Broca’s area of the brain

The Broca’s Area was named in 1861 as the center of speech articulation. It is considered to be responsible for controlling the movement of the muscles of the speaking apparatus and related movements of lips, tongue, larynx, and pharynx (1). 

Broca’s area plays the primary role in the creation of programs of spoken production as well as phonetic-phonological, syntactic and semantic aspects of language. Moreover, it enables the adoption of grammatical rules. 

Except for the language production process, Broca's zone or area is involved in the process of understanding the language, as well as in the exercise of other language functions.

In this article, we will focus on its functions and anatomy, but we will also address certain damages to this area and its consequences.

Anatomy of the Broca’s Area

The Broca’s area is located in the posterior part of the lower frontal winding of the left hemisphere. These are the 44. and 45. Brodman areas. Area 44 is divided into dorsal and ventral regions by composition of neurons and receptors and is slightly larger in the left than in the right hemisphere of the brain.

Area 45 is divided into the anterior and posterior parts and is approximately the same size in both hemispheres of the brain (1).

The Broca’s area is connected to the temporal cortex in the following way: the Brodman Area 44 is connected by a dorsal pathway containing both fasciculus arcuatus and fasciculus longitudinalis superior.

The Brodman area 45 is connected via the capsulaeextremae fiber system, and the frontal operculum via fasciculus uncinatus with the anterior temporal cortex (1). 

Functions of the Broca’s Area

Broca's zone is assumed to be responsible for creating programs for production of language symbols and executing commands for the primary motor field (Broca’s Area 4) where the impulses are sent from into the muscles of the larynx, palate, tongue, and lips, which enable processes of articulation and phonation.  

Also, this area does not play a role only in serial phoneme stacking, morphemes, and flexural extensions, but also in syntactic editing of a sentence. Characteristics of language disorders that occur due to the lesion of Broca's zone show that it is responsible for the phonetic-phonological and syntactic aspects of the language (2).

Brodman's field 44 (area opercularis) in the left hemisphere is part of the Broca's zone and is responsible for the production of speech, more precisely, the integration of speech elements into meaningful sequences, choice of information between different sources, syntax and phonological aspects, complex semantics and verbal working memory.

In this field, motor programs are created for speech activity, control muscle movements of the speech apparatus and related movements of the lips, tongue, larynx, pharynx. The impulse for oral speech goes through the premotor and motor areas of the muscles of the speech apparatus and face.

Brodman Field 45 (area triangularis) also forms part of the Broca’s Area, but it has more complex functions than the Area 44.

It plays a role in many speech functions such as speech attention, word production, grammar application, speech fluency, semantic deciding between concrete and abstract, as well as a role in the association of the verb with the noun, in the pronunciation of voices while reading aloud, understanding affective prosody, etc.

There are also assumptions that this area mediates working memory in the domain of semantics, non-verbal information, and long-term declarative memory (2). 

There are also assumptions that this area mediates working memory in the domain of semantics, non-verbal information, and long-term declarative memory (2). 

As the key Broca’s Area parts, the Fields or Areas 44 and 45 also have many things in common. Those include common functions, e.g. production and understanding of speech, thematic aspects of speech, role in working and episodic memory, motor inhibition within executive functions, recognizing known scents and enjoying music. 

There are other important areas too. It is considered that BA 44 and BA 46 have roles in phonological processes, 44 and 45 in syntactic processes and 45 and 47 in semantic processes. The entire frontal speech area in the lower frontal gyrus is the integration site.

The Broca’s Area mediates sequencing as well in general, even in the visuospatial domain. It also has a role in achieving motor functions, e.g. catching objects and object manipulation. 

Speech perception

Although Broca's Area itself has been identified as the seat of phonology, grammar, and even specific grammar operations, studies of functional brain visualization link this brain zone with a whole range of linguistic and non-linguistic processes.

Broca’s Area key function was long considered to be limited to linguistic production rather than to linguistic comprehension, but modern research shows that the Broca’s Area plays a significant role in understanding the language, as much as it does in language production (2). 

Studies have shown that the left lower frontal gyrus participates in processes that take place outside the language function, more precisely beyond its basic functions in word production, semantic decision-making, and syntactic understanding.

Nevertheless, they feel that there is a connection between non-verbal auditory discrimination and linguistic processes, that is, that formant transitions analogous to the frequency changes are important for phonemic discrimination.

Frequency changes can be compared with format changes in syllables and words the same as in the suprasegmental ones the prosodic information they make the difference between a question and a sentence.

Studies that used functional visualization and transcranial magnetic stimulation (TMS) methods have determined the activation of the triangularis gyrus (Brodman Area 45) in both hemispheres during semantic processing during linguistic comprehension. More specifically, the role was related to the controlled retrieval of semantic knowledge or the choice between the competing alternatives to semantic interpretation.

If Brodman Area 45 is included in semantic finding or selection, then we conclude that it is also extremely active during lexical or sentence ambiguity. Scientists have also determined, with the help of fMRI, that the Brodman field 45 is activated while listening to sentences that have high semantic ambiguity (2).

Damage of the Broca’s Area

Lesions in the Broca's zones lead to severe expressive language deficits. There are also non-linguistic difficulties that lead to problems in understanding signs, gestures, and pantomime. 

Impairment of the Brodman areas 44 and 45 lead to a whole group of language disorder symptoms that represent Broca's aphasia syndrome. Brodman areas 44 and 45 are found in both cerebral hemispheres, but almost all patients with this type of aphasia have lesions in the lower left frontal cortex.

We now know that the Broca zone plays a primary role in the creation of speech production programs and the syntactic aspects of language. It also enables the adoption of grammatical rules. That's why damage or lesions in these zones have severe consequences to speech and language production. 

Damage of the Brodman Area 44 leads to milder motor aphasic disorders, but if the lesion engages deeper parts of the white mass and adjacent cortex area, the classic Broca's motor aphasia occurs. 

Broca’s Aphasia

Broca's aphasia is a condition resulting from damage to the Broca's zone in the third frontal wind of the left hemisphere. It is also called motor aphasia, efferent motor aphasia and expressive aphasia (3). There are two main Broca's aphasia varieties. Those are the so-called big aphasia and small aphasia.

The so-called big Broca's aphasia results from a lesion of the frontal operculum dominant hemispheres with expansion in deeper brain parts engaging the basal ganglia and often begins as global aphasia and evolves into the classic type. 

Small Broca's aphasia has a more benign flow and usually evolves into a more fluent variant with a smaller occurrence of hesitation, agrammatism, and dysprosodia, and with difficulty in understanding complex syntactic structures.

To sum up, the Broca's aphasia has been described as a syndrome characterized by extremely strenuous speech production, damage to melody and articulation, semantic and phonemic paraphasia, production of telegraph sentences, as well as reduced and abnormal grammatical form. According to Benson, Broca's aphasia is characterized by non-fluent speech, rare literary paraphrases, poor repetition, good understanding, and poor naming ability.  

Conclusion

The Broca’s area was named after Pierre Paul Broca, a French physician, and anthropologist who discovered that damage to that part of the brain caused problems in grammatical processing during speech production, while generally having no negative consequences for language comprehension. This discovery took place in the 19^th century. The disorder is called Broca's aphasia. 

More recent studies examine the role of this brain region in a broad sense, as an interface between action and perception, the central role of which is to reconcile the perceptual and motor functions underlying verbal and non-verbal communication.

References

1. Stinnett TJ, Zabel MK. Neuroanatomy, Broca Area. [Updated 2018 Oct 27]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan Found online at: https://www.ncbi.nlm.nih.gov/books/NBK526096/
2. Hickok G, Costanzo M, Capasso R, Miceli G. The role of Broca's area in speech perception: evidence from aphasia revisited. Brain Lang. 2011 Dec;119(3):214-20. doi: 10.1016/j.bandl.2011.08.001. Epub 2011 Sep 13. PMID: 21920592; PMCID: PMC3195945. Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3195945/
3. Acharya AB, Wroten M. Broca Aphasia. [Updated 2019 Jun 29]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan-.  Found online at: https://www.ncbi.nlm.nih.gov/books/NBK436010/