Language Processing in the Primary Cortex

Table of Contents

List of figures

1.Introduction

2. Terminology: Language and Language Processing

3. Basic Ideas about Brain and Language: A Historical Review
3.1 Antiquity
3.2 Middle ages to 1800
3.3 Localism of Gall and Broca
3.4 Associationism: Wernicke and Lichtheim
3.5 Jackson’s Evolutionary Model
3.6 The Concept of Holism
3.7 Dynamic Localization of Function
3.8 Geschwind’s Concept of Connectionism

4. Brain Anatomy
4.1 The Meninges
4.2 The Brain stem
4.3 The Cerebellum
4.4 The Diencephalon
4.5 The Cerebrum
4.6 The Cerebral Cortex
4.7 Gray and White Matter
4.8 The Ventricular System
4.9 Blood Supply

5. The Core Cortical Language Areas
5.1 Language Lateralization
5.2 Broca’s Area
5.3 Wernicke’s Area
5.4 Angular and Supramarginal Gyrus

6. Linguistic and Neural Mechanism of Language Components
6.1 The Dual Loop Model
6.2 Linguistic Organization of Speech Production
6.3 Linguistic Organization of Speech Comprehension
6.4 Linguistic Organization of Written Language Comprehension
6.5 Neuroanatomical Organization of Speech Production
6.6 Neuroanatomical Organization of Speech Comprehension
6.7 The Neuroanatomical Organization of Written Language Comprehension

7. Aphasia: Syndromes and Symptoms
7.1 Broca’s Aphasia
7.2 Wernicke’s Aphasia
7.3 Conduction Aphasia

8. Methods of Investigation
8.1 Cerebral Angiography
8.2 Neural Stimulation
8.3 Wada Test
8.4 Structural Neural Imaging: CT and MRI
8.5 Functional Neural Imaging: EEG, ERP, MEG, PET, rCBF, and fMRI

9. Conclusion

Bibliography

List of figures

Figure 1: Brain Anatomy, Crystal, D. 2010, 268

Figure 2: Overview of Lobes, Sulci, and Gyri, Rohen et al. 2016, 102

Figure 3: Brodmann Classification. Marcus/Jacobson 2003, 17-7

Figure 4: Speech Production Model, Tremblay et al. 2016, 742

Figure 5: Speech Comprehension Model, Rauschecker/Scott 2009, 719

Figure 6: Activated Regions in Reading, Eden et al. 2016, 816

1.Introduction

The study of language related to issues regarding the nature and structure of language had been subject to the philosophy of language until the modern era. However, the study of language form and functions became increasingly important in the course of the development of the independent discipline of linguistics in the 19th century. Yet, it was only in the 20th century that language was regarded from a neurocognitive point of view and the cooperation of multiple disciplines, such as linguistics, psychology, neurosciences, and medicine, led to a remarkable progress in the investigation of the neural substrates underlying specific language processes (Müller/Rickheit 2003: 1). As language plays a key role in the majority of actions carried out by humans, its thorough investigation is justified and reasonably necessary (Whitaker 1971: vii). Another reason to pay attention to language is the fact that it is reserved for human beings. Although animals are able to communicate, their communicative system cannot be called a language as the system is unable to fall back on linguistic capacities that humans may have, such as the existence of fixed rules and the creativity in word combination. Nevertheless, there is abundant evidence that the neural substrates involved in human language evolved from structures that were already, even though partly, present in non-human primates. Consequently, the current understanding of the neuroanatomical architecture of language has benefited from ape research revelations (Rauschecker/Scott 2009: 718).

The present thesis aims to transmit knowledge about which cortical brain areas may be involved in language processing and how these mechanisms may be structured. Furthermore, an overview of how language-brain relations were seen in the past is given, followed by a basic anatomical description of the whole brain to fully understand neural language mechanisms. In addition, major language disorder types whose examinations have contributed to the location of language functions are described. The work concludes with an outline of the most important investigative methods by means of which detailed scientific language-related research could be conducted.

2. Terminology: Language and Language Processing

Starting with the problematic issue of defining the term language, is significant depicting the necessity of knowing which elements are included in language to be able to correctly analyze the underlying neural basis. The term language is characterized by its creative nature, meaning the ability to combine linguistic units in an infinite manner. Furthermore, it has to be differentiated between competence, the theoretical knowledge about language, and performance, the practical use of language (Harris/Coltheart 1986: 7, 8). Bussmann (1996: 253) refers to it as a “vehicle for expression or exchanging of thoughts, concepts, knowledge, and information as well as the fixing and transmission of experience and knowledge”. According to Bussmann, the term is thought to be limited to human beings, excluding animal and artificial communication systems, and is subject to mental mechanisms. Crystal (2010: 275) includes all linguistic levels, such as speaking, hearing, reading, and writing.

Language processing indicates cognitive mechanisms that are associated with language production and comprehension (Bussmann 1996: 264). Various different mental processes are involved in expressing and understanding language, marking a complex interplay of brain structures including the processing of all linguistic components, e.g. phonology, grammar, and semantics (Crystal 1997: 218, 309).

3. Basic Ideas about Brain and Language: A Historical Review

Ideas about the relation of brain and language can be traced back to the antiquity. Over the course of time, scientists developed several different theories about brain function. Some were more influential than others. But together they form an essential basis of today’s understanding of language-brain connections.

3.1 Antiquity

Starting early in the history, the seat of language was assigned to the brain around 3500 B.C. in the Edwin Smith papyrus which includes 48 lesion descriptions containing at least eight cases of brain injuries (Walsh 1978: 1). In the script cranial lesions were considered to be responsible for disorders in other body regions. Language deficiencies were explained to be evoked by a divinity’s spirit entering the human head leaving the affected person “silent in sadness”. It was claimed that the method of trepanation serves as remedy.

As a result, the god’s breath could stream out of the skull (Ahlsén 2006: 11). Language problems were also described in other Egyptian texts from 3000 B.C. and in cuneiform writings of the Hittites around 1500 B.C. (Whitaker 1998: 27).

To sum up Greek contributions, Hippocrates, Democritus, Herophilus, Plato, and Aristotle have to be named. The Hippocratic corpus contains a collection of ancient Greek medical works. At around 400 B.C. Hippocrates in person depicted a man subject of a letter memory deficiency and stated that hemiparesis and language abnormalities were linked. He shared the view of the brain being seat of the intellect and the heart being the sensitive center. Simultaneously, Democritus claimed that the brain has an interior and an exterior faculty: it serves as keeper of intelligence and sense. Localizing intelligence in the ventricles of the brain was an attempt of Herophilus- a prevalent idea called to be consistent until the late 18th century. Plato allotted the various capacities of the soul to certain brain regions, whereas Aristotle was convinced of the heart playing the main role (Ahlsén 2006: 11, 12).

At around 200 A.D., Galen expanded the idea of localization of functions in the ventricles. He developed the concept of pneuma, a psychic gas, which is kept in the ventricles conducting essential brain functions (Walsh 1978: 6).

3.2 Middle ages to 1800

In this period of time the description of case reports and language disorder syndromes went on. The ventricle theory was still supported. Memory was considered to be placed in the fourth ventricle. According to Antonio Guainerio, word sparsity and errors in naming were provoked by phlegm in the fourth ventricle as those symptoms were associated to memory deficiencies. Varolius and Vesalius disagreed with the prevailing theory emphasizing the relevance of brain volume and soft brain substance (Ahlsén 2006: 13). Vesalius observations mark the beginning of detailed lesion descriptions of meticulous care that have been absent beforehand. Attempts of localizing mind and soul continued. The ideas of unitarism were developed by Descartes in the 17th century. He localized the soul, which he described as indivisible, in the pineal gland. Willis assigned imagination to the corpus callosum (Walsh 1978: 10-13). Further important contributions in the 17th century were made by Wepfer.

He is known for his description of agrammatism (Whitaker 1998: 30). First associationist ideas came up with Johann Gesner’s case studies in 1776, in which language deficiencies as subordinates of memory disorders were called to provoke problems connecting images with linguistic signs (Ahlsén 2006: 13).

3.3 Localism of Gall and Broca

The localization of mental function roots in the craniology of Gall, later to be known as the phrenology approach elaborated by Spurzheim. The theory is based on the association of the skull’s shape, or more precisely the bumps on the skull, with mental abilities (Fromkin/Rodmann 1993: 439). Gall stated that the cortex consists of many different organs. Every organ has its unique responsibility. A strong adversary of the phrenology approach was Flourens who did not share localist views but believed that brain regions work in concert (Walsh 1978: 14). However, Flourens was not the only person to criticize Gall’s claims. The theory was target of criticism as localist ideas were incompatible with religious views (Ahlsén 2006: 16).

However, localism was acknowledged with the publications of Paul Broca at the latest, even if Bouillaud and Dax had placed human language faculty in the left frontal lobe beforehand (Sadownik 2010: 28). After examining brain lesions of a patient called Leborgne, who had strong speaking problems, and further patients with the same symptoms in the third frontal gyrus of the left hemisphere, Broca advanced a hypothesis published in 1865. He claimed that mental functions could be localized in the brain gyri and that damages to regions in the left side of the brain might be responsible for language deficiencies (Ahlsén 2006: 17). As mentioned before, Broca was not the first to localize language in the left frontal lobe but according to Caplan (1987: 46) his work is “[…] the first truly scientific paper on language-brain relationships.”

3.4 Associationism: Wernicke and Lichtheim

The description of a damage in the left superior temporal gyrus (Wernicke’s area), resulting in problems of language comprehension, was made by Carl Wernicke in 1874 (Walsh 1978: 16). Wernicke discovered a second language center and described the syndrome of sensory aphasia. He did not want to draw his attention on the description of undetected aphasia forms alone.

Instead he tried to explain the existence of different types of aphasia by means of a theoretical model comprising brain pathways and at least two language centers. His model depicts a motor language center (Broca’s area), a sensory language center (Wernicke’s area) and association fibers connecting the two centers (arcuate fasciculus). He presumed that complex processing could be managed not by one region alone, but only by several brain regions connected with each other. As a consequence, motor and sensory areas are involved in the language faculty and work in concert, which represents the idea of associationism (Sadownik 2010: 37-39). Lichtheim elaborated Wernicke’s model in 1885 adding a further language center, called concept center. Auditory stimuli can be transmitted to the concept center to receive semantic information or speech expression can by started by the concept center sending concepts to Broca’s region. The model contains supplementary forms of aphasia and was expanded to take reading and writing disorders into consideration (Ahlsén 2006: 21).

3.5 Jackson’s Evolutionary Model

Localization of mental functions was not in Hughlings Jackson’s interest. His main focus lay on the application of evolutionary foundations, established in 1867 by Herbert Spencer, to the language-brain subject. Jackson proposed that brain evolution suggests a hierarchy concerning mental functions from simple reflexes to logical reasoning (Ingram 2007: 55). The model reflects the hierarchical operation and development of the nervous system described by Ahlsén (2006: 22) as following:

1. from simple to more complex,
2. from lower centers to higher centers,
3. from more organized centers to more complex centers,
4. from automatic to intentional.

Jackson’s model, which has already been developed in 1874, gained prominence around 1920 as it was taken over by Head (Ahlsén 2006: 23).

3.6 The Concept of Holism

In the early 20th century holistic views experienced a revival. Holism is characterized by the claim of an absence of localization in the brain and of an involvement of all cerebral regions in all processes. Proponents of the holistic school, such as von Monakow, Head, and Goldstein, argued that language loss is mostly the expression of a disturbance of the symbolic faculty.

They regarded the examination of the brain as less important and emphasized behavioral descriptions and psychological studies (Sadownik 2010: 46,47).

3.7 Dynamic Localization of Function

The dynamic localization of function is a Russian theory that emerged in the course of the 20th century. Taking the aspect of ontogeny into account represents the basic thought of the concept. Opposing to localist views, Pavlov claimed in 1949 that complex processes could only be managed considering the ontogeny of dynamic systems (Ahlsén 2006: 28). The systems comprise interconnected structures arranged in different places of the nervous system. Another representative of the Russian theory was Lev Vygotsky, who stressed the fact to initially focus on the object of research before posing the question of localization. He argued that various organs work in concert while performing a complex process. Neural mechanisms undertake the control of cooperating organs. To complete the main supporters of the approach, Alexander Luria has to be named who carried out his aphasia research within the context of the dynamic localization theory (Walsh 1978: 21).

3.8 Geschwind’s Concept of Connectionism

Norman Geschwind resumed and modernized associationism around 1965- henceforth to be found under the name of “connectionism” or neoclassical school (Ahlsén 2006: 27). Geschwind revived Wernicke’s model of language functions elaborating it to a more complex theory based on a higher amount of aphasic lesion studies. The concept is working as a system of language areas that is interconnected within a common circuit. The language areas portray the central links of the circuit. Language processing is possible if the links cooperate as a whole. The model gained influence as it served as a principle to explain how different types of aphasia develop. Although, it has to be mentioned that the model’s absolute validity is not given in recent times as it is criticized for its far too simple character (Sadownik 2010:44-46).

4. Brain Anatomy

illustration not visible in this excerpt

Figure 1, Crystal, D. 2010, 268.

Before reaching the actual matter of interest of the given research topic, a general anatomical overview of the brain is provided to guarantee a total comprehension of complex processing structures.

4.1 The Meninges

The three membranes, the meninges, contain fluid, envelop the central nervous system (CNS), and act as protectors. The outermost and thickest membrane is the dura Mater, followed by the arachnoid and the pia mater (Jacobson/Marcus 2008: 399).

4.2 The Brain stem

Medulla oblongata, pons, and midbrain denote the components of the brain stem located at the most posterior aspect of the skull base adjacent to the interbrain and the spinal cord. Each of the components consists of the tectum involved in sensory and motor functions, the tegmentum containing cranial nerves and sets of interconnected nuclei, and the basis constituting cortical fibers extending to the cerebellum and spinal cord. The medulla transmits sensory stimuli to the thalamus and to the other parts of the brain stem and collaborates on the control of autonomous functions, such as heart rate, blood pressure, and digestion (Kaplan 1960: 23-25).

4.3 The Cerebellum

The cerebellum is located in the inferior posterior region of the head. It lies directly on the back of the pons and can be split into a left and a right halve. The cerebellar nuclei are surrounded by the cerebellar cortex, which is made up of three layers and which can further be divided into ten lobules. The cerebellar cortex is thought to play a major role in regulating motor activities (Squire et al. 2013: 677, 678). More precisely, it is responsible for automatically adapting motor activities with regard to sensory information and acquired movement patterns (Martini et al. 2012: 425).

4.4 The Diencephalon

Located above the midbrain between brain stem and cerebrum, the diencephalon or interbrain is formed by four structures, namely the thalamus, epithalamus, hypothalamus, and subthalamus (Augustine 2008: 79). The thalamus is designated as the relay- and processing center for sensory stimuli, whereas the hypothalamus acts as controlling structure for autonomous functions, emotion, and hormone production. The epithalamus constitutes the pineal gland which secretes hormones (Martini et al. 2012: 425).

4.5 The Cerebrum

Also known under the name of telencephalon, the cerebrum consists of two hemispheres with the longitudinal fissure acting as dividing line. A cluster of neural fibers, the corpus callosum, connects the two halves and represents the hugest white matter structure in the brain, made up of about 250 million contralateral axonal projections (Augustine 2008: 71). Both brain halves consist of a corpus striatum situated at the frontal sides of each hemisphere, a rhinencephalon depicting a structure related to olfaction, and a pallium which is grouped into the gray layer of the cerebral cortex and the white layer of the medulla (Kaplan 1960: 13). The cerebrum’s domain comprises tasks related to conscious thinking and intellectual functions. It plays a central role in the storage and processing of memories, and in the regulation of the muscle tone (Martini et al. 2012: 425).

[...]