ordering (8) biases initial attention to the left side of both physical and
number space. Together with a preference
for increasing over decreasing order—
already apparent in 4-month-old human
infants (9, 10)—the biological default of a
number line would point from left to right.
The chicks’ positioning of a numerosity to
either the left or the right side of space de-
pending on a given standard gives testimony
to a flexibility that is far from bird-brained.
Such flexible assignment is in direct anal-
ogy to the relativity of number-space asso-
ciations in the human brain. Neurological
patients who neglect the left side of space
are also impaired in processing numerals
located to the left of a given standard (see
the figure, panel B) (11). It is the flexible
classification of extents, amounts,
and magnitudes as left-sided or
right-sided that may have allowed
situational and possibly cultural
variations in the directionality of
number lines to arise.
Rugani et al. offer a key lesson
in how informative a simple but
elegantly designed behavioral experiment with a precocious species can be for the interpretation
of number-space associations in
the human mind. They provide
a provocative set of hypotheses
to be tested in future research.
What is the role of emotions in
the spatialization of magnitudes?
In both natural environments and
laboratory situations, “more” is
commonly equivalent to “better.”
Chicks, like other animals, prefer
more over less food and prefer to
follow many rather than few companions (12). In animals and humans, the left cerebral hemisphere
is specialized for the processing of positive
emotions, the right hemisphere for negative
emotions (13). If more feels better, could the
left hemisphere’s positivity bias favor an association of larger magnitudes with the right
side of space, which it primarily controls?
With respect to human number-space associations, what environmental or epigenetic
factors have contributed to the development
of a right-to-left orientation in only a minority of cultures with a horizontally organized
script? Such questions need to be tackled
by many disciplines jointly, including behavioral ecology, developmental psychology,
comparative linguistics, and culture-sensi-tive neuroscience (14). ■
1. S. Dehaene, The Number Sense (Oxford Univ. Press, New
2. S. M. Göbel, S. Shaki, M. H. Fischer, J. Cross Cult. Psychol.
42, 543 (2011).
3. R. Rugani, G. Vallortigara, K. Priftis, L. Regolin,Science
347, 534 (2015).
4. D. J. White, L. Ho, G. Freed-Brown, Psychol. Sci. 20, 1140
5. J. Low, K. C. Burns, M. E. Hauber, Int.J.Avian Sci. 151, 775
6. M.D.de Hevia,V.Izard, A.Coubart,E.S.Spelke, A.Streri,
Proc. Natl. Acad. Sci. U.S.A. 111, 4809 (2014).
7. M. H. Fischer, P. Brugger, Front. Psychol. 2, 260 (2011).
8. A. Knops, K. Willmes,Neuroimage 84, 786 (2014).
9. V. Macchi Cassia, M. Picozzi, L. Girelli, M. D. de Hevia,
Cognition 124, 183 (2012).
10. M. D. de Hevia, L. Girelli, M. Addabbo, V. Macchi Cassia,
PLOS ONE 9, e96412 (2014).
11. P. Vuilleumier, S. Ortigue, P. Brugger,Cortex 40, 399 (2004).
12. R. Rugan et al., Front. Psychol. 5, 150 (2014).
13. L.J.Rogers, G.Vallortigara, R.J.Andrew, Divided
Brains: The Biology and Behavior of Brain Asymmetries
(Cambridge Univ. Press, New York, 2013).
14. S.Kazandjian, S.Chokron, Nat. Rev. Neurosci. 9,965
Of chicks and humans. (A) Rugani et al. show that chicks trained to find food behind a panel representing an abstract number
(dots differed in color and shape but were matched for area and circumference) expect food behind the left of two panels
representing a smaller number, but behind the right for a larger number. In the example, the numerosity representing 8 is
smaller than that experienced during training with 20 and thus associated with the left side of space. After exposure to a training
numerosity of 5, the same numerosity of 8 is associated with the right side of space. (B) In humans, patients who neglect the left
side of space after right hemisphere damage are slow in classifying 6 as smaller or larger than 7, but fast if the reference number is
5 ( 11). The representation of increasing quantities from left to right may be the biological default, modulated (but not caused) by
situational demands or cultural habits.
32 > 20
8 < 20
8 > 5
2 < 5
Reference number for
decision task “smaller
or larger than 5?”
Reference number for
decision task “smaller
or larger than 7?”
Neuropsychology Unit, Department of Neurology, University
Hospital Zürich, CH-8091 Zürich, Switzerland. E-mail: peter.
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