Available online at www.sciencedirect.com
ScienceDirect
Comparative primate neurobiology and the evolution of brain
language systems
1,2,3,4,5
James K Rilling
Human brain specializations supporting language can be our capacity for language. As discussed below, compara-
identified by comparing human with non-human primate tive studies of primate neurobiology have helped to
brains. Comparisons with chimpanzees are critical in this identify some of these specializations. Comparisons with
endeavor. Human brains are much larger than non-human our closest living relative, the chimpanzee, have been
primate brains, but human language capabilities cannot be crucial in this endeavor, for we cannot conclude that a trait
entirely explained by brain size. Human brain specializations has uniquely evolved in humans unless we also demon-
that potentially support our capacity for language include firstly, strate its absence in modern chimpanzees [4].
wider cortical minicolumns in both Broca’s and Wernicke’s
areas compared with great apes; secondly, leftward
Brain size
asymmetries in Broca’s area volume and Wernicke’s area
The human brain averages 1330 cc in size, a value that far
minicolumn width that are not found in great apes; and thirdly,
exceeds that for the brain of any other living primate
arcuate fasciculus projections beyond Wernicke’s area to a
species [5]. Rhesus macaque brains average only 88 cc [6].
region of expanded association cortex in the middle and inferior
Chimpanzee brains average 405 cc and gorilla brains
temporal cortex involved in processing word meaning.
average 500 cc [5]. These numbers suggest the possibility
Addresses that our unique capacity for language is simply a product
1
Department of Anthropology, Emory University, 1557 Dickey Drive, of our large brain size. However, evidence from high-
Atlanta, GA 30322, United States
functioning human microcephalics, some of whom have
2
Department of Psychiatry and Behavioral Sciences, Emory University,
brain volumes within the great ape range, suggests other-
Atlanta, GA 30322, United States
3 wise since their linguistic abilities can eclipse those of
Center for Behavioral Neuroscience, Emory University, Atlanta, GA
30322, United States chimpanzees. Thus, there are apt to be qualitative differ-
4
Yerkes National Primate Research Center, Emory University, Atlanta, ences between human and non-human primate brains
GA 30322, United States
5 that support our capacity for language [7].
Center for Translational Social Neuroscience, Emory University,
Atlanta, GA 30322, United States
Language circuitry
Corresponding author: Rilling, James K. ([email protected])
In 1970, Geschwind outlined a model of the functional
neuroanatomy of human language [8 ]. This model is a
useful starting point for comparative analysis, however it
Current Opinion in Neurobiology 2014, 28:10–14
is important to note that it does not include features that
This review comes from a themed issue on Communication and
we now know to be important, such as the involvement of
language
the thalamus, basal ganglia and cortical areas beyond
Edited by Michael Brainard and Tecumseh Fitch
Wernicke’s and Broca’s areas in language [9 ,10 ,11,12].
In this model, Wernicke’s area, the posterior portion of
the left superior temporal gyrus, is essential for speech
comprehension, and Broca’s area, a region in the left
Available online XXX
inferior frontal cortex, is responsible for speech pro-
0959-4388/$ – see front matter, # 2014 Elsevier Ltd. All rights
duction. These two areas are connected by a large white
reserved.
matter fiber tract known as the arcuate fasciculus. Perhaps
http://dx.doi.org/10.1016/j.conb.2014.04.002
other primate species lack human language skills because
they lack homologues of human Wernicke’s and Broca’s
areas [13]. However, when defined based on cytoarchi-
Introduction tecture and shared non-linguistic functional properties,
Language is among the most distinctive attributes of such homologues have been identified in non-human
Homo sapiens. Other primate species communicate, but primate brains [14]. This leads to the question of whether
none does so by combining thousands of symbols accord- there might be microstructural differences within these
ing to a defined set of rules to generate phrases with a areas between humans and non-human primates.
nearly infinite variety of meanings [1,2]. Although chim-
panzees raised in human linguistic environments develop Broca’s area
limited symbolic abilities, none has eclipsed the language Not only is Broca’s area involved in human speech, it also
competence of a 2 and 1/2-year old human child [3]. Thus, mediates aspects of communication in non-human
there must be human brain specializations that support primates [15]. Mirror neurons are found in macaque area
Current Opinion in Neurobiology 2014, 28:10–14 www.sciencedirect.com
Comparative primate neurobiology and language evolution Rilling 11
F5, part of which is homologous to the posterior part of greater integration of information by providing more
human Broca’s area (area 44). Mirror neurons ostensibly space for connections. Finally, Broca’s area is larger in
allow monkeys to improve their understanding of actions the left cerebral hemisphere of the human brain and this
they observe in others by mapping those observed actions may be related to the strong tendency for many aspects of
onto their own motor system and simulating the action in language function to be lateralized to the left hemisphere.
their own brain. Mirror neurons are well known for their Importantly, this Broca’s area asymmetry is not present in
response to reaching and grasping movements of both self chimpanzees [29].
and others, but they also fire when producing or observing
communicative mouth movements [16]. Macaque mon- Wernicke’s area
keys communicate extensively with orofacial expressions, Whereas production of human and macaque monkey voca-
so mirror neurons are likely vital to macaque communi- lizations depend on different neural substrates, compre-
cation. hension of species-specific vocalizations seems to depend
on similar neural substrates in humans and macaque mon-
There is evidence that mirror neuron regions are also keys [30]. Human speech comprehension depends on the
involved in orofacial communication in humans [17], how- left posterior superior temporal gyrus (i.e. Wernicke’s area)
ever human Broca’s area is also responsible for the gram- [8 ]. Similarly, the left superior temporal gyrus is respon-
matical aspects of speech [18 ,19]. Perhaps, just as Broca’s sible for discriminating species-specific vocalizations, but
area is involved in human speech production, Broca’s area not other types of auditory stimuli, in Japanese macaques
homologue in non-human primates is involved in the [31]. Single cell electrophysiology similarly implicates the
production of non-human primate vocalizations. However, superior temporal gyrus in processing macaque species-
this does not seem to be the case, since lesioning the specific calls. Monkey auditory cortex consists of three
monkey homologue of Broca’s area does not impair voca- longitudinal cytoarchitectonic streams in the superior
lization [20,21]. Monkey calls instead depend upon the temporal lobe: a core region, a surrounding belt region,
limbic system and brainstem. This neurological difference and an adjacent parabelt region [32]. Single cell electro-
reflects differences in the nature of human speech and non- physiology studies have revealed that unlike core areas that
human primate vocalizations. In contrast to human speech, respond best to pure tones, lateral belt areas respond best to
monkey calls are largely involuntary expressions of complex sounds, including species-specific vocalizations
emotional arousal [22], implying that they may not be [33]. Further evidence for similarity between humans and
under voluntary cortical control. On the other hand, there macaques in the neural substrates for processing species-
is some evidence that captive chimpanzees will exhibit specific vocalizations comes from a comparative PET neu-
volitional calls [23], and that Broca’s homologue is involved roimaging study. In a small sample of macaque monkeys,
in their production [24]. This raises the possibility that blood flow responses were more pronounced for species-
Broca’s area was recruited for volitional vocal, in addition to specific calls compared with non-biological sounds within
orofacial communication, before the divergence of humans cortical area Tpt as well as the dorsal frontal operculum,
and chimpanzees some 5–7 million year ago. presumed homologues of Wernicke’s and Broca’s areas,
respectively [34 ]. However, it should be noted that
Despite these similarities, there are obvious functional another monkey PET study found that the anterior tip
differences between human and chimpanzee Broca’s of the left superior temporal gyrus, rather than Wernicke’s
areas, not just in the motor aspects of speech but also area homologue, was specialized for processing species-
in the involvement of human Broca’s area with syntax specific calls [35]. The difference could be attributable to
[11,25]. These functional differences are likely to be the former study measuring blood flow with a temporal
supported by underlying anatomical differences. It is resolution of 60 s and the latter measuring glucose metab-
now known that Broca’s area has wider cortical minicol- olism occurring over a period of about 25 min [35].
umns in humans than in great apes [26 ], whereas mini-
column width does not differ between humans and great Like Broca’s area, human Wernicke’s area has functional
apes in primary somatosensory or motor cortex [27]. properties not found in its non-human homologue, such as
Cortical minicolumns are vertical columns consisting of phonological processing [11]. Are there anatomical differ-
approximately 80–100 neurons with strong interconnec- ences that support these expanded functional capabili-
tions that constitute a coherent functional unit. In sensory ties? A portion of Wernicke’s area known as the planum
cortex, for example, neurons within a given minicolumn temporale is leftwardly asymmetric in most humans [36].
share a peripheral receptive field. Cortical minicolumns However, this same asymmetry is also present in great
consist of vertically aligned cells separated by vertically apes [37,38]. While these findings are consistent with the
defined cell poor spaces where most connections are notion that human brain language systems evolved from
made. Minicolumns contrast with macrocolumns that pre-existing neural systems present in nonhuman primate
consist of many minicolumns bound together by short- ancestors, they also suggest that planum temporale asym-
range horizontal connections [28]. The larger human metries are not the neural substrate supporting human-
minicolumns within Broca’s area are thought to facilitate specific linguistic abilities. Nevertheless, potentially
www.sciencedirect.com Current Opinion in Neurobiology 2014, 28:10–14
12 Communication and language
relevant specializations of human Wernicke’s area are identified, but the pathway is weak [44 ,45] and its
apparent at the microstructural level. As with Broca’s function unknown. In fact, the dominant frontal connec-
area, planum temporale minicolumns are wider in tion of Wernicke’s homologue (area Tpt) is not with
humans compared with chimpanzees, and planum tem- Broca’s area homologue, but rather with dorsal prefrontal
porale minicolumn width is leftwardly asymmetric cortex [42]. This auditory ‘where’ pathway is involved
(left > right) in humans but not chimpanzees [39 ]. with localizing sounds in space [46 ]. On the other hand,
the dominant connection of Broca’s area homologue is
Against this backdrop of comprehension-related sim- with the cortex of the middle superior temporal gyrus [43]
ilarity in brain activation patterns, there is also emerging via a ventral pathway known as the extreme capsule that
evidence for differences between humans and macaques. is involved in auditory object recognition [46 ].
A recent comparative fMRI study showed that whereas
both humans and macaques activate the lateral sulcus and The tracer methods that have been used to delineate
superior temporal gyrus (STG) when listening to monkey white matter connections in macaque monkeys are inva-
and human vocalizations, only in humans did the STS also sive, terminal procedures that cannot be used in humans.
respond to intelligible human utterances. Macaque STS Instead, human white matter pathways have traditionally
did not respond to monkey calls. Notably, the human STS been described based on gross dissections of post-mortem
activations spanned nearly the entire length of the sulcus, brains [47]. However, it is difficult to precisely define
and the authors conclude that the evolution of language cortical terminations of fascicles using these methods.
appears to have recruited most of STS in humans [40 ] Diffusion tensor imaging provides a non-invasive alterna-
(see also [41]). tive method that can be applied to humans and non-
human primates [48–50]. Although DTI has demon-
Connections between Wernicke’s and Broca’s strated connections between Broca’s and Wernicke’s
areas areas (or their homologues) in humans, chimpanzees,
Beyond these human specializations within human Bro- and rhesus macaques, the human pathway bears one
ca’s and Wernicke’s areas, there are differences between major difference from that of the non-human species.
humans and non-human primates in the white-matter In both rhesus macaques and chimpanzees, the posterior
connections that link these two regions. Anterograde terminations of the arcuate are focused on the homologue
and retrograde tracer techniques have been used to of Wernicke’s area in the posterior superior temporal
describe the connections of Wernicke’s and Broca’s areas gyrus. Humans, however, also possess a massive projec-
homologues in macaque monkeys [42,43,44 ,45]. Direct tion of the arcuate into the middle and inferior temporal
connections between the two regions have been gyri, ventral to classic Wernicke’s area [51 ].
Figure 1
• Wider minicolumns • Wider minicolumns • L>R volume • L>R minicolumns
CS IPS
40 PrCS 39 46 9 10 IFS 22 45 44 6 • Posterior and medial 47 37 STS 21 displacement of visual cortex
• Arcuate fasciculus projection into region of expanded temporal association cortex involved in processing word meaning
Current Opinion in Neurobiology
Potential neurological specializations supporting human language Broca’s area has wider minicolumns in humans compared with great apes, and the
volume of human Broca’s area is leftwardly asymmetric, whereas this is not the case in chimpanzees. Wernicke’s area also has wider minicolumns in
humans compared with great apes, and there is a leftward asymmetry in the column width in humans that is absent in chimpanzees or macaque
monkeys. Finally, the human arcuate fasciculus pathway includes a massive projection to the cortex ventral to classic Wernicke’s area that is involved
in lexical-semantic processing and that has markedly expanded in human evolution, displacing adjacent visual cortex in a posterior and medial
direction in the process.
Current Opinion in Neurobiology 2014, 28:10–14 www.sciencedirect.com
Comparative primate neurobiology and language evolution Rilling 13
4. Preuss TM: Who’s afraid of Homo sapiens? J Biomed Discov
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which direcetly or indirectly support cortical language functions.
capsule pathway was recruited into the language system
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This study critiques the traditional model of brain language processing
expansion of the arcuate fasciculus than there has been that is focused on Broca’s and Wernicke’s areas, presents evidence that
the basal ganglia sequence the discrete elements that constitute a
of the extreme capsule. These data suggest that the
complete motor act, syntactic process or thought process. It argues
arcuate fasciculus was a more important substrate for for the importance of cortical–-cortical circuits in walking, talking and
comprehending the meaning of sentences.
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