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Broca's area, or the Broca area (/ˈbroʊkə/,[1][2][3]alsoUK: /ˈbrɒkə/, US: /ˈbroʊkɑː/[4]), is a region in the frontal lobe of the dominant hemisphere, usually the left, of the brain[5] with functions linked to speech production.
Language processing has been linked to Broca's area since Pierre Paul Broca reported impairments in two patients.[6] They had lost the ability to speak after injury to the posterior inferior frontal gyrus (pars triangularis) (BA45) of the brain.[7] Since then, the approximate region he identified has become known as Broca's area, and the deficit in language production as Broca's aphasia, also called expressive aphasia. Broca's area is now typically defined in terms of the pars opercularis and pars triangularis of the inferior frontal gyrus, represented in Brodmann'scytoarchitectonic map as Brodmann area 44 and Brodmann area 45 of the dominant hemisphere.[8]
Functional magnetic resonance imaging (fMRI) has shown language processing to also involve the third part of the inferior frontal gyrus the pars orbitalis, as well as the ventral part of BA6 and these are now often included in a larger area called Broca's region.[9]
Studies of chronic aphasia have implicated an essential role of Broca's area in various speech and language functions. Further, fMRI studies have also identified activation patterns in Broca's area associated with various language tasks. However, slow destruction of Broca's area by brain tumors can leave speech relatively intact, suggesting its functions can shift to nearby areas in the brain.[10]
Broca's area is often identified by visual inspection of the topography of the brain either by macrostructural landmarks such as sulci or by the specification of coordinates in a particular reference space. The currently used Talairach and Tournoux atlas projects Brodmann'scytoarchitectonic map onto a template brain. Because Brodmann's parcelation was based on subjective visual inspection of cytoarchitectonic borders and also Brodmann analyzed only one hemisphere of one brain, the result is imprecise. Further, because of considerable variability across brains in terms of shape, size, and position relative to sulcal and gyral structure, a resulting localization precision is limited.[11]
Nevertheless, Broca's area in the left hemisphere and its homologue in the right hemisphere are designations usually used to refer to the triangular part of inferior frontal gyrus (PTr) and the opercular part of inferior frontal gyrus (POp). The PTr and POp are defined by structural landmarks that only probabilistically divide the inferior frontal gyrus into anterior and posterior cytoarchitectonic areas of 45 and 44, respectively, by Brodmann's classification scheme.[12]
Area 45 receives more afferent connections from the prefrontal cortex, the superior temporal gyrus, and the superior temporal sulcus, compared to area 44, which tends to receive more afferent connections from motor, somatosensory, and inferior parietal regions.[12]
The differences between area 45 and 44 in cytoarchitecture and in connectivity suggest that these areas might perform different functions. Indeed, recent neuroimaging studies have shown that the PTr and Pop, corresponding to areas 45 and 44, respectively, play different functional roles in the human with respect to language comprehension and action recognition/understanding.[12]
Brodmann area 44 (BA44) is strongly connected to premotor and supplementary motor regions, and is therefore involved in phonological processing, syntactic structure building, and the motor sequencing of speech production. Researchers also highlight that BA44 is highly engaged in hierarchical processing, the ability to organize and understand multi-level structured information. Lesions to BA44 have been associated with impairments in grammatical morphology, reduced fluency of speech, and articulation planning.[13]
Brodmann area 45 (BA45) is often associated with controlled semantic retrieval and top-down processing during language comprehension. Neuroimaging studies found greater BA45 activation during tasks that require choosing among multiple words of the same meaning. In addition, cytoarchitecturally, because BA45 receives stronger input from temporoparietal areas involved in semantic processing, it has a more developed granular layer (layer IV) than BA44.[14]
For a long time, it was assumed that the role of Broca's area was more devoted to language production than language comprehension. However, there is evidence to demonstrate that Broca's area also plays a significant role in language comprehension. Patients with lesions in Broca's area who exhibit agrammatical speech production also show inability to use syntactic information to determine the meaning of sentences.[15] Also, a number of neuroimaging studies have implicated an involvement of Broca's area, particularly of the pars opercularis of the left inferior frontal gyrus, during the processing of complex sentences.[16] Further, functional magnetic resonance imaging (fMRI) experiments have shown that highly ambiguous sentences result in a more activated inferior frontal gyrus.[17] Therefore, the activity level in the inferior frontal gyrus and the level of lexical ambiguity are directly proportional to each other, because of the increased retrieval demands associated with highly ambiguous content.
There is also specialization for particular aspects of comprehension within Broca's area. Work by Devlin et al. (2003)[18] showed in a repetitive transcranial magnetic stimulation (rTMS) study that there was an increase in reaction times when performing a semantic task under rTMS aimed at the pars triangularis (situated in the anterior part of Broca's area). The increase in reaction times is indicative that that particular area is responsible for processing that cognitive function. Disrupting these areas via TMS disrupts computations performed in the areas leading to an increase in time needed to perform the computations (reflected in reaction times). Later work by Nixon et al. (2004)[19] showed that when the pars opercularis (situated in the posterior part of Broca's area) was stimulated under rTMS there was an increase in reaction times in a phonological task. Gough et al. (2005)[20] performed an experiment combining elements of these previous works in which both phonological and semantic tasks were performed with rTMS stimulation directed at either the anterior or the posterior part of Broca's area. The results from this experiment conclusively distinguished anatomical specialization within Broca's area for different components of language comprehension. Here the results showed that under rTMS stimulation:
To summarise, the work above shows anatomical specialisation in Broca's area for language comprehension, with the anterior part of Broca's area responsible for understanding the meaning of words (semantics) and the posterior part of Broca's area responsible for understanding how words sound (phonology).
Experiments have indicated that Broca's area is involved in various cognitive and perceptual tasks. One important contribution of Brodmann's area 44 is also found in the motor-related processes. Observation of meaningful hand shadows resembling moving animals activates frontal language area, demonstrating that Broca's area indeed plays a role in interpreting action of others.[21] An activation of BA 44 was also reported during execution of grasping and manipulation.[22]
It has been speculated that because speech-associated gestures could possibly reduce lexical or sentential ambiguity, comprehension should improve in the presence of speech-associated gestures. As a result of improved comprehension, the involvement of Broca's area should be reduced.[12]
Many neuroimaging studies have also shown activation of Broca's area when representing meaningful arm gestures. A recent study has shown evidence that word and gesture are related at the level of translation of particular gesture aspects such as its motor goal and intention.[23] This finding helps explain why, when this area is defective, those who use sign language also have language deficits.[24]:494–7 This finding, that aspects of gestures are translated in words within Broca's area, also explains language development in terms of evolution. Indeed, many authors have proposed that speech evolved from a primitive communication that arose from gestures.[21][25] (See below.)
Damage to Broca's area is commonly associated with telegraphic speech made up of content vocabulary. For example, a person with Broca's aphasia may say something like, "Drive, store. Mom." meaning to say, "My mom drove me to the store today." Therefore, the content of the information is correct, but the grammar and fluidity of the sentence is missing.[26]
The essential role of the Broca's area in speech production has been questioned since it can be destroyed while leaving language nearly intact. In one case of a computer engineer, a slow-growing glioma tumor was removed. The tumor and the surgery destroyed the left inferior and middle frontal gyrus, the head of the caudate nucleus, the anterior limb of the internal capsule, and the anterior insula. However, there were minimal language problems three months after removal and the individual returned to his professional work. These minor problems include the inability to create syntactically complex sentences including more than two subjects, multiple causal conjunctions, or reported speech. These were explained by researchers as due to working memory problems. They also attributed his lack of problems to extensive compensatory mechanisms enabled by neural plasticity in the nearby cerebral cortex and a shift of some functions to the homologous area in the right hemisphere.[10]
A speech disorder known as stuttering is seen to be associated with underactivity in Broca's area.[27][28]
Aphasia is an acquired language disorder affecting all modalities such as writing, reading, speaking, and listening and results from brain damage. It is often a chronic condition that creates changes in all areas of one's life.[29]
Patients with expressive aphasia, also known as Broca's aphasia, are individuals who know "what they want to say, they just cannot get it out".[29] They are typically able to comprehend words, and sentences with a simple syntactic structure (see above), but are more or less unable to generate fluent speech. Other symptoms that may be present include problems with fluency, articulation, word-finding, word repetition, and producing and comprehending complex grammatical sentences, both orally and in writing.[7] For example, an individual with expressive aphasia might attempt to say "I went to the store and bought groceries," but instead unintentionally produce, "go store...buy food." This type of speech omits grammatical markers and is commonly described as telegraphic speech.[30]
この特定の症状群によって、表現性失語症の患者と他のタイプの失語症の患者が区別されます。失語症にはいくつかの明確な「タイプ」があり、それぞれのタイプは異なる一連の言語障害によって特徴付けられます。表現性失語症の患者は良好な話し言葉の理解を維持する傾向がありますが、他のタイプの失語症では、患者はまったく言語を理解できなくなり、話し言葉を理解できなくなります（聴覚言語失認）[ 31 ] [ 32 ] [ 33 ]。一方、他のタイプでは言語理解は維持されますが、障害があります。表現性失語症の人は、他のタイプの失語症の人よりも読み書きに苦労することが少ないかもしれません（失読症を参照）。[ 24 ] : 480–500表現性失語症の人は自分の言語出力を自己監視する能力が高い傾向があるが（「自分が言っていることを聞き」、修正する）、他のタイプの失語症患者は自分の言語障害に全く気づいていないように見えることがある。
古典的な意味では、表現性失語症はブローカ野の損傷の結果であり、特定の脳領域の病変が特定の分離可能な症状を引き起こすことが多いが、[ 34 ]症例研究では、病変の位置と失語症の症状の間に常に1対1の対応関係があるとは限らないことが示されている。[ 32 ]特定の脳領域（通常は左半球）の損傷と特定のタイプの失語症の発症との相関関係により、特定のタイプの失語症の存在（および重症度）のみに基づいて疑われる脳病変の位置を（非常に大まかではあるが）推測することが可能となるが、患者が複数の脳領域に損傷を受けており、複数のタイプの失語症の症状を示す可能性があるため、これは複雑である。認知の特定の側面の正常な機能にどの脳領域が不可欠であるかを推測するために病変データを調べることは、欠損病変法と呼ばれる。この方法は、失語症学として知られる神経科学の分野で特に重要である。認知科学、具体的には認知神経心理学は、欠損病変法を広く利用する神経科学の分野である。[ 35 ]
Since studies carried out in the late 1970s[36] it has been understood that the relationship between Broca's area and Broca's aphasia is not as consistent as once thought.[37] Lesions to Broca's area alone do not result in Broca's aphasia, nor do Broca's aphasic patients necessarily have lesions in Broca's area.[38][39] Lesions to Broca's area alone are known to produce a transient mutism that resolves within 3–6 weeks. This discovery suggests that Broca's area may be included in some aspect of verbalization or articulation; however, this does not address its part in sentence comprehension. Still, Broca's area frequently emerges in functional imaging studies of sentence processing.[40] However, it also becomes activated in word-level tasks.[41] This suggests that Broca's area is not dedicated to sentence processing alone, but supports a function common to both. In fact, Broca's area can show activation in such non-linguistic tasks as imagery of motion.[42]
ブローカ野が発音に最も関与しているという仮説を考慮すると、これらのすべての課題におけるブローカ野の活性化は、被験者が応答を準備する際の潜在的な発音によるものかもしれない。この注意点にもかかわらず、ブローカ野がどのような役割を担うにせよ、前頭葉の既知のワーキングメモリ機能に関連している可能性があるというコンセンサスが形成されつつあるようだ。（機能画像文献で報告されているタライラッハ座標[ 43 ]は広範囲に分布しており、ブローカ野の一部とされている。）たとえば、受動態の文の処理には、文の他の関連部分が操作されている間（つまり、項への主題役割の割り当てを解決するため）、情報を一時的に保持するのに役立つワーキングメモリが必要になる可能性がある。ミヤケ、カーペンター、ジャストは、文処理はこのような一般的な言語ワーキングメモリ機構に依存していると提唱しているが、カプランとウォーターズは、ブローカ野は特に構文処理のためのワーキングメモリに関与していると考えている。フリーデリチ（2002）はブローカ野を構成要素に分解し、ブロードマン野44が音韻構造[ 44 ]と統語構造の両方のワーキングメモリに関与していると示唆している。この領域は、理解プロセスの時間経過が進むにつれて、まず音韻で、次に統語で活性化する。ブロードマン野45とブロードマン野47は、統語的再分析と修復のプロセスが必要な意味的特徴と主題構造のワーキングメモリに特に関与していると考えられている。これらの領域は、ブロードマン野44が処理の役割を終えた後に活性化し、複雑な文の理解が一般的な記憶リソースに頼らなければならないときに活性化する。これらの理論はすべて、統語的理解の問題は概念的欠陥ではなく計算的欠陥から生じるという見方への動きを示している。新しい理論は、脳が異なる言語的および認知的要素をどのように統合するかについてより動的な見方を取り、これらの操作の時間経過を調べている。
神経認知研究では、ブローカ野に隣接する前頭葉領域が、言語課題だけでなく非言語課題においてもワーキングメモリに重要であることがすでに示唆されている。[ 45 ]カベサとナイバーグによるワーキングメモリの画像研究の分析は、BA45/47が情報の選択や比較に利用され、BA9/46はワーキングメモリ内の情報の操作により深く関わっている可能性があるという見解を支持している。ブローカ失語症を引き起こすには通常大きな病変が必要であるため、これらの領域も一部の患者で障害を受け、複雑な形態統語構造の理解障害の一因となっている可能性がある。
ブローカ野はこれまで、音韻分節、統語処理、統合など、さまざまなプロセスと関連付けられてきました。これらはすべて、異なる種類の言語情報を分節化してリンクすることを含みます。[ 46 ] [ 47 ] [ 48 ]単語を繰り返したり読んだりすることは意味処理や統語処理を伴いませんが、音素シーケンスと運動動作をリンクする操作が必要です。研究結果によると、このリンクは、音素表現と調音表現をそれぞれ担当する側頭皮質と前頭皮質との相互作用、および実際の発話行為の前に運動皮質との相互作用を通じて、ブローカ野によって調整されます。これらの独自の発見に基づいて、ブローカ野は調音の座ではなく、むしろ発話生成の主要な要素を担当する大規模な皮質ネットワーク全体に神経情報を操作および転送する重要なノードであると提唱されています。
2007年に発表された研究では、ブローカ症候群の患者であるルボルニュとルロンの保存された脳が高解像度体積MRIを使用して再検査されました。この研究の目的は、脳を3次元でスキャンし、皮質および皮質下病変の範囲をより詳細に特定することでした。この研究ではまた、皮質下病変の範囲とともに、現在ブローカ野と呼ばれている領域との関連で、前頭葉の病変の正確な位置を特定しようとしました。 [ 49 ]
ルボルニュはブローカの患者だった。30歳の時、彼はほとんど言葉やフレーズを発することができなかった。[ 50 ]質問されても、「tan」という音節を繰り返し発することしかできなかった。[ 7 ]彼の死後、左前頭葉の表面に神経梅毒病変が発見された。[ 51 ]
ルロンもブローカの患者の一人でした。彼もまた発話能力の低下を示しました。彼は「はい」「いいえ」「3」「いつも」「レロ」（自分の名前の誤った発音）の5つの単語しか言えませんでした。ルロンの剖検で外側前頭葉に病変が発見されました。ブローカの以前の患者であるルボルニュも、前頭葉の同じ領域に病変がありました。[ 52 ]これらの2つの症例から、ブローカは発話はこの特定の領域に局在していると考えるようになりました。[ 49 ]
Examination of the brains of Broca's two historic patients with high-resolution MRI has produced several interesting findings. First, the MRI findings suggest that other areas besides Broca's area may also have contributed to the patients' reduced productive speech. This finding is significant because it has been found that, though lesions to Broca's area alone can possibly cause temporary speech disruption, they do not result in severe speech arrest. Therefore, there is a possibility that the aphasia denoted by Broca as an absence of productive speech also could have been influenced by the lesions in the other.[7] Another finding is that the region, which was once considered to be critical for speech by Broca, is not precisely the same region as what is now known as Broca's area. This study provides further evidence to support the claim that language and cognition are far more complicated than once thought and involve various networks of brain regions.[53]
The pursuit of a satisfying theory that addresses the origin of language in humans has led to the consideration of a number of evolutionary "models". These models attempt to show how modern language might have evolved, and a common feature of many of these theories is the idea that vocal communication was initially used to complement a far more dominant mode of communication through gesture. Human language might have evolved as the "evolutionary refinement of an implicit communication system already present in lower primates, based on a set of hand/mouth goal-directed action representations."[21]
"Hand/mouth goal-directed action representations" is another way of saying "gestural communication", "gestural language", or "communication through body language". The recent finding that Broca's area is active when people are observing others engaged in meaningful action is evidence in support of this idea. It was hypothesized that a precursor to the modern Broca's area was involved in translating gestures into abstract ideas by interpreting the movements of others as meaningful action with an intelligent purpose. It is argued that over time the ability to predict the intended outcome and purpose of a set of movements eventually gave this area the capability to deal with truly abstract ideas, and therefore (eventually) became capable of associating sounds (words) with abstract meanings. The observation that frontal language areas are activated when people observe hand shadows[21] is further evidence that human language may have evolved from existing neural substrates that evolved for the purpose of gesture recognition.[54] The study, therefore, claims that Broca's area is the "motor center for speech", which assembles and decodes speech sounds in the same way it interprets body language and gestures. Consistent with this idea is that the neural substrate that regulated motor control in the common ancestor of apes and humans was most likely modified to enhance cognitive and linguistic ability.[25] Studies of speakers of American Sign Language and English suggest that the human brain recruited systems that had evolved to perform more basic functions much earlier; these various brain circuits, according to the authors, were tapped to work together in creating language.[55]
Another recent finding has shown significant areas of activation in subcortical and neocortical areas during the production of communicative manual gestures and vocal signals in chimpanzees.[56] Further, the data indicating that chimpanzees intentionally produce manual gestures as well as vocal signals to communicate with humans suggests that the precursors to human language are present at both the behavioral and neuronanatomical levels. More recently, the neocortical distribution of activity-dependent gene expression in marmosets provided direct evidence that the ventrolateral prefrontal cortex, which comprises Broca's area in humans and has been associated with auditory processing of species-specific vocalizations and orofacial control in macaques, is engaged during vocal output in a New World monkey.[57][58] These findings putatively set the origin of vocalization-related neocortical circuits to at least 35 million years ago, when the Old and New World monkey lineages split.
