Neuroscience Research 46 (2003) 273 /279
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Sentence processing is uniquely humanKuniyoshi L. Sakai a,b,*, Fumitaka Homae a, Ryuichiro Hashimoto aaDepartment of Cognitive and Behavioral Science, Graduate School of Arts and Sciences, The University of Tokyo, Komaba, 3-8-1 Komaba, Meguro-ku,
Tokyo 153-8902, Japanb
SORST, Japan Science and Technology Corporation, Kawaguchi-shi 332-0012, Japan
Received 20 January 2003; accepted 26 March 2003

Abstract
In this article, we will focus on three fundamental issues concerning language processing in the human brain, and update recent
advances made by functional neuroimaging and magnetic stimulation studies of language. First, we will provide the firstexperimental evidence that the neural basis of sentence comprehension is indeed specialized. Specifically, our recent functional
magnetic resonance imaging (fMRI) study has clarified that the human left prefrontal cortex (PFC) is more specialized in thesyntactic processes of sentence comprehension than other domain-general processes such as short-term memory. Second, thedistinction between explicit and implicit syntactic processes will be clarified, based on our fMRI studies that elucidate syntactic
specialization in the left PFC. Third, we will advance a hypothesis stating that distinct subregions of the left PFC are recruited forthe syntactic integration of lexico-semantic information. The current direction of research in the neuroscience of language is
beginning to reveal the uniqueness of the human mind.
# 2003 Elsevier Science Ireland Ltd and Japan Neuroscience Society. All rights reserved.
Keywords: Language; Syntax; Semantics; Universal grammar (UG); Functional magnetic resonance imaging (fMRI); Optical topography (OT);
Transcranial magnetic stimulation (TMS)

1. The uniqueness of sentence processing
There is a tacit assumption in neuroscience from thegenetic to the systemic level, which holds that thebiological foundations of humans are essentially similar
to those of non-human primates, and that even humanlanguage can be understood by extending experimentswith monkeys and apes (Trefil, 1997; Gannon et al.,
1998; Ramus et al., 2000; Cantalupo and Hopkins, 2001;
Fujiyama et al., 2002). Language function closelyinteracts with other cognitive faculties of the mind,
such as perception, memory, and consciousness, whichhave hampered the study of essential processes involving
linguistic components alone (Sakai et al., 2001). Previous studies have examined various cognitive functionsin the prefrontal cortex (PFC) of monkeys to findhomologs of human cognitive functions, and haveclaimed that there are anatomical and functional* Corresponding author. Tel./fax: /81-3-5454-6261.
E-mail address: sakai@mind.c.u-tokyo.ac.jp (K.L. Sakai).

commonalities between the PFCs of monkeys andhumans (Barbas and Pandya, 1989; Rizzolatti and
Arbib, 1998; Nakahara et al., 2002). Both monkey andhuman studies have reported that the PFC plays acritical role in executive processes, which operate on
short-term memory and contribute to general cognitive
functions (Fuster, 1989; Smith and Jonides, 1999; Levy
and Goldman-Rakic, 2000). In accordance with theseprevailing views, it has been assumed that language
processing is, in principle, not entirely different from the
combination of other cognitive functions commonly
found in monkeys and humans.
However, it has been known that human language isradically different from what is known as animalcommunication. ‘Ape language’ should not be called alanguage, because there is no convincing evidence ofsyntactic structures or word orders in ape language.
According to critical assessment of a chimpanzee’s signs,
the mean length of each utterance is no more than 1.6
words, and even longer utterances lack rule-governed
combinations of signs (Terrace, 1979). Although dissenting views continue to be voiced (Savage-Rumbaugh

0168-0102/03/$ - see front matter # 2003 Elsevier Science Ireland Ltd and Japan Neuroscience Society. All rights reserved.
doi:10.1016/S0168-0102(03)00122-6

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and Lewin, 1994), the evidence of Kanzi’s ‘protogrammar’ is far from rule systems found in natural languages,
and it has not been shown that Kanzi’s understanding of
human utterances is grammatical in nature, which
would be based on associative mechanisms of learninginstead (Sakai and Miyashita, 1993). It is known that
apes are able to use a few hundred words, but the
numbers of words are clearly limited, while children incontrast exhibit the explosive expansion of words intheir first several years (Lieberman, 1991). Given thesecontrasting abilities between humans and non-humanprimates, we have to ask if there is something uniqueboth to humans and to human language. Recently,
Hauser, Chomsky, and Fitch have proposed that recursion is the only uniquely human component of thefaculty of language (Hauser et al., 2002), which is also
known as the property of discrete infinity, the ability togenerate an infinite range of discrete expressions from afinite set of elements. Sentences are indeed such infinite
expressions generated from a limited set of sounds,
signs, or letters. Chomsky has proposed that thisproperty is instantiated in human languages by syntactic
mechanisms (Chomsky, 2000). Our position is that
sentence comprehension characterizes human languages,
and that its neural basis is uniquely human.
The most critical question to be addressed here iswhether there exists a specialized (domain-special )neural system for human language processing that is
separable from other domain-general systems. Activations of the left PFC have been attributed to the
executive processes for working memory or to cognitive
demands in general (Smith and Jonides, 1999; Duncan
and Owen, 2000). However, there is no direct proof of
one-to-one correspondence between brain functions andcortical regions, and it is possible that the left PFC ismodulated primarily by domain-special factors andsecondarily by domain-general factors. We recently
tested this hypothesis by directly comparing the brain
activations in syntactic decision tasks with those inverbal short-term memory tasks (Hashimoto and Sakai,2002). In each trial of syntactic decision tasks, one
complete sentence was presented to judge whether the
subject of a verb corresponded to one of two persons, orto judge whether a pronoun was able to refer to one oftwo persons in the sentence. These tasks explicitly
required to utilize the structure-dependent rules. One
example of a Japanese sentence used to represent thesyntactic decision tasks was ‘Taro-wa Saburo-ga kare-wo
homeru-to omou ’ (‘Taro thinks that Saburo praises him ’).
In a verbal short-term memory task for words, phrasesof one sentence were rearranged into separate groups ofnouns and verbs, so that they were syntacticallyunrelated. A pair of phrases were then presented tojudge their temporal orders in the original sequence. Itstask requirements including memory encoding andretrieval are commonly involved in standard verbal

short-term memory tasks. One example used in thistask was ‘kare-ni Taro-ni Saburo-ni omou homeru-to’(‘him Taro Saburo thinks praises ’). Using functional
magnetic resonance imaging (fMRI), we found that theleft dorsal prefrontal cortex (DPFC, mainly in Brodmann’s area (BA) 8), as well as the left pars opercularisand pars triangularis of the inferior frontal gyrus (F3op/
F3t, BAs 44/45), show selective activation for syntactic
decision tasks when they were directly compared withthe verbal short-term memory task (Fig. 1A; redregions). The activation of these regions is related toprocesses of analyzing syntactic structures, and it cannot
be explained either by task difficulty or by verbal shortterm memory components such as the phonologicalstore, the subvocal rehearsal system, and the central
executive. Activation due to those domain-generalfactors was observed in the left precentral gyrus, theleft parietal operculum, and the right anterior cingulate
cortex (Fig. 1A; green regions). To our knowledge, thisstudy is the first to clarify that the human left PFC ismore specialized in the syntactic processes of sentencecomprehension than other domain-general processes.
These results further demonstrate that sentence processing is predominantly controlled by the left hemisphere,
whereas there is no such clear functional lateralization
for the general cognitive factors commonly found inprimates, suggesting that the cerebral dominance may
represent the uniqueness of human language processing.
Linguists have provided a clear conceptualization ofwhat distinctions need to be made between syntactic
processing and other cognitive processes in order toaccount for linguistic competence (Chomsky, 1980). We
believe that the identification of the cortical regions
responsible for the essential properties of syntactic
processing eventually leads to an understanding of
how language is specialized and instantiated in thehuman brain.
Recently, we further demonstrated a causal link
between syntactic processing and activation in the left
F3op/F3t by using transcranial magnetic stimulation(TMS) (Sakai et al., 2002). In this study, we employed aminimal-pair paradigm (Table 1; syntactic and semantic
decision tasks with visual stimuli), in which only one
linguistic element differs between a stimulus pair. This
TMS study has provided new findings that are striking
in three ways: (1) event-related TMS pulses selectively
reduced reaction times (RTs) in explicit syntactic
decisions but not in explicit semantic decisions, whichis unexpected according to the known TMS effects on
cognitive processes, (2) this effect was observed during
syntactic decisions regarding both normal and anomalous sentences, and (3) this effect was observed whenmagnetic stimulation was administered to the left F3op/
F3t at a specific timing (150 ms from a verb stimulus),
but not to the left F2 (the middle frontal gyrus) (Fig.
1B). As to the first point, the temporally-restricted andK.L. Sakai et al. / Neuroscience Research 46 (2003) 273 /279

275

Fig. 1. (A) Selective activation for syntactic processing in the left PFC. Regions identified by directly comparing syntactic decision tasks and a shortterm memory task for words (red) are projected together onto the left hemisphere of a surface-rendered standard brain (Hashimoto and Sakai, 2002).
When the short-term memory task for words was compared with an easier short-term memory task, activation was observed in the left precentral
gyrus and the left parietal operculum (green). (B) The sites for TMS in the left F3op/F3t (site I) and in the left F2 (site II) on the left lateral surface
image of the standard brain (Sakai et al., 2002). a, the precentral sulcus; b, the inferior frontal sulcus; c, the superior frontal sulcus.
Fig. 2. (A) Selective activation for explicit syntactic processing in the left F3op/F3t. Activated regions were identified by comparing a syntactic
decision task (Syn) and other tasks (Sem, Pho, and Voi; Table 1), combining normal and anomalous sentence conditions for each task (Suzuki andSakai, 2003). They are projected in three orthogonal planes and onto the left hemisphere (L) of a surface-rendered standard brain. A voice-pitch
comparison (Voi) task was also included in the comparison. (B) A syntactic specialization in the left F3op/F3t for detecting syntactic anomaly.
Activation in a grammatical error-detection task (GR, explicit syntactic processing of ungrammatical sentences) was directly compared with that in aspelling error-detection task (SP, implicit syntactic processing of sentences containing spelling errors), revealing a single locus in the left F3op/F3t(Embick et al., 2000). The threshold was set at P B/0.05, corrected for multiple comparisons.

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K.L. Sakai et al. / Neuroscience Research 46 (2003) 273 /279

syntax-selective reduction of RTs suggests the physiological effects of facilitation or priming, such that thestimulation of the left F3op/F3t at a specific timing
enhances its normal function. Indeed, the mean reduction of RTs for syntactic decisions (normal sentences: 14
ms, anomalous sentences: 25 ms) were comparable tothat of a previous behavioral study, in which the RTs for
lexical decision were reduced when primed by a syntactically appropriate word (Goodman et al., 1981). It is
possible that a TMS temporarily raises the overall
excitability of neurons, thus making an alerted state of‘stand-by’, which leads to more effective activation when
specific responses of those cells are required for syntactic
decisions. With respect to the second point, the TMS
effects on normal and anomalous sentences were inagreement with the left F3op/F3t activation observed inour event-related optical topography (OT) study using
the same minimal-pair paradigm (Table 1) with auditory
stimuli (Noguchi et al., 2002), indicating that common
processes were involved both in judging a sentence to benormal and in judging it to be anomalous for each task.
Therefore, explicit syntactic processing subsists not only
in detecting syntactic anomalies per se, but in analyzing
syntactic consistency in a sentence. This finding wasachieved by utilizing event-related TMS and OT,
thereby differentiating trials with normal and anomalous sentences. Even if ‘normal’ sentences are physically
identical stimuli used in both the syntactic and semantic
tasks, we have demonstrated that TMS resulted indifferential effects on normal sentences that paralleled
the effects on anomalous sentences, depending on thetypes of explicit linguistic decisions being made.

2. Explicit versus implicit syntactic processing
The results of Hashimoto and Sakai (2002) havefurther suggested that there is a distinction between
unconscious, obligatory sentence processing and conscious, controlled sentence processing. In that study, we
found that the left DPFC is activated when syntactic
information is processed at the sentence level without
explicit instructions, and that the left F3op/F3t is
additionally activated when explicit syntactic decisions
are required. Consistent with these findings, our eventrelated fMRI study (Suzuki and Sakai, 2003) using theminimal-pair paradigm (Table 1) with auditory stimuli
clearly established that the left F3op/F3t is involved

more in explicit syntactic processing (as in the syntactic
decision task) than in implicit syntactic processing (as in
the semantic and phonological decision tasks) (Fig. 2A).
Moreover, its activation is specific to syntactic judgements regarding both normal and anomalous sentences,which is consistent with the above-mentioned TMS
study (Sakai et al., 2002) and OT study (Noguchi etal., 2002). The left F3op/F3t activation in Suzuki andSakai (2003) matched with that of our previous fMRI
study using a block design (Embick et al., 2000). In thelatter fMRI study, we used an error-detection paradigm
that contrasted explicit syntactic processing of ungrammatical sentences and implicit syntactic processing ofsentences containing spelling errors, using the same
lexical material across these conditions. We found thatthe ungrammatical sentences produced more activationin the cortical language areas than did the sentences with
spelling errors, and that the difference in activation was
significantly greater in the left F3op/F3t than in otherlanguage areas (Fig. 2B). Recent imaging studies fromother labs have accumulated evidence of the involvement of the left DPFC and F3op/F3t in syntactic
processing (Stromswold et al., 1996; Just et al., 1996;
Caplan et al., 1998; Dapretto and Bookheimer, 1999;
Kang et al., 1999; Friederici et al., 2000b; Moro et al.,
2001; Indefrey et al., 2001). In these studies, thesyntactic knowledge of English, German, or Italian
languages was tested in native speakers. Althoughvarious aspects of sentence processing are substantiallydifferent, the consistent activation of the left DPFC andF3op/F3t in these studies and in our study using
Japanese, a non-Indo-European language, suggeststhat syntactic specialization of the left PFC reflects theexistence of universal grammar (UG) in the brain.
As proposed by Fodor, modular processes are by
their very nature automatic and implicit (Fodor, 1983).
The suggested specialization of the left DPFC for
implicit information processing in the syntactic domain
indicates that it is a putative syntactic module. The
activation of the left DPFC due to non-syntactic tasks
and non-automatic syntactic tasks reported in someprevious studies does not necessarily contradict thishypothesis, because it is always difficult to control
automatic syntactic factors involved in inner speech
while thinking. In normal language comprehension, one
does not explicitly but automatically process syntactic
information. On the other hand, explicit syntactic
processing is employed when sentence comprehension

Fig. 3. Sentence-processing selective activation in the left F3t/F3O. An activated region identified by comparing a sentence comprehension task and alexical decision task under the auditory condition is projected in three orthogonal planes and onto the left hemisphere (L) of a surface-rendered
standard brain (a red region) (Homae et al., 2002). An activated region identified by comparing a sentence comprehension task and a lexical decisiontask under the visual condition is also rendered on the standard brain (a green region). Almost all voxels of the activated region under the auditory
condition, which were localized within the left F3t/F3O, coincided with those under the visual condition.

K.L. Sakai et al. / Neuroscience Research 46 (2003) 273 /279
Table 1
Examples of stimuli used in the syntactic, semantic, and phonologicaldecision tasks
Task

Normal stimuli

Anomalous stimuli

Syntactic decision(Syn )‘yuki-wo sawaru ’snow-Acc touch(vt)(Someone ) touchessnow‘yuki-wo tsumoru ’snow-Acc lie (vi)

Semantic decision(Sem )(Something ) lies snow

‘iro-wo mazeru ’color-Acc blend (vt)
(Someone ) blends
colors‘iro-wo medatsu ’color-Acc stand out (vi)(Something ) stands out
the color‘yuki-ga tsumoru ’snow-Nom lie (vi)Snow lies (on the
ground )‘yuki-ga nigeru ’snow-Nom escape (vi)Snow escapes‘iro-ga medatsu ’‘iro-wo sawaru ’color-Nom standcolor-Acc touch (vt)out (vi)
The color stands out (Someone ) touches color
Phonological decision(Pho )‘yuki-wo sawaru ’(L /H /L) (L /H /
H)(Someone ) touchessnow‘yuki-wo sawaru ’(L /H /L) (H /L /L)
(Someone ) touches snow

‘iro-ga medatsu ’‘iro-ga medatsu ’(L /H /L) (L /H / (L /H /L) (H /L /L)
L)
The color stands out The color stands out
A minimal-pair paradigm that distinguishes the difference among
explicit syntactic, semantic, and phonological processing in sentencecomprehension (Suzuki and Sakai, 2003). In the syntactic decision(Syn) task, the subjects were asked to judge whether a presented
sentence was syntactically normal (N) or anomalous (A). In thesemantic decision (Sem) task, the subjects judged whether a presented
sentence was semantically N or A. In the phonological decision (Pho)task, the subjects judged whether a presented stimulus was phonologically N or A. In the Syn task, nouns with accusative (Acc) caseparticles can produce either N or A stimuli, depending on whether atransitive verb (vt) or intransitive verb (vi) is used. In the Sem task,
nouns with Acc or nominative (Nom) case particles can produce either
N or A stimuli, depending on the denotative meaning of the verb used.
In the Pho task, verbs with different accent patterns (L: low, H: high)can produce either N or A stimuli.

becomes more difficult, as in the case of garden-path or
ambiguous sentences (e.g., ‘The horse raced past the barnfell ’). In the presence of explicit task requirements likethe syntactic task employed in our experiments, explicit
syntactic processing is employed to judge the syntactic
correctness of an utterance. Thus it follows that theregions activated by explicit syntactic processing involve
all regions related to implicit syntactic processing as wellas any additional regions. Indeed, our fMRI studies
demonstrated that both the left DPFC and the left
F3op/F3t are activated when the explicit use of syntactic
rules is required by the syntactic tasks (Hashimoto and277

Sakai, 2002; Suzuki and Sakai, 2003). Recent imaging
studies using Jabberwocky sentences, in which allcontent words were replaced with pseudowords while
inflections and function words were maintained topreserve syntactic structures, have indicated that theleft DPFC and/or the left F3op/F3t are involved insyntactic processing (Friederici et al., 2000a; Moro et al.,
2001; Indefrey et al., 2001). These results support the
involvement of at least two distinct prefrontal regions insyntactic processing.
The distinction between explicit and implicit syntactic
processing raises a further question as to whether the left
F3op/F3t is always dormant in implicit syntactic processing. Activation of the left F3op has been reported tooccur in implicit syntactic processing (simple reading) ofsyntactically anomalous phrases (Kang et al., 1999). In
another study, which required listening to a pair ofsentences to determine whether they had the same literalmeaning, the left F3op/F3t was activated for implicit
syntactic processing of syntactically normal sentences(Dapretto and Bookheimer, 1999). The activation of theleft F3op/F3t for syntactic processing may stem fromthe fact that the subjects explicitly paid attention tosyntactic processing in spite of the absence of task
requirements. The results of Suzuki and Sakai (2003)
indicate that explicit tasks are more effective forclarifying the left F3op/F3t involvement in syntactic
processing than are implicit tasks.

3. The syntactic integration of semantic information
Sentences convey not only lexico-semantic information for each word, but sentence meaning based onsyntactic structures (Caplan, 1992; Sakai et al., 2001;
Townsend and Bever, 2001; Friederici, 2002). The
importance of syntactic structures has been elucidated,since Chomsky’s discussion of the sentence ‘Colorlessgreen ideas sleep furiously ’, in that sentence structure canbe understood independently of meaning (Chomsky,1957). Although this sentence conveys no meaningful
information, it is clearly different from a list of words
without syntactic structures: ‘Furiously sleep ideas green
colorless ’. Moreover, semantic processing at the sentence level differs from a simple summation of lexicosemantic processing for each word. For example, themeaning of ‘John thinks that David praises his son ’clearly differs from that of ‘John thinks that his sonpraises David ’, though the lexical items involved in eachof these sentences are identical. Thus, the processing ofsyntactic structures plays a critical role in the selective
integration of lexico-semantic information into sentence
meaning. In summary, syntactic analyses are performedin the service of semantics, and sentence meaning isderived from syntactic analyses of the sentence structures.

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We have recently proposed that a ventral region in theleft PFC extending from the pars triangularis (F3t, BA45) to the pars orbitalis (F3O, BA 47) is a putative
region for the selection and integration of semantic
information, which are separable from simple lexicosemantic processing (Homae et al., 2002) (Fig. 3). In thisfMRI study, we directly compared cortical activations insentence comprehension tasks with those in lexicaldecision tasks. One sentence example translated into
English is: ‘May I take a picture here? ’ */‘Yes, if you canbreak the flash’, wherein the original phrase ‘do not use ’is replaced with ‘can break ’, to which the subjects
respond by button-pressing. In the lexical decisiontask, the same phrases used in the sentence comprehension tasks were presented in a completely randomizedorder, thereby detecting pronounceable non-words. We
found sentence-processing selective activation in the left
F3t/F3O, and confirmed that the left F3t/F3O isactivated under both auditory and visual conditions.
These results demonstrate that the sentence-related
processes in this region are essentially amodal andthus beyond modality-dependent lexical processing.
The remaining critical question to be answered is how
lexico-semantic information is integrated by syntacticstructure analyses.
Previous neuroimaging studies have reported controversial results as to whether or not distinct corticalregions subserve the critical processes of syntax andlexico-semantics. While Röder et al. (2002) have reported that the activation of a left region of F3op/F3t
reflects both syntactic and semantic effects in sentences,
Dapretto and Bookheimer (1999) have claimed that
F3op and F3O are differentially involved in syntactic
processing and lexico-semantic processing, respectively.
Regarding this controversy, we believe that the following five points should be addressed. First, each of thestudies by Dapretto and Bookheimer (1999) and Röder
et al. (2002) employed a single task requiring either
semantic or syntactic processing under all of thecontrasting conditions tested. Dapretto and Bookheimer(1999) asked the subjects to judge whether or not themeaning of two sentences differ, while Röder et al.
(2002) asked the subjects to count the number ofgrammatical errors. It is possible that the resultant
activation patterns were biased by one type of processing explicitly required in the tasks. Second, semantic
processing at the sentence level is not identical to lexicosemantic processing for each word. Dapretto and Bookheimer (1999) have regarded lexico-semantic information as sentence meaning, but the two are qualitatively
different, as discussed above. Röder et al. (2002) haveexamined sentence meaning for all-or-none conditions,in which the contribution of lexico-semantic processing
cannot be separated from sentence meaning. Third, a
direct comparison of syntactic versus semantic conditions, which was employed by Dapretto and Book-

heimer (1999), eliminates all regions related to thesyntactic processing that is involved in both conditions.
Fourth, a direct comparison between normal andJabberwocky (nonsense) sentence types, which was
employed by Röder et al. (2002), does not cancel out
all syntactic processes as they proposed, but leaves
behind syntactic reanalyses required for the interpretation of sentence meaning. Indeed, they reported significant interaction between sentence types and syntactic
difficulty in the left F3op/F3t. Finally, it is still possiblethat activations related to syntactic and semantic
processing of sentences reflect domain-general cognitive
factors such as task difficulty and short-term memory. It
has been suggested that the left F3O is related to themaintenance of letter information (Henson et al., 2000).
In contrast to the static models of separate processes(Dapretto and Bookheimer, 1999) and overlapping
processes (Röder et al., 2002) for syntactic and lexicosemantic information, we propose an alternative dynamic model, such that separate processes for building
syntactic structures and evaluating lexico-semantic information interact with each other in the course of
sentence processing. Because semantic interpretation ofsentences is clearly dependent on syntactic information,
this integration process is crucial in sentence comprehension (Smith and Wilson, 1979). One theoreticalmodel has proposed an initial stage for building
syntactic structures on the basis of word-category
information, and a later stage for the integration ofsyntactic and lexico-semantic information (Frazier,1987). In contrast, another model has assumed aninteraction between syntactic and lexico-semantic processes from an early stage (Marslen-Wilson and Tyler,1980). Taking either model, it is true that syntactic andlexico-semantic information is integrated in the courseof sentence processing, which may subsist in the functional network of the left PFC. It would therefore benecessary to elucidate the neural mechanisms responsible for the syntactic integration of lexico-semantic
information. Future studies would clarify the individual
roles of the subregions in the left PFC that work inconcert for human-unique sentence processing.

Acknowledgements
We would like to thank Dr Yasushi Miyashita for his
encouragement, Dr David Embick and Mr Kei Suzuki
for collaboration in fMRI experiments, and Ms Hiromi
Matsuda for her administrative assistance. This work
was supported by a Solution Oriented Research for
Science and Technology (SORST) grant from JST, a
Young Investigators’ Grant from Human Frontier
Science Program (HFSP), and a grant from the Mitsubishi Foundation to KLS.

K.L. Sakai et al. / Neuroscience Research 46 (2003) 273 /279

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