The Cognitive Neuroscience of Human Communication

Vesna Mildner

• Publisher: Psychology Press
• Year: 2010
• Print ISBN: 9780805854350
• eBook ISBN: 9781136875281
• Topic: Languages & Linguistics
• Subtopic: History & Theory in Psychology
• Index: Languages & Linguistics

This is a book about speech and language. It is primarily intended for those interested in speech and its neurophysiological bases: phoneticians, linguists, educators, speech therapists, psychologists, and neuroscientists. Although speech and language are its central topic, it provides information about related topics as well (e.g. structure and functioning of the central nervous system, research methods in neuroscience, theories and models of speech production and perception, learning, and memory). Data on clinical populations are given in parallel with studies of healthy subjects because such comparisons can give a better understanding of intact and disordered speech and language functions.

There is a review of literature (more than 600 sources) and research results covering areas such as neuroanatomy, neurophysiology, development of the nervous system, sex differences, history of neurolinguistics, behavioral, neuroimaging and other research methods in neuroscience, linguistics and psychology, theories and models of the nervous system function including speech and language processing, kinds of memory and learning and their neural substrates, critical periods, various aspects of normal speech and language processes (e.g. phonetics, phonology, syntax, semantics, reading), bilingualism, speech and language disorders, and many others.

Newcomers to the field of neurolinguistics will find it as readable as professionals will because it is organized in a way that gives the readers flexibility and an individual approach to the text. The language is simple but all the technical terms are provided, explained, and illustrated. A comprehensive glossary provides additional information.

1
Central Nervous System

This chapter is an overview of the development, structure, and functioning of the central nervous system, with special emphasis on the brain. All areas that are discussed later, in the chapter on speech and language, are described and explained here, in addition to the structures that are essential for the understanding of the neurobiological basis of speech and language. More information and details, accompanied by excellent illustrations, may be found in a number of other sources (Drubach, 2000; Gazzaniga, Ivry, & Mangun, 2002; Kalat, 1995; Kolb & Whishaw, 1996; Pinel, 2005; Purves et al., 2001; Thompson, 1993; Webster, 1995). For easier reference and navigation through these descriptions, several figures are provided in the appendix. In Figure A.1 there are the major directions (axes): lateral—medial, dorsal—ventral, caudal—rostral, superior—inferior, and anterior—posterior. Brodmann’s areas and cortical lobes are shown in Figure A.2. The most frequently mentioned cortical structures are shown in Figures A.3 through A.6. These and other relevant figures are included in the text itself. At the end of this chapter there is a section on the neural bases of speech production and perception and their interrelatedness.

THE DEVELOPMENT OF THE CENTRAL NERVOUS SYSTEM

Immediately after conception a multicellular blastula is formed, with three cell types: ectoderm, mesoderm, and endoderm. Bones and voluntary muscles will subsequently develop from mesodermal cells, and intestinal organs will develop from endodermal cells. The ectoderm will develop into the nervous system, skin, hair, eye lenses, and the inner ears. Two to 3 weeks after conception the neural plate develops on the dorsal side of the embryo, starting as an oval thickening within the ectoderm. The neural plate gradually elongates, with its sides rising and folding inward. Thus the neural groove is formed, developing eventually, when the folds merge, into the neural tube. By the end of the 4th week, three bubbles may be seen at the anterior end of the tube: the forebrain (prosencephalon), the midbrain (mesencephalon), and the hindbrain (rhombencephalon). The rest of the tube is elongated further and, keeping the same diameter, becomes the spinal cord (medulla spinalis). The forebrain will eventually become the cerebral cortex (cortex cerebri). During the 5th week the forebrain is divided into the diencephalon and the telencephalon. At the same time the hindbrain is divided into the metencephalon and myelencephalon. In approximately the 7th gestation week the telencephalon is transformed into cerebral hemispheres, the diencephalon into the thalamus and related structures, while the metencephalon develops into the cerebellum and the pons, and the myelencephalon becomes the medulla (medulla oblongata).

During the transformation of the neural plate into the neural tube, the number of cells that will eventually develop into the nervous system is relatively constant— approximately 125,000. However, as soon as the neural tube is formed, their number rises quickly (proliferation). In humans that rate is about 250,000 neurons per minute. Proliferation varies in different parts of the neural tube with respect to timing and rate. In each species the cells in different parts of the tube proliferate in unique ways that are responsible for the species-specific folding patterns. The immature neurons that are formed during this process move to other areas (migration) in which they will undergo further differentiation. The process of migration determines the final destination of each neuron. The axons start to grow during migration and their growth progresses at the rate of 7 to 170 μm per hour (Kolb & Whishaw, 1996). Between the eighth and tenth week after conception the cortical plate is formed; it will eventually develop into the cortex. Major cortical areas can be distinguished as early as the end of the first trimester. At the beginning of the third month, the first primary fissures are distinguishable, for example, the one separating the cerebellum from the cerebrum. Between the 12th and the 15th week the so-called subplate zone is developed, which is important for the development of the cortex. At the peak of its development (between the 22nd and the 34th week) the subplate zone is responsible for the temporary organization and functioning. During that time the first regional distinctions appear in the cortex: around the 24th week the lateral (Sylvian) fissure and the central sulcus can be identified; secondary fissures appear around the 28th week; tertiary fissures start to form in the third trimester and their development extends into the postnatal period (Judaš & Kostovi

, 1997; Kostovi

, 1979; Pinel, 2005; Spreen, Tupper, Risser, Tuokko, & Edgell, 1984). Further migration is done in the insideout manner: the first cortical layer to be completed is the deepest one (sixth), followed by the fifth, and so on, to the first layer, or the one nearest to the surface. This means that the neurons that start migrating later have to pass through all the existing layers. During migration the neurons are grouped selectively (aggregation) and form principal cell masses, or layers, in the nervous system. In other words, aggregation is the phase in which the neurons, having completed the migration phase and reached the general area in which they will eventually function in the adult neural system, take their final positions with respect to other neurons, thus forming larger structures of the nervous system. The subsequent phase (differentiation) includes the development of the cell body, its axon and dendrites. In this phase, neurotransmitter specificity is established and synapses are formed (synaptogenesis). Although the first synapses occur as early as the end of the 8th week of pregnancy, the periods of intensive synaptogenesis fall between the 13th and the 16th week and between the 22nd and the 26th week (Judaš & Kostovi

, 1997). The greatest synaptic density is reached in the first 15 months of life (Gazzaniga et al., 2002). In the normal nervous system development these processes are interconnected and are affected by intrinsic and environmental factors (Kostovi

, 1979; Pinel, 2005; Spreen et al., 1984). In most cases the axons immediately recognize the path they are supposed to take and select their targets precisely. It is believed that some kind of a molecular sense guides the axons. It is possible that the target releases the necessary molecular signals (Shatz, 1992). Some neurons emit chemical substances that attract particular axons, whereas others emit substances that reject them. Some neurons extend one fiber toward the surface and when the fiber ceases to grow, having reached the existing outer layer, the cell body travels along the fiber to the surface, thus participating in the formation of the cortex. The fiber then becomes the axon, projecting from the cell body (now in the cortex) back to the original place from which the neuron started. This results in the neuron eventually transmitting the information in the direction opposite to that of its growth (Thompson, 1993). The neurons whose axons do not establish synapses degenerate and die. The period of mass cell death (apoptosis) and the elimination of unnecessary neurons is a natural developmental process (Kalat, 1995). Owing to great redundancy, pathology may ensue only if the cell death exceeds the normal rate (Strange, 1995). The number of synapses that occur in the early postnatal period (up to the second year of life) gradually decreases (pruning) and the adult values are reached after puberty. Since these processes are the most pronounced in the association areas of the cortex, they are attributed to fine-tuning of associative and commissural connections in the subsequent period of intensive cognitive functions development (Judaš & Kostovi

, 1997). Postmortem histological analyses of the human brain, as well as glucose metabolism measurements in vivo, have shown that in humans, the development and elimination of synapses peak earlier in the sensory and motor areas of the cortex than in the association cortex (Gazzaniga, Ivry, & Mangun, 2002). For example, the greatest synaptic density in the auditory cortex (in the temporal lobe) is reached around the third month of life as opposed to the frontal lobe association cortex, where it is reached about the 15th month (Huttenlocher & Dabholkar, 1997; after Gazzaniga et al., 2002). In newborns, glucose metabolism is highest in the sensory and motor cortical areas, in the hippocampus and in subcortical areas (thalamus, brainstem, and vermis of the cerebellum). Between the second and third month of life it is higher in the occipital and temporal lobes, in the primary visual cortex, and in basal ganglia and the cerebellum. Between the 6th and the 12th month it increases in the frontal lobes. Total glucose level rises continuously until the fourth year, when it evens out and remains practically unchanged until age 10. From then until approximately age 18 it gradually reaches the adult levels (Chugani, Phelps, & Mazziotta, 1987). Myelination starts in the fetal period and in most species goes on until well after birth.

From the eighth to the ninth month of pregnancy brain mass increases rapidly from approximately 1.5 g to about 350 g, which is the average mass at birth (about 10% of total newborn’s weight). At the end of the first year, the brain mass is about 1,000 g. During the first 4 years of life it reaches about 80% of the adult brain mass—between 1,250 and 1,500 g. This increase is a result of the increase in size, complexity, and myelination—and not of a greater number of neurons (Kalat, 1995; Kostovi

, 1979; Spreen, Tupper, Risser, Tuokko, & Edgell, 1984; Strange, 1995). Due to myelination and proliferation of glial cells, the brain volume increases considerably during the first 6 years of life. Although the white matter volume increases linearly with age and evenly in all areas, the gray matter volume increases nonlinearly and its rate varies from area to area (Gazzaniga et al., 2002). Brain growth is accompanied by the functional organization of the nervous system, which reflects its greater sensitivity and ability to react to environmental stimuli. One of the principal indicators of this greater sensitivity is the development of associative fibers and tracts; for example, increasing and more complex interconnectedness is considered a manifestation of information storage and processing. Neurophysiological changes occurring during the 1st year of life are manifested as greater electrical activity of the brain that can be detected by EEG and by measuring event-related potentials (ERPs; Kalat, 1995). Positron emission tomography (PET) has revealed that the thalamus and the brainstem are quite active by the fifth week postnatally, and that most of the cerebral cortex and the lateral part of the cerebellum are much more mature at 3 months than at 5 weeks. Very little activity has been recorded in the frontal lobes until the age of about 7.5 months (Kalat, 1995). Concurrent with many morphological and neurophysiological changes is the development of a number of abilities, such as language (Aitkin, 1990). In most general terms, all people have identical brain structure, but detailed organization is very different from one individual to the next due to genetic factors, developmental factors, and experience. Genetic material in the form of the DNA in the cell nucleus establishes the basis for the structural organization of the brain and the rules of cell functioning, but development and experience will give each individual brain its final form. Even the earliest experiences that we may not consciously remember leave a trace in our brain (Kolb & Whishaw, 1996).

Changes in cortical layers are closely related to changes in connections, especially between the hemispheres. Their growth is slow and dependent on the maturation of the association cortex. Interhemispheric or neocortical connections (commissures) are large bundles of fibers that connect the major cortical parts of the two hemispheres. The largest commissure is the corpus callosum, which connects most cortical (homologous) areas of the two hemispheres. It is made up of about 200 million neurons. Its four major parts are the trunk, splenium (posterior part), genu (anterior part), and the rostrum (extending from the genu to the anterior commissure). The smaller anterior commissure connects the anterior parts of temporal lobes, and the hippocampal commissure connects the left and the right hippocampus. The hemispheres are also connected via massa intermedia, posterior commissure and the optic chiasm (Pinel, 2005). Most interhemispheric connections link the homotopic areas (the corresponding points in the two hemispheres; Spreen et al., 1984), but there are some heterotopic connections as well (Gazzaniga et al., 2002). Cortical areas where the medial part of the body is represented are the most densely connected (Kolb & Whishaw, 1996). It is believed that neocortical commissures transfer very subtle information from one hemisphere to the other and have an integrative function for the two halves of the body and the perceptual space. According to Kalat (1995), information reaching one hemisphere takes about 7 to 13 ms to cross over to the opposite one. Ringo, Doty, Demeter, & Simard (1994), on the other hand, estimate the time of the transcortical transfer to be about 30 ms. Ivry and Robertson (1999) talk about several milliseconds. In their experiments on cats, Myers and Sperry (as cited in Pinel, 2005) have shown that the task of the corpus callosum is to transfer the learned information from one hemisphere to the other. The first commissures are established around the 50th day of gestation (anterior commissure). Callosal fibers establish the interhemispheric connections later and the process continues after birth until as late as age 10 (Kalat, 1995; Lassonde, Sauerwein, Chicoine, & Geoffroy, 1991). Corpus callosum of left-handers was found to be about 11% thicker than that of the right-handers, which was attributed to greater bilateral representation of functions (Kalat, 1995; Kolb & Whishaw, 1996). There is disagreement among authors considering the sex differences in callosal size (for more information, see chap. 2, this volume). Myelination of corpus callosum proceeds during postnatal development and it is one of the parts of the nervous system whose myelination begins and ends last. It is thought that the callosal evolution has an impact on hemispheric specialization (Gazzaniga et al., 2002). In Alzheimer’s patients, the area of corpus callosum, especially of its medial part (splenium), is significantly smaller than in healthy individuals (Lobaugh, McIntosh, Roy, Caldwell, & Black, 2000).

After the age of 30 the brain mass gradually decreases and by the age of 75 it is approximately 100 g smaller (Kolb & Whishaw, 1996). Although the brains of people in their seventies have fewer neurons than the brains of younger people, in healthy elderly individuals the decrease is compensated for by the dendrites of the remaining neurons becoming longe...

Table of contents

• Contents
• Foreword
• Preface
• 1 Central Nervous System
• 2 Sex Differences
• 3 Brief History of Neurolinguistics from the Beginnings to the 20th Century
• 4 Research Methods
• 5 The Central Nervous System
• 6 Lateralization and Localization of Functions
• 7 Learning and Memory
• 8 Speech and Language
• Epilogue
• Glossary
• Appendix
• References
• Author Index
• Subject Index