Brain structure and language

Contents

1. Introduction

2. Brain Structure
2.1 General Structure
2.2 Hemispheric Structure and Function
2.3 Main Language Areas in the Brain
2.4 The Auditory System
2.5 The Visual System

3. Methods of Investigation
3.1 Early Attempts
3.2 The Wada Test
3.3 The Tachistoscopic Presentation
3.4 The Dichotic Listening Technique
3.5 The CAT-Scan
3.6 The PET-Scan
3.7 MRI and fMRI
3.8 ERP

4. Conclusion

References

1. Introduction

The brain is probably the most important part of the body. Its function is to control almost all body movements which also include the vegetative system like the digesting organs or the heart. Of course, it is also responsible for producing and receiving language.

This paper will introduce the main structure of the brain with special attention to those areas of the brain which are involved in language. It starts with the general structure and then explains the different hemispheric functions and the main language areas in the brain. These not only include those which are necessary for speaking because language means more. That is the reason for explaining the auditory and visual pathways from their origin, the ear respectively the eye, all the way to their associated processing areas in the brain, as well (chapters 2.4 and 2.5).

The second main part of this paper will give a short overview of the investigation methods which were and are used to map the brain and to gain knowledge about how the brain works in correlation with language. This overview reaches from the early attempts of post-mortem examination to the high-tech methods of today neurolinguistics.

2. Brain structure

2.1 General Structure

The weight of an average human adult brain is around 1 to 1.75kg but it almost needs one-fifth of the body’s blood supply (Gleason & Bernstein Ratner, 1998, p.60). But neither the size of the brain nor the relation between body size and brain size is important for the intelligence. For example, this relation in a 13 year old child and in a three year old chimpanzee is about the same and there is no doubt that the child should be more intelligent than the chimpanzee (Steinberg et al. 2001, p.311). A special part of the brain can be seen as a hint for higher intelligence because it has developed later in evolution than other parts. It is the cerebral cortex. This part is barely visible in lower vertebrates like fish or birds but much more developed in higher vertebrates like humans, apes or dolphins. The cerebral cortex is concerned with higher intellectual functions as well as language (Steinberg et al. 2001, p.312).

The brain itself is organised in different structures which are relatively easy to locate. The biggest parts of the brain are the cerebral hemispheres. They are divided horizontally into a left and a right hemisphere and are only connected by the corpus callosum and some fibre bundles, so there is still an exchange of information between the two hemispheres (Obler & Gjerlow 1999, p.18). Together, they are also called the cerebrum. Furthermore, there are several other subcortical structures which are located below the cerebrum and the further down one goes the more basic are the functions of the brain parts. At first, there are the basal ganglia which work together with the cerebrum and are responsible for the fine-tune body movements. Surrounded by the basal ganglia is the interbrain or diencephalon which mainly consists of the thalamus and hypothalamus. The main function of these parts is the control of the central vegetative systems, for example body core temperature regulation. Below that as the upper part of the brainstem is the midbrain or mesencephalon (3 àsee next picture). The brainstem also consists of the pons or metencephalon and the medulla oblongata or myelencephalon. All of these parts have something to do with vital functions for the body. Even further down, already at the level of the neck, is the spinal cord (7) and behind the brainstem is the cerebellum (4). The cerebellum controls the coordination of the muscles and the bodily equilibrium and it has its own two hemispheres (Garman 2000, pp.50-51; Merriam-Webster Online Dictionary; healthline.com).

Abbildung in dieser Leseprobe nicht enthalten(Garman, 2000, p.51)

The surface of the brain, also called cortex, is covered with valleys and ridges which are called sulci or fissures for small and bigger valleys and gyri for ridges. Because of all the valleys and ridges the cortex is the biggest part of the brain even though it only consists of a thin layer of neurons which are specialised cells (Garman 2000, p.50) but if one unfolds the cortex it would unfold to a surface of about 2.5 square feet (Gleason & Bernstein Ratner, 1998, p.60). Furthermore, the brain surface can be divided into lobes. The frontal lobe is the anterior region of the brain and is confined by the Rolandic or central fissure of the parietal lobe. The Sylvian fissure separates the temporal lobe from the first two. Finally, at the back of the brain is the occipital lobe. In general, one can say that the occipital lobe is concerned with the functions of vision and the parietal lobe with spatial orientation as well as body-surface senses which means touch. The processing for hearing and time-relations is found in the temporal lobe. General functions like cognition are located in the frontal lobe. All these lobes can be found in both hemispheres but there are also some specific functions which can only be found in one of the hemispheres (Steinberg et al. 2001, p.311). The important part which controls all body movements, for example the movement of the lips and tongue while speaking, is laid in front of the central fissure and is called the motor cortex. Again, in front of the motor cortex is the premotor cortex which is probably responsible for planning movements. The sensory cortex is located at the other side of the central fissure (Garman 2000, p.74).

Abbildung in dieser Leseprobe nicht enthalten(Garman, 2000, p.75)

A further distinction of the brain can be made by its biological components. The cortex and some other parts like the thalamus and hypothalamus consist of the so called grey matter because after death it turns grey. This matter consists mostly of cell bodies. The other matter is the white one which is located mostly in the sub cortical regions and it consists of nerve cell fibres. (Obler & Gjerlow 1999, p.18)

2.2 Hemispheric Structure and Function

As it is said before, the brain is divided into two hemispheres. The nerve fibres which descent from and ascent to the brain cross somewhere in the spinal cord and so the right hemisphere controls the left side of the body and the left hemisphere controls the right side of the body. For example, if a person suffers a stroke at the motor cortex of the left hemisphere he probably will be lamed at the right side of his body. This regards the vast majority of the nerve fibres and they are called contralateral; but there are also some ipsilateral or same-side muscles and nerves. For example, every ear has nerves to both hemispheres (Obler & Gjerlow 1999, p.23).

Now, there is the so called hemispheric dominance which means that one hemisphere controls the other in almost all humans. Apparently, most people prefer their right hands for writing or their right foot for kicking a ball. This means that they have a left-hemispheric dominance. Only about nine percent of the world population prefers to write with the left hand but as studies have shown only 30 percent of them have right-hemispheric dominance (Klar, 1999 in Steinberg et al. 2001, p.315). This is surprising but in most of them the hemispheric dominance is not as strong as in right-handed persons. Some scientists argue that the lack of strong hemispheric dominance in left-handers might be a reason for speech disorders and reading or writing difficulties like stuttering and dyslexia. Another interesting thing about left-handers is that there are more males than females (at least in the USA) and that the proportion of artists among them is bigger than among right-handers (Steinberg et al. 2001, pp.314-316).

Both hemispheres look quite similar but in both are areas which are only in one of them or bigger than in the other one and which process different stimuli. This is called lateralization. For example, the right hemisphere is concerned with the recognition of emotions and faces and it also deals with music and non-human sounds like animal sounds or noises from our environment. On the other hand, the areas for logical and analytical operations and higher mathematics are located in the left hemisphere. Furthermore, the main areas to process language are located there, as well (for more details see chapter 2.3), even in most left-handed writers (Steinberg et al. 2001, pp.318-320).

Damasio & Damasio stated in 1992: “For most people, language is in the left hemisphere: for roughly 99 percent of right-handers and about two thirds of left-handers.” (in Steinberg et al. 2001, p.319).

2.3 Main Language Areas in the Brain

There are several areas of the brain which are involved in processing language and they are all located at the cortex. Of course, language includes speaking but also listening, reading, singing, writing and signing for sign language and all these areas which decode, plan, produce or receive language are somehow connected. Finally, to produce language one needs muscles for articulation. It was estimated that one needs at least about 100 muscles for speech production and that every of those muscles is innervated by around as many motoneurons. This would mean that there are 140 000 muscular events when we speak with a normal speech rate of 14 sounds per second (Darley et al. 1975b in Gleason & Bernstein Ratner, 1998, p.64).

[...]