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Symbolic representation of number in chimpanzeesTetsuro Matsuzawa
This paper aims to summarize the existing evidence for the
symbolic representation of number in chimpanzees.
Chimpanzees can represent, to some extent, both the cardinal
and the ordinal aspect of number. Through the medium of
Arabic numerals we compared working memory in humans and
chimpanzees using the same apparatus and following the same
procedure. Three young chimpanzees outperformed human
adults in memorizing briefly presented numerals. However, we
found that chimpanzees were less proficient at a variety of
other cognitive tasks including imitation, cross-modal
matching, symmetry of symbols and referents, and one-to-one
correspondence. In sum, chimpanzees do not possess human-
like capabilities for representation at an abstract level. The
present paper will discuss the constraints of the number
concept in chimpanzees, and illuminate some unique features
of human cognition.
Address
Primate Research Institute, Kyoto University, 41 Kanrin, Inuyama, Aichi
484-8506, Japan
Corresponding author: Matsuzawa, Tetsuro
Current Opinion in Neurobiology 2009, 19:92–98
This review comes from a themed issue on
Cognitive neuroscience
Edited by Michael Platt and Elizabeth Spelke
Available online 14th May 2009
0959-4388/$ – see front matter
# 2009 Elsevier Ltd. All rights reserved.
DOI 10.1016/j.conb.2009.04.007
IntroductionUsing the Arabic numerals 0 through 9, the chimpanzee
Ai can represent both the cardinal and the ordinal aspect
of number to some extent [1�]. Similar skills have now
been partially confirmed in six other chimpanzees [2��,3].
A recent study has used numerical stimuli to demonstrate
an extraordinary memory capability in young chimpan-
zees [2��]. In contrast with the memory test, chimpanzees
show poor performance in many other cognitive tasks that
require some form of abstraction. The present paper will
address the following five topics. First, it describes the
logic and framework for the study of the number concept
in nonhuman animals. Second, it reviews the long-run-
ning Ai project, focusing in particular on studies dealing
with the symbolic representation of number. Third, it
describes the extraordinary memory capacity that young
Current Opinion in Neurobiology 2009, 19:92–98
chimpanzees possess for numerals. Fourth, it introduces
common characteristics of the difficulties experienced by
chimpanzees in various kinds of cognitive tasks. Finally,
it discusses the number concept within the larger context
of cognition, illuminating cognitive development in
chimpanzees and the unique features of human cognition.
The Ai project: psychophysical measurementand the underlying logicThe Ai project – encompassing a series of studies whose
principal subject has been a female chimpanzee named Ai
– began when Ai, at the age of 1.5 years, first touched an
experimental keyboard on April 15th 1978 [3–5]. The last
in a succession of ape-language projects launched in the
second half of the 20th century, the Ai-project was also
the forerunner of a new research paradigm called Com-
parative Cognitive Science (CCS) [6]. CCS is the com-
bined study of psychophysics and ape-language, using
computer-controlled apparatus. In its original form, CCS
aimed to compare perception, memory, and cognition in
humans with those of closely related species such as
chimpanzees. Crucially, such inter-species comparisons
rely on identical methods: subjects, irrespective of
species, use the same apparatus and follow the same
procedure during testing.
Psychophysics is the classic psychological study of
measuring human sensation, perception, memory, and
cognition in general. Psychophysics tries to unravel the
relationship between physical events and psychological
events. Psychophysical researchers have established
many laws, including, for example, Weber–Fechner’s
law and Steven’s power law. There exist well-established
methods for measuring the relationship between the
stimuli presented and the corresponding internal psycho-
logical states. Thus, psychophysics can be extended to
issues related to concept formation, such as the under-
standing of number.
The Ai project has two important features: the research
presents humans and chimpanzees with exactly the same
tasks so that their performance can be compared in detail,
and that the research undertakes that comparison by
using the methods of psychophysics. There is the syner-
gistic connection between these two features for the
enterprise of CCS. CCS aims to compare different species
at the level of cognitive mechanisms, and it is the great
virtue of psychophysical method that serves not only to
describe the performance of humans and nonhuman
animals but also to analyze that performance into its
component mechanisms. By adopting psychophysical
approach, you can go beyond questions of what trained
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Symbolic representation of number in chimpanzees Matsuzawa 93
chimpanzees can do and ask how chimpanzees and
humans do what they do: What cognitive capacities are
shared by the two species and what capacities distinguish
them.
Ai was the first chimpanzee who learned to discriminate
the 26 letters of the alphabet during tests of shape
perception and visual acuity [7]. She also mastered
additional visual symbols such as specially devised geo-
metric shapes (called lexigrams) and Japanese Kanji
characters signifying different colors, in the course of
experiments designed to examine her classification of
various colors [8].
Ever since the beginning of the Ai-project, my colleagues
and I have been focusing on the mathematical skills of
chimpanzees rather than on any form of bilateral com-
munication between humans and chimpanzees—the latter
having been the central theme of previous ape-language
studies. There were four reasons why we decided to
concentrate on issues related to number concepts.
First, the world of logico-mathematical skills was small.
Mathematical cognition is a part of human cognition in
general, but the number system is narrower and more
clearly and precisely defined in comparison to the lin-
guistic system. The number system includes the levels of
Integer that is a part of Real number, and that is a part of
Complex number. The scales of the number system
advance from Nominal to Ordinal, Interval, and Ratio
scales. Therefore, focusing on the Integer, for example,
we can examine both cardinal aspects (‘one, two, three,
four. . .’) and ordinal aspects (‘first, second, third,
fourth. . .’) of the number concept in chimpanzees. From
the perspective of future work, we can examine the
combination of numerical elements to create new mean-
ing: such as putting 1 and 0 together to create ‘10’ in the
decimal number system. Moreover, further questions
open up regarding numerical operations such as addition
and subtraction. These kinds of numerical manipulations
can be viewed as corresponding to the syntactical struc-
ture of the language system.
Our second reason for focusing on number concepts was
the accumulation of knowledge about mathematical
thinking in human subjects. Several important papers
had just been published about the child’s understanding
of number, including Gelman and Gallistel’s landmark
article [9]. These helped us to discern what we ought to
study in chimpanzees. Since then, further studies on
number concepts in humans have continued to shed light
on many relevant issues and promoted our understanding
of various aspects of numerical cognition (see reviews in
[10–13]) including its neural mechanisms [14,15].
Third, a handful of studies had reported on topics related
to the number concept in nonhuman animals, such as
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enumeration, quantity discrimination, and sequential
responding. Today, much attention continues to be paid
to the concept of number in nonhuman animals [16,17].
Various researchers have published studies that they
described as tests of numerical competence in macaque
monkeys [18–20], cotton-top tamarin [21], elephant [22�],parrot [23], pigeons [24], even salamanders [25] and bees
[26�]. However, none of these compared the performance
of human and nonhuman subjects directly, using the
same methodology. Almost all the literature on nonhu-
man animals has focused on rudimentary forms of the
number concept. By contrast, the Ai-project’s series of
number concept studies stands out clearly for two reasons.
First, it focuses on the symbolic representation of number
using Arabic numerals. This symbolic representation
allowed us to conduct direct comparisons between
humans and chimpanzees. Second, we applied psycho-
physical measurements using a fully automated compu-
ter-controlled apparatus that completely excluded any
kind of social cueing.
The fourth reason why we decided to concentrate on
issues related to number concepts is the connection to the
psychophysical approach. Most domains of human cogni-
tion are not yet amenable to psychophysical analysis,
because the proper mathematical characterization of
the cognitive domain is not clear. Number, however, is
highly amenable to psychophysical analysis. Indeed, the
Weber–Fechner’s law of psychophysics holds in the
numerical domain.
Cardinal and ordinal aspects of number andintroducing zeroAi is the first chimpanzee who mastered the use of Arabic
numerals to represent numbers [4]. She learned both
cardinal and ordinal aspects of the number system. She
used the numerals to label real-life items, shown to her in
a display window, in terms of their numerosity. She also
became proficient at responding to Arabic numerals in
ascending order, selecting them from a touch-screen in
the correct sequence. When the numeral ‘0’ was intro-
duced, Ai mastered its use in both the cardinal and the
ordinal domains; however, the acquisition process and the
generalization tests in this case clearly highlighted the
constraints of the symbolic representation of number
acquired by the chimpanzee [1�]. Further detail on Ai’s
experience with numerals follows below.
Cardinal number training began when Ai was about five
years old. She had already learnt to use lexigrams corre-
sponding to object and color names. Ai mastered the skill
of naming 1 through 6, in addition to the naming of 14
objects and 11 colors. Although the ‘word order’ of
describing a particular sample (such as ‘five red tooth-
brushes’) was free, she spontaneously adopted a favored
sequence, assigning the object and color names first and
then naming the number last. This rule of describing the
Current Opinion in Neurobiology 2009, 19:92–98
94 Cognitive neuroscience
number in the final position was preserved in the case of
new objects and new colors. It was preserved even when
the number of objects was limited to just one. In a sense,
the chimpanzee spontaneously developed her own ‘gram-
mar’ to describe the world [4].
Patterns in the ease with which Ai acquired object, color,
and numerical labels provide an interesting contrast.
During the initial phase of learning to use visual symbols
to label corresponding objects, Ai experienced consider-
able difficulties: the first two objects required a long
period of training. However, once past this early hurdle,
learning to name a third object became easier, and the
fourth easier yet. The same pattern was evident in the
case of color naming [8]. This suggests that the chim-
panzee ‘learned to learn’ in terms of objects and color
naming. On the contrary, no comparable improvement
was observed in the case of numerical naming: The
sessions required to reach the learning criterion did not
decrease as the numerals 3, 4, 5, and 6 were successively
introduced.
After becoming proficient at the numerical naming of real
life objects, Ai learned to label white dots projected onto a
touch-screen monitor [27,28]. Her range of numerosities
and associated labels was also extended, up to nine.
Transferring the task on to a fully automated, computer-
ized system further allowed us to measure precisely Ai’s
response latencies in numerical naming. We found that the
response latency remained constant up to about five dots;
thereafter we saw a gradual increase in the time taken to
label progressively larger sets. At a look, this tendency
reminded us of the two phases of numerical labeling in the
case of human subjects, subitizing and counting. However,
Ai’s response latency reached its maximum at eight dots,
rather than the largest of the sets, at nine. In fact, this
tendency was clear throughout the process of acquisition:
The latency was always at maximum at n � 1 dots, when
the largest set being tested was of size n. This strongly
suggests that instead of counting the dots one by one, the
chimpanzee seemed to be performing analog magnitude
estimation of the number of dots. Such magnitude esti-
mation of quantity is well documented not only in chim-
panzees [29] but also in other animals [18]. Of course, Ai’s
performance does not necessarily imply that chimpanzees
cannot count; it simply proves that it is not a straightforward
task to teach them to solve problems by counting items one
by one.
In addition to the cardinal aspect of number, we also
trained Ai in the ordinal domain. First she learned to
select Arabic numerals from 1 to 9 on a touch-screen, in
ascending order [30]. After initial training with consecu-
tive items, we introduced sequences of nonadjacent
numerals—these tests revealed both a positional and a
symbolic distance effect in Ai’s performance. The pos-
itional effect means that Ai’s latency to select the first
Current Opinion in Neurobiology 2009, 19:92–98
item in a sequence was short when this item was a small
number; she took longer to decide on the first item when
this was large. The symbolic distance effect describes the
finding that latency decreases when two numerals being
compared are far apart (in terms of their position in the
sequence); the task becomes more difficult when the
numerals are close. In sum, Ai (as well as other chimpan-
zees we subsequently trained) clearly mastered the
sequencing of numerals along the ordinal scale [30,31].
In a follow-up study, we attempted to add the number 0 to
Ai’s repertoire, training her in the meaning of ‘zero’ first in
the cardinal and then in the ordinal domain [1�]. She was
initially required to use the numeral 0 to describe the
absence of dots in the display frame. After she had learned
to use zero in this cardinal setting, we introduced the
numeral 0 in the ordinal sequence task. She did not
spontaneously place the numeral 0 before 1; there was,
thus, no immediate transfer of the meaning of zero from
the cardinal to the ordinal domain. However, once again,
these negative data do not imply that chimpanzees cannot
possess the concept of zero—in fact, Ai eventually suc-
ceeded in mastering both the cardinal and the ordinal
meaning of the numeral. Nevertheless, whether chim-
panzees are capable of bilateral transfer between cardinal
and ordinal domains of number remains an open question.
Working memory of numerals in chimpanzeesPeople’s general view of the chimpanzee mind is that it is
inferior to that of humans [32]. My colleagues and I
showed for the first time that young chimpanzees have
an extraordinary working memory capacity for numerical
recollection [33,2��]. Their performance was better than
that of human adults tested on the same apparatus fol-
lowing the same procedure.
The subjects of this study were six chimpanzees, three
mother–offspring pairs, including Ai. We started training
the three young chimpanzees at the age of four years; the
two mothers who were also naı̈ve to the task received the
same training in parallel. All of them succeeded in learn-
ing the sequence of Arabic numerals from 1 to 9, using a
touch-screen monitor connected to a computer. A mem-
ory task called ‘masking task’ was then introduced, at
around the time when the young chimps reached five
years of age. In this task, after touching the first numeral,
all other numerals were replaced by white squares. The
subjects had to remember which numeral had appeared in
which location, and then touch the squares in the correct
ascending order. Ayumu, one of the three young chim-
panzees, can memorize the nine numerals in 0.67 s. His
performance is much faster and more accurate than
human adults (Figure 1).
We next invented a novel variation of the memory test,
the so-called ‘limited-hold memory task’ [2��]. In this
task, after touching the initial white circle that served as
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Symbolic representation of number in chimpanzees Matsuzawa 95
Figure 1
Chimpanzee Ayumu, at the age of 5.5 years, performs the masking task in which the subject has to remember which numeral appeared in which
location of the monitor. The latency to touch the first numeral was 0.67 s. The young chimpanzees can memorize the nine numerals much faster and
more accurate than human adults [2��].
the start key to each trial, the numerals appeared only for
a certain limited duration, and were then automatically
replaced by white squares. We tested both chimpanzee
subjects (Ai and her son Ayumu) and human subjects on
three different hold duration conditions: 650, 430, and
210 ms.
The number of numerals was limited to five. In human
subjects and Ai, the percentage of correct trials decreased
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as a function of hold duration. However, from the very
first session, Ayumu’s performance remained at almost
the same level irrespective of hold duration, and showed
no decrement. Ayumu outperformed all of the human
subjects in both speed and accuracy. Our most recent
(unpublished) data show that Ayumu can remember eight
numerals shown for only 210 ms with an accuracy of 80%.
Such memory performance has never been obtained in
human subjects, even after intensive training. Seeing is
Current Opinion in Neurobiology 2009, 19:92–98
96 Cognitive neuroscience
believing: Please enter the keywords ‘chimpanzee mem-
ory’ in YouTube.com’s search engine, and watch the 10-
min video clip entitled ‘Ayumu’ that we have uploaded.
Not only Ayumu but also the other two young chimpan-
zees we tested showed the same extraordinary memory
capability [2��]. Young chimpanzees thus appear to have
an extraordinary working memory capacity for numerical
recollection—better than that of human adults. The
results may be reminiscent of the phenomenon known
as ‘eidetic imagery’ or the feats of so-called ‘idiot savants’.
Such abilities are known to be present in some human
children among thousands, with the ability generally
declining with age.
A recent study presented the first examination of eye
tracking in chimpanzees by a computer-based automatic
detection system [34�]. The eye movements of chimpan-
zees as they viewed naturalistic pictures were recorded.
The results were compared with those of humans in the
same apparatus following the same procedure. Although
the two species shared many common features such as
looking at faces and eyes, chimpanzees shifted the fix-
ation location more quickly and more broadly than
humans. The chimpanzees are good at quickly grasping
the pattern as a whole.
Cognitive tasks that present particulardifficulties for chimpanzeesDuring my past three decades with the chimpanzees [35],
it became clear that some cognitive tasks easy for humans
were not so easy for chimpanzees. For example, although
infant chimpanzees show neonatal imitation of facial
gestures just like humans, this does not develop into
generalized imitation [36]. Chimpanzees can master
the ‘Do this!’ game to some extent. Although they can
imitate actions toward objects, it is difficult for them to
imitate actions toward their own bodies, and even more
difficult to imitate simple actions without objects [37].
Cross-modal matching is also a challenging task for chim-
panzees. They require intensive training to master
matching auditory stimuli to corresponding visual stimuli
[38]. Similar difficulties can be observed in symbolic
matching-to-sample tasks. Very little evidence exists
regarding the symmetry of symbols and their referents.
Let us consider a symbolic matching-to-sample task in
which a referent needs to be matched to the correspond-
ing symbol. For example, the color ‘red’ must be matched
to the symbol ‘red’, and the color ‘green’ to the symbol
‘green’. After intensive training on the task, chimpanzees
can become proficient at selecting the correct symbol in
response to being presented with a specific color. In turn,
you can now test the reverse scenario: showing a symbol
first and then providing two choice alternatives, the color
‘red’ pitted against the color ‘green’. Chimpanzees,
trained only on color-to-symbol matching, find it difficult
Current Opinion in Neurobiology 2009, 19:92–98
to match the symbol to the corresponding color. This kind
of bi-directional correspondence between symbols and
referents is referred to as ‘symmetry’. Symmetry is almost
automatically established in the early phase of human
language development. However, this is not the case in
chimpanzees, who do not transfer spontaneously but need
further intensive training [39].
Let us return to the issue of number. In true counting
behavior, one-to-one correspondence should provide
the essential basis for further establishment of the
number concept. Imagine that you place five cups
upside down in front of a chimpanzee, and then provide
her with small wooden blocks. Using a gesture, you
then ask the chimpanzee to put the wooden blocks on
top of the cups. It is extremely difficult for the chim-
panzee to place the wooden blocks – which can be seen
as representing ‘counters’ – on the cups one by one. She
will more probably stack the blocks on top of one
another, the chimpanzees’ preferred way of manipulat-
ing objects in such situations. Of course, chimpanzees
may learn to solve this particular task, however, it will
not automatically generalize to variations thereof. One-
to-one correspondence looks easy to us, but it is not so
for the chimpanzees.
Human infants assume stable supine posture from right
after birth [40�]. This enhances face-to-face visual com-
munication, vocal exchange, as well as bimanual object
manipulation since the hands are freed from having to
cling to the mother. By contrast, chimpanzee infants
continuously cling to their mothers at least throughout
the first three months of life. It is the stable supine
posture, not bipedal locomotion, that makes human hands
free and provides important clues to the complexity of our
object manipulation, a precursor of tool use. When we
analyze the development of object manipulation in chim-
panzees using standard tests involving wooden blocks,
nesting cups, and so forth, we see interesting differences
between the two species in terms of action grammar [41].
Only one of the three infant chimpanzees we studied
started to stack blocks spontaneously—she did so at a
little less than three years of age, much later than human
infants [42]. The other two never stacked blocks, despite
having had extensive experience of watching their
mothers’ stacking behavior. No chimpanzees, not even
Ai, succeeded in making a copy of a simple three-block
structure, such as a ‘bridge’ or ‘arch’ in which two blocks
are placed side by side a short distance apart and a third is
placed on top of both [43�]. Chimpanzees can manipulate
blocks into one-dimensional structures, in a vertical or
horizontal alignment. However, they find two-dimen-
sional structures difficult, and three-dimensional con-
struction more difficult still. These kinds of
hierarchical constraints are seen throughout all examples
of object manipulation by chimpanzees both in the
laboratory and in the wild [44–47].
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Symbolic representation of number in chimpanzees Matsuzawa 97
Conclusions: a trade-off theory of memoryand symbolic representationWhy do chimpanzees have a better memory than humans
for immediately capturing visual stimuli? Why do chim-
panzees have difficulties representing things at an
abstract level? The existing data can be interpreted
according to an evolutionary trade-off hypothesis [40�].The common ancestor of humans and chimpanzees five to
six million years ago may have possessed an extraordinary
memory capability. At a certain point in evolution,
because of limitations on brain capacity, the human brain
may have acquired new functions in parallel with losing
others—such as acquiring language while losing visuo-
spatial temporal storage ability.
Humans may have developed unique cognitive capa-
bilities such as symbolic representation or the ability to
handle abstract concepts. Imagine that you witness a
creature that passes quickly in front of you. You notice
that body is covered in brown hairs, and the creature has
a white streak on its forehead and a black spot on its
right front leg. This, in a sense, is one useful way of
dealing with stimuli that you encounter. However,
there might be a different way to view the world. On
the basis of the various features you observed, you may
summarize this detailed information and label the
creature as a ‘horse’. This kind of representation may
be efficient because it can save you having to memorize
details, allow you to generalize the experience to similar
encounters in future, and share the information with
others. In the case of the number concept, there is
adaptive value in being able to report the number of
enemies, count the number of allies, and communicate
about the number of potential prey.
Communication is likely to have been more important
than immediate memory in the social life of early homi-
nids. Humans are creatures who can learn from the
experiences of others. The trade-off theory has support
from not only a phylogenetic but also an ontogenetic
perspective. In humans, youngsters often outperform
adults on certain memory tasks. In the course of cognitive
development, human children may acquire linguistic
skills while losing a chimpanzee-like photographic mem-
ory [40�]. This may be due to the time lag of myelination
of neuronal axons in each part of the brain. It is known
that the association cortex responsible for complex
representation and linguistic skills develops more slowly
than other primary areas. Further study on the mind–brain relationship will illuminate this kind of trade-off
across phylogeny and ontogeny. In sum, the present
paper summarized the potentials and constraints of sym-
bolic representation of number in chimpanzees, as
revealed through the medium of Arabic numerals and
through comparative studies using humans undergoing
identical test procedures. In conclusion, the number
concept in terms of symbolic representation is uniquely
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human, and comparable levels of abstraction have yet to
be demonstrated in the rest of the animal kingdom—
even in our closest evolutionary neighbor, the chimpan-
zee.
AcknowledgementsThe present study was supported by grants from the Ministry of Education,Science, and Culture in Japan (#12002009, #16002001, #20002001), andJapan Society for the Promotion of Science (gCOE-A06, gCOE-D07, ITP-HOPE). Thanks are due to the staff of the Primate Research Institute,Kyoto University, and to keepers and veterinarians who took care of thechimpanzees at the institute. I thank Dora Biro for her help given in thecourse of revising the manuscript.
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Using a y-maze, the authors found that bees not only can differentiatebetween patterns containing two and three elements but also can use thisprior knowledge to differentiate three from four, without any additionaltraining. This is the first report of number-based visual generalization byan invertebrate.
27. Murofushi K: Numerical matching behavior by a chimpanzee(Pan troglodytes): subitizing and analogue magnitudeestimation. Jpn Psychol Res 1997, 39:140-153.
28. Tomonaga M, Matsuzawa T: Enumeration of briefly presenteditems by the chimpanzee (Pan troglodytes) and humans (Homosapiens). Anim Learn Behav 2002, 30:143-157.
29. Tomonaga M: Relative numerosity discrimination bychimpanzees (Pan troglodytes): evidence for approximatenumerical representations. Anim Cogn 2008, 11:43-57.
30. Tomonaga M, Matsuzawa T: Sequential responding to Arabicnumerals with wild cards by the chimpanzee (Pantroglodytes). Anim Cogn 2000, 3:1-11.
31. Biro D, Matsuzawa T: Numerical ordering in a chimpanzee (Pantroglodytes): planning, executing, and monitoring. J CompPsychol 1999, 113:178-185.
32. Premack D: Human and animal cognition: continuity anddiscontinuity. Proc Natl Acad Sci U S A 2007, 104:13861-13867.
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33. Kawai N, Matsuzawa T: Numerical memory span in achimpanzee. Nature 2000, 403:39-40.
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Kano F, Tomonaga M: How chimpanzees look at pictures: acomparative eye-tracking study. Proc R Soc B 2009 doi:10.1098/rspb.2008.1811.
The authors present the first examination of eye tracking in chimpan-zees. The eye movements of chimpanzees as they viewed naturalisticpictures were recorded. The results were compared with those fromhumans. Although the two species shared many common features,chimpanzees shifted the fixation location more quickly and morebroadly than humans.
35. Matsuzawa T: Q and A Tetsuro Matsuzawa. Curr Biol 2009,19:R310-R312.
36. Myowa-Yamakoshi M, Tomonaga M, Tanaka M, Matsuzawa T:Imitation in neonatal chimpanzees (Pan troglodytes). Dev Sci2004, 7:437-442.
37. Myowa-Yamakoshi M, Matsuzawa T: Factors influencingimitation of manipulatory actions in chimpanzees. J CompPsychol 1999, 113:128-136.
38. Hashiya K, Kojima S: Acquisition of auditory-visual intermodalmatching-to-sample by a chimpanzee (Pan troglodytes):comparison with visual-visual intramodal matching.Anim Cogn 2001, 4:231-239.
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Matsuzawa T: Comparative cognitive development. Dev Sci2007, 10:97-103.
Human infants right after the birth take the stable supine posture. Thisenhances the face-to-face visual communication, vocal exchange, andalso the bimanual object manipulation because the hands are free fromclinging to the mothers. The paper pointed out the significance of stablesupine posture as the important clue for hominid evolution.
41. Hayashi M: A new notation system of object manipulation inthe nesting-cup task for chimpanzees and humans.Cortex 2007, 43:308-318.
42. Hayashi M, Matsuzawa T: Cognitive development in objectmanipulation by infant chimpanzees. Anim Cogn 2003,6:225-233.
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Poti P, Hayashi M, Matsuzawa T: Spatial construction skills ofchimpanzees (Pan troglodytes) and young human children(Homo sapiens sapiens). Dev Sci 2009 doi: 10.1111/j.1467-7687.2008.00797.x. Published online.
The authors have assessed spatial construction skills in chimpanzees andyoung children with a block modeling task. Chimpanzees do not developin the direction of constructing in two dimensions as human children dostarting from the age of 30 months. The pattern of development ofconstruction skills indicates that spatial analysis and spatial representa-tion are partially different in the two species.
44. Matsuzawa T: Chimpanzee intelligence in nature and incaptivity: isomorphism of symbol use and tool use. In GreatApe Societies. Edited by McGrew W, Marchant L, Nishida T.Cambridge University Press; 1996:196-209.
45. Poti P: Chimpanzees’ constructional praxis (Pan paniscus, Pantroglodytes). Primates 2005, 46:103-113.
46. Hayashi M: Stacking of blocks in chimpanzees. Anim Cogn2007, 10:89-103.
47. Crast J, Fragaszy D, Hayashi M, Matsuzawa T: Dynamic in-handmovements in adult and young juvenile chimpanzees.J Phys Anthropol 2009, 138:274-285.
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