13 - Brain Development and Literacy

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CDEV 94


Lou Morgan


Research Paper

November 8, 2004



Brain Development

and Literacy



This paper will concentrate on the neural structures which underlie literacy and on the development of those structures from conception to maturity. Since our literary tradition was originally oral, I will begin with the structures which mediate our oral literary tradition and then move on to consider the additional structures which allowed our oral tradition to be written down and read by others.


Although all mammals, including humans, have a Cerebral Cortex, our ancestors which preceded the evolutionary emergence of the mammals got along quite well without one. The neural structures which mediate the behavior of our ancestors, such as the amphibians, still exist within we humans, and they still play a vital role in our behavior [1]. Since these pre-mammalian structures are found at the central core of the mammalian brain and are clearly demarcated from the cortex on the exterior, they are termed "subcortical" [2].


It is quite clear that frogs participate in oral communication with one another. It is generally agreed that, at the very least, their croaking helps them to find a mate. It has been suggested that it may also serve to establish territorial boundaries, and it could even serve additional functions which we have not yet identified. We will begin our study of brain development and literacy by looking at the subcortical structures in human brains which subserve oral communication.


Communication between individuals requires first expression, as by vocalization, and then reception, as by hearing. Although this paper is more concerned with the receptive half of the cycle, the communication must first be expressed before it can be received, so we will start with vocalization.

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Most overt behavior, such as vocalization, requires some degree of metabolic arousal. The sleep-wake cycle is dependent on the cyclic release of  Melatonin from the Pineal Gland which only occurs during the hours of darkness [3]. In amphibians, the pineal receives direct photic input through the Parietal Eye on the top of the head [4]. In mammals the photic input is supplied by the ordinary eyes to the
Suprachiasmatic Nucleus
which in turn passes the information on to the pineal [5]. Melatonin released by the pineal during darkness then coordinates the release of adrenocorticotropic hormone (
ACTH) by the   Pituitary which in turn causes the release of the arousal hormone Cortisol by the Adrenal Cortex [6]. In nocturnal animals such as the frog and the mouse, the ACTH is released by the pituitary at night. In animals such as humans which tend to be awake during the day, the ACTH is released during the day.


In both the frog's croak and the human's speech, the exhalation which carries the sound is controlled by nerves which leave the spinal cord and control the muscles of the ribcage and the diaphragm [7]. The vocal cords, which are responsible for the sound itself, are controlled by the  Inferior Laryngeal Nerve  which is a branch of the Vagus Nerve [8] and controlled by the Nucleus Ambiguus  [9]. All of the above are subcortical, as are the nuclei which give rise to the nerves controlling articulation. The role of the mammalian Cerebral Cortex will be discussed later.


Like vocalization, hearing requires some degree of metabolic arousal, and the mechanism for this is the same as above. Hearing begins in the Cochlea, which is the spiral shaped organ in the inner ear in which sound waves vibrate the hairs of sensory cells. The lowest tones are sensed by the hairs at the apex of the spiral, and the highest tones by the hairs at its base. The auditory input is processed through two distinct stages, each consisting of several nuclei, before reaching its major subcortical targets, but the tonal separation which begins at the cochlea is maintained at each stage [10].


The final subcortical recipients of acoustic input are the inferior colliculus [11] and the medial geniculate body [12]. These structures project, in turn, to the reticular formation [13], which then projects both to the nucleus

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ambiguus and to the spinal cord. It is the nerves which leave the spinal cord to activate muscles that result in overt behavior. For example, if a frog sitting silently hears another frog croak and is in a high enough state of arousal, he may well croak in response, via the circuits discussed above, and we have the beginnings, on a very primitive level, of oral communication.


Before we turn our attention to the cortex, we need to consider a very important part of the brain called the hippocampus [14]. Since the hippocampus was initially identified among animals with a cortex, it came to be considered as a part of the cortex. When it was belatedly discovered that the brains of precortical animals such as amphibians have a homologous structure, this structure was named the "hippocampal primordium" [15]. The hippocampus and its primordium are important to us for two reasons.


First the hippocampus and its primordium are essential for the transition from short-term to long-term memory. It is quite common among our early ancestors for the newly hatched young to leave the place of their birth in search of food, only to return many years later in order to breed. They seem to find their way back primarily through their sense of smell. They remember what the river or lake of their birth smelled like. The hippocampal primordium evolved between the primary olfactory nucleus, which mediates the sense of smell, and preoptic-amygdala area [16], which has receptors for the sex hormones testosterone and estrogen [17], and it receives input from both. It evolved to encode the smell of home so that when the gonads signal the brain that the time for breeding has come, the animal can find its way back to where it was born and where its young will be able to hatch and develop safely.


The second reason that the hippocampus and its primordium are important is that it is here that we first see the evolution of the cortex [18]. As already mentioned, all the subcortical structures discussed so far are bunched up in the central core of our brains. The cortex came into existence when neurons began migrating out from the central core and eventually positioned themselves on the outer surface. Although these newly "cortical" neurons developed

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axons which radiate widely, their primary connection remains with their place of origin. All this began in the hippocampus, which I believe is Brodmann's area 34 (see the cortical map at the end of this paper).


As mentioned earlier, the final subcortical recipients of acoustic input are the inferior colliculus and the medial geniculate body. In addition to both of them projecting to the reticular system, the inferior colliculus projects to the medial geniculate body, and both project to the cerebral cortex [19]. The medial geniculate body projects to the primary auditory area, which is located on the middle part of the anterior transverse gyrus in the floor of the lateral sulcus and is known as Brodmann's area 41. The inferior colliculus projects to the secondary auditory area, which is located on the posterior transverse gyrus and adjacent portions of the superior temporal gyrus and is known as Brodmann's area 42. Both of these areas project back, not only to the medial geniculate body and inferior colliculus, but also to the several nuclei which have already been mentioned as forming two distinct signal processing stages between the cochlea and the highest subcortical level. This feedback allows the cortex to participate in the signal processing which takes place at lower levels [20]. The tonal separation which began in the cochlea continues into the primary auditory area.


Although we appear superficially to be divisible down the middle into two halves, left and right, which are mirror images of one another, beneath the surface there are major differences between our two halves. This includes the two halves of our brain. Our left hemisphere specializes in fine details, while the right deals with the context in which the details are embedded. Thus the left hemisphere understands and provides the complex articulations necessary to form words [21], while the right understands and provides the broad changes in intonation that convey emotion [22].


The cortical center which articulates words was first identified on the left hemisphere by Broca and is therefore called Broca's area. It is located in part of the inferior frontal gyrus approximately equivalent to Brodmann's areas 44 and 45 [23]. It's output is to the many subcortical

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nuclei that control the complex acts of verbal articulation. Emotional tone is provided to speech by the homologue of Broca's area in the right hemisphere.


The emotions themselves are mediated by a portion of the brain called the limbic system, which includes both subcortical and cortical structures and has close connections with the autonomic nervous system. The subcortical elements are generally thought to include the amygdaloid complex, septal nuclei, hypothalamus, epithalamus, anterior thalamic nuclei and parts of the basal ganglia. The cortical elements includes the subcallosal, cingulate and parahippocampal gyri on the medial surface of the hemisphere, designated Brodmann's areas 23, 24, 25, 31 and 32, as well as the underlying hippocampal formation and dentate gyrus [24].


The center which understands words was first identified on the left side by Wernicke and is therefore called Wernicke's area [25]. It consists of the posterior superior temporal, supramarginal and angular gyri and is equivalent to the posterior part of Brodmann's area 22 as well as Brodmann's areas 39 and 40 [26]. It's input is from the primary and secondary auditory areas, Brodmann's areas 41 and 42. Emotional tone is understood by the homologue of Wernicke's area in the right hemisphere and perceived by the limbic system.


Thus far we have outlined the neural structures which underlie verbal communication. I have no solid information on the evolutionary progression from oral communication to oral literature, but it seems pretty clear that a key feature would be memory. I would guess that the first oral literature consisted of simple stories which reflected our attempt to understand the world around us. Thus all cultures seem to have creation myths and stories which provide explanations for natural phenomena such as the progression of day and night. The recounting of hunting trips or conflicts with neighboring tribes would have eventually evolved into more complex stories such as the Iliad and the Odyssey. As already mentioned, the part of the brain central to the conversion of short-term to long-term memory is the hippocampus. Although the memories themselves may be stored throughout the neuroaxis, an

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especially important storage area seems to be the temporal cortex [27], Brodmann's areas 20 and 21, among others. Specific details are processed by the left hippocampus and stored in the left temporal cortex. The overall shape of the story and its emotional import are processed by the right hippocampus and stored in the right temporal cortex.


Here in the West, the transition from oral literature to written literature required the encoding of sounds into written symbols. It should be noted that Chinese writing encodes visual symbols rather than sounds. Vision, of course, begins with the eyes. Unlike auditory input, which passes through two signal processing stages on its way to the inferior colliculus and the medial geniculate body, visual input travels directly to the visual correlates, the superior colliculus [28] and the lateral geniculate body [29]. Like their auditory correlates, the superior colliculus and the lateral geniculate body project to the motor nuclei of the reticular formation, and it is this circuitry which mediates the response of our precortical ancestors to their visual environment [30].


All the subcortical visual input to the cortex seems to be via the lateral geniculate body which projects to the primary visual cortex, Brodmann's area 17, located in the walls of the calcarine sulcus and adjacent portions of the cuneus and lingual gyrus on the posterior medial surface of each hemisphere [31]. This cortical input is then projected back to a subcortical structure called the pulvinar which is located at the rear of the thalamus [32]. The pulvinar then relays the visual input to the secondary visual cortex, Brodmann's areas 18 and 19, which are adjacent to the primary visual cortex.


Unlike the inferior colliculus, which provides auditory information to the secondary auditory cortex, the superior colliculus does not appear to have any direct input to the secondary visual cortex. It does, however, provide input both to the lateral geniculate body and the pulvinar [33]. Visual signal processing then takes place in a series of back and forth projections between the pulvinar and the secondary visual cortex.


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The association between the visual symbols of the 26 letters of the alphabet and the 44 phonemes of American English takes place in the left angular gyrus, Brodmann's area 39, which has already been mentioned as part of Wernicke's area. Whole-word reading takes place, and visual symbols such as Chinese writing are understood, in the right angular gyrus [34].


Brain development occurs in a sequence which parallels brain evolution, with the evolutionarily older structures developing first. The first stage of development is migration of the primitive neuroblasts from the embryonic neural tube to the site of their eventual maturation. This is complete at birth [35]. The neuroblasts then sprout receptive dendrites, which generally remain fairly close to the cell body, and a single axon, which conducts impulses away from the cell. The axon may be very, very long in comparison with the dendrites and may divide into several collaterals, thereby providing input to several different parts of the brain. Most, but not all, axons become covered with a myelin sheath which speeds conduction. Even before myelination, connective synapses have begun to form between axons and dendrites. This formation of synapses can continue throughout life but is most intense in the early years. Synapses, and even entire neurons, which are not used regress and die. Synapses and neurons which are used frequently become and remain strong and healthy [36].


The most readily visible indication of neuronal maturation is myelination. At the time of full-term birth, all of the subcortical neurons are myelinated, but the only parts of the cortex which are myelinated are the very restricted parts related to sensory input from the area around the mouth and the related activity of sucking [37]. Therefore, except for feeding, all of the new born baby's behavior is subcortical.


The primary sensory areas for hearing, vision and touch mature next, followed by the secondary sensory areas and then still later by the association areas such as Broca's and Wernicke's. The prefrontal cortex, which helps us guide our behavior in response to our emotional needs, matures the most slowly [38].

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References


1. Herrick, C.J. (1965). The Brain of the Tiger Salamander. Chicago. University of Chicago Press. p. 16.


2. Carpenter, M.B. and Sutin, J. (1983). Human Neuroanatomy, 8th Ed. Baltimore. Williams & Wilkins. p. 40.


3. Nussey, S.S. and Whitehead, S.A. (2001). Endocrinology, an Integrated Approach. Oxford. BIOS Scientific Publishers Ltd. p. 41.


4. Butler, A.B. and Hodos, W. (1996). Comparative Vertebrate Neuroanatomy. New York. John Wiley & Sons. P. 30.


5. Carpenter, p. 499.


6. Nussey, p. 132.


7. Carpenter, p. 196.


8. Hollinshead, W.H. (1974). Textbook of Anatomy, 3rd. Ed. New York, Harper & Row. p. 945.


9. Carpenter, p. 349.


10. Carpenter, p. 362-363.


11. Carpenter, p. 412-413.


12. Carpenter, p. 524.


13. Carpenter, p. 332-335 & 438-441.


14. Carpenter, p. 621.


15. Herrick, p. 101.


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16. Herrick, p. 323.


17. Adler, N.T. (1981). Neuroendocrinology of Reproduction. New York. Plenum Press. p. 530.


18. Herrick, p. 102.


19. Carpenter, p. 372.


20. Carpenter, p. 678.


21. Springer, S.P. and Deutch, G. (1989). Left Brain, Right Brain, 3rd Ed. New York. W.H. Freeman and Company. p. 67.


22. Springer, p. 184.


23. Carpenter, p. 703.


24. Carpenter, p. 639.


25. Springer, p. 177.


26. Dorland's Illustrated Medical Dictionary, 25th ed. (1974). Philadelphia. W.B. Saunders. p. 131.


27. Carpenter, p. 632.


28. Carpenter, p. 417.


29. Carpenter, p. 526.


30. Carpenter, p. 423.


31. Carpenter, p. 665.


32. Carpenter, p. 494.


33. Carpenter, p. 422.


34. Springer, p. 48.


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35. Carpenter, p. 71.


36. Cook, J.L. and Cook, G. (2005). Child Development, Principles and Perspectives. Boston. Pearson Education. p. 149.


37. Hansen, P.E., Ballesteros, M.C., Soila, K., Garcia, L. and Howard, J.M. (1993). MR Imaging of the Developing Human Brain, Part 2. Postnatal Development. RadioGraphics. May, p. 611-622.


38. Herrick, C.J. (1930). An Introduction to Neurology. Philadelphia. W.B. Saunders. p. 346.




   








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