Language Learnability And Development: Evidence Of Hemispherical Lateralization

Neurobiological Basis of Language

Discuss About The Language Learnability Language Development.

The most complex body organ is the human brain. It forms the central nervous system together with the spinal cord. It has different specialized parts that work in coordination to carry out different functions; some of which are termed as basic while others as complex. There are two hemispheres of the brain-the right and the left hemisphere. Each hemisphere is divided into lobes that carry out specific functions that are mediated by neuronal and hormonal connections to other body parts. This paper aims at discussing the evidence of language lateralization and the processes involved to the left cerebral hemisphere. The main discussion will base on research and experiments by psychologists, behavioral scientists, psychiatrists, neurobiologists and neuroanatomists.

Insights into the study of the human brain have been ongoing since time immemorial. Psychologists and neuroanatomists have made significant findings in the study and mapping of the brain. They have employed different methods of study including but not limited to; electroencephalogram, positron emission scans, magnetoencephalography, functional magnetic resonance imaging, photon migration tomography and transcranial magnetic scanning. Most methods for testing and qualifying lateralization of functions are too invasive to be performed on normal healthy people. Most of the findings are thus performed on people with functional deficits secondary to a pathological process. These findings have mapped the brain into functional areas associated with behavior, memory, speech, language, learning, executive control processes and other higher functions.

Language is diverse. Akmajian (2017), describes it as a rational and logical way of communication and transmitting information to others with the consequent response. Pinker (2009), states, “One is born not knowing any form of language”. However, as he/she grows and interacts with the environment, he acquires, develops, modifies and maintains certain systems involved in communication. The founder father of modern linguistics is Noam Chomsky. His contribution towards formulating a formal theory of grammar and language design have been significant. Language is not strictly entrenched to its communication purposes alone. Language also serves as a mode of group cultural identity, entertainment, social stratification and grooming. Liu et al., (2009) say that processes of language are multiple and integrate with each other to form an intricate function.

The knowledge concerning the neurobiological basis of language has been enhanced over the past years. Hemispherical lateralization refers to the unlikeliness of the two hemispheres of the brain to perform similar functions. Luria, (2012) confirmed that each hemisphere is specialized for a specific function because of location of strong neural connections and mechanisms that suit the processing of the intended function. The best example to support this statement is the functionality of speech and language. The right hemisphere is involved in processes which require intuition, holistic thought, creativity, imagination, 3-dimensional forms, art and music awareness. Conversely, the left hemisphere is more concerned with processes that are associated with logic, analytic thought, language, reasoning, numbers and science skills (Hervé et al., 2013). Processing of language involves different neuronal pathways of the brain. These pathways have been mapped to be located in Broca’s and Wernicke’s area of the most dominant hemisphere. The area involved in comprehension of spoken and written language is Wernicke’s. It is located in the dominant hemisphere specifically in the superior temporal lobe mapped as Brodmann’s topographical area number 22. On the other hand, the role of specialization in language production is played by Broca’s area. Its mapping is the posterior inferior gyrus of the frontal lobe of the most dominant cerebral hemisphere. Brodmann named it as area number 44, pars opercularis and 45, pars triangularis. These two speech regions are linked by structural fibers, the dominant one being the arcuate fasciculus.

Hemispherical Lateralization

The Wada test provides evidence that 70% of left-handed people and 98% of the left-handed people have left cortical cerebral hemisphere as the dominant part in speech and language processing (Baxendale, 2009)). This shows that only less than 2% of the population have right hemisphere dominance thus develop motor and sensory aphasia in lesions that affect the right hemisphere. Further evidence is provided by the anatomic differences between the right and left hemisphere. The left Sylvian sulcus is longer and less steep than the right hemisphere. This difference gave anatomists the desire to know why the difference exists. Therefore exploring the brain through use of neuro-architectonic processes was applied.

Different neuro-architectonic processes have given evidence of differences in neuronal density between right and left right hemisphere. Also, different neuroreceptors have been identified in both hemispheres. These studies also provide that both the left and right hemisphere are involved to support different language processes with the right hemisphere playing subtle roles. For instance, the prosodic element of speech and language is a concern of right cerebral hemisphere whereas the left hemisphere is lateralized for support of syntactic and semantic function (Gleason & Ratner, 2016).

The processes that take place during language comprehension are hierarchically structured from perception of auditory stimulus, integration and final comprehension. Analyses of auditory processes takes place bilaterally in the primary auditory cortices. This does not apply to the semantic and syntactic processes. Syntactic processes are more supported by the temporo-frontal networks of left hemisphere while networks for semantics have a moderate support from this region. Sentence prosody and intonation are specialized to be a function of the right hemisphere. Comprehending an auditory input and providing a rational answer is an end-result that arises from a number of sub-processes. These processes include acoustic-phonological, semantic and syntactic processes.

Spoken language comprehension begins by analyzing of the acoustic-phonological component of the auditory input. This is a process that is supported by the auditory cortical area and the adjacent regions. According to Da Costa et al., (2011), this areas have a role of differentiating acoustic signals that are speech from those that are non-speech. These function is computed with the Heschl’s gyrus. It is further described that the analysis of sound signals is a role of Heschl’s gyrus whereas the region adjacent to it called planum temporale is associated with the categorization process. Despite the left and right primary auditory cortices responding to speech, they have different preferences in computation. The right primary auditory cortex is characteristic to tonal pitch whereas the left PAC has a preference to speech sound characteristics. This kind of specialization arises due to the type of frequency that is needed to work optimally. The right hemisphere utilizes the theta range frequency while the left hemisphere utilizes the gamma range type hence the reason for lateralization.

Language Comprehension Processes

The approach used to understand the semantic and syntactic processes of language is designed to test language “intelligibility” (Morgan & Demuth, 2014).  The altering of the acoustic signals was the approach designed to test intelligibility. This was done by normal speech spectral rotation in order to render the signal as unintelligible. This kind of study made it easier to discover that the anterior superior temporal sulcus was systematically specialized for the function of speech intelligibility.

On the other hand, the posterior superior temporal sulcus was found to be similarly stimulated by rotated speech, noise-vocoded speech and normal speech. These findings led to the conclusion that the posterior superior temporal gyrus is mainly involved in representation of short term sound sequences that at least contain some phonetic information even if they are not intelligible (Emmorey & Braun, 2011). This differentiation in function takes us to the double pathways that begin in the primary auditory cortex. One pathway is directed from the Heschl’s gyrus to the anterior superior temporal sulcus/superior temporal gyrus and the other going to the Heschl’s gyrus to the posterior superior temporal sulcus/superior temporal gyrus. These findings correlate with the observations realized in clinical cases of patients with deficits in speech comprehension secondary to focal cerebral pathologies in the foremost temporal region of the dominant hemisphere. Processes such as word retrieval have been strongly associated with the left hemisphere.

Findings from several studies through neuroimaging techniques have shown that the regions involved in this function are posterior regions in the left middle and inferior temporal gyri. The superior temporal gyri may sometimes take part in this processes. When a task difficulty in word retrieval is increased, brain regions associated with solving this difficulty increase their functional activity. This activation has been observed mainly in the frontal and temporal areas of left hemisphere with a few cases showing right hemisphere activation. However, McGettigan & Scott (2012), affirms that even in those who are presumed to have a left laterality of speech and language, parts of the right frontal and temporal lobes are also activated in some language processes. This gives the evidence that whereas language is commonly associated with the left hemisphere, some components are integrated in the right hemisphere. Language studies by transcranial Doppler stimulation over the Wernicke’s area identified an increase in blood flow when word-picture matching activity administered. Stadthagen-Gonzalez (2009), reiterates that the stimulation was almost entirely on the left hemisphere in those who were right-handed.

Syntactic Processes

Syntactic processes as a function of Broca’s area have as well been studied in both hemispheres through use of learning artificial grammar. Pulvermüller (2010), rationalize that artificial grammar learning was appropriate in such a study because in this approach, all necessary inputs are well controlled thus allowing the process of learning the language to be constant and equal among the subjects under the pilot plan. This means that there was no subject with upfront information or an advantage over the others. Therefore, every brain was a new slate waiting for information to be inscribed (De Vries et al., 2010).

The contribution of Broca’s area with reference to learning syntax proved that subjects were capable of acquiring the new language as long as the universal principles governing the acquisition of a natural language were adhered to. During the initial study process, majority of the participants had a low activation frequency in Broca’s area and a high frequency of activation in hippocampal area. However, as the reinforcement of syntax learning and language acquisition went on, the activation of Broca’s area systematically increased while that of the hippocampal area decreased gradually. After a few days of language study, the Broca’s area was assessed and found to be sensitive to classifications of grammaticality.

Rogalsky & Hickok (2011), supports this study and thus concludes, “Broca’s area takes part in acquisition of syntactic knowledge and learning syntactic rules together with the appropriate processing and integration”. It thus follows that learning and processing of syntax is a crucial role of Broca’s area. Further studies compared the ability of sentence comprehension and processing of artificial grammar sequences. These studies ended up activating a large portion of the perisylvian cortex hence the left prefrontal cortex was assigned functionality basing on Brodmann areas. It was thus concluded that the posterior and superior regions are specialized in structural and sequential facet of processing while the inferior and anterior regions were tasked with insertion of contextual information into structural matrixes. It was emphasized that these special features of syntax processes were mainly located in the left hemisphere (Teichmann et al., 2015).

Several psycholinguistics propose that syntactic and sentence parser processes are developed from preliminary stages where simple syntactic structures are formed basing on word grouping. These simplest forms are used as foundation from which complexities and references are established. Further experiments have taken place using functional MRI in assessing language in children with and without dyslexia. These children were assigned language-related tasks and then a functional MRI is performed bilaterally. Heim et al., (2010), found that dyslexic children had a limited activity in Broca’s area on the left in the course of task performance. There was also a significant limitation of activity in Wernicke’s area of the left cortex. This was a proof that language and speech-sensitive functions and processes are strongly lateralized to the left cerebral cortex.

Semantic Processes

Psychologists and linguistics provide that there are structural connections between Wernicke and Broca’s areas of speech and language. The dominant fiber pathway is called arcuate fasciculus. The language-relevant regions (temporal and prefrontal cortex) have been established to be linked by two pathways, the dorsal and ventral pathway. Each of this pathway support a different function of language (Yeh et al., 2013). The ventral pathway has been mapped to support the language component of sound-meaning whereas the dorsal pathway supports auditory-motor integration. There are different approaches that track this connecting fibers. The deterministic fiber tracking approach provides that it is the functional data that predefines the endpoints of the two fiber pathways. This approach maintains that pars orbicularis and triangularis are connected to the temporal cortex by the ventral fiber bundle through the system of extreme capsule to support sound-meaning mapping. The premotor cortex and temporal lobe are connected to pars orbicularis by dorsal fiber bundle thus supporting the sensory-motor mapping component of sound articulation.

The other approach is the probabilistic fiber tracking approach. Klein et al., (2013), describe that this approach provides presence of one end of connection point termed the seed point. Defining two seed points in inferior frontal gyrus leads to the basis of two functionally different activations. It is presumed that the dorsal pathway has 2 components; one associated to higher-level language processes while another supports sound-motor mapping. Insights into study of tracts have shown that there are other several tracts which have not been explored to suffice a discussion. Among these tracts are the short- and long-range structural connecting pathways. These integrated connectivity pathways give a clue that language and speech processes are complex component of the brain which requires input from several functional regions for integration and interpretation (Segalowitz, 2014).

Functional connections between different default brain networks have also been described in depth by different anatomists and psychologists. In every study regarding brain functionality and specialization, there is at least a report about activation of a function-related region in the left perisylvian cortex. This cortex is formed by 4 parts which are; parietal, temporal, frontal and prefrontal cortices. Functional studies provide evidence that each area in the perisylvian cortex has a specific function with regards to language processing (Friederici, 2012).

From the above discussion on the relationship of speech and language in relation to hemispherical lateralization, it can be concluded that language is a complex processes that requires almost every brain component for integration. Language lateralization varies among individuals due to a number of factors. However, despite the shortcomings noted in the theories and experiments that focus to prove lateralization, it still remains that language processes are functions of the left hemisphere while the right hemisphere is effective for visuospatial processes. However, this is not to say that the left hemisphere can solely carry out the functions and processes related to language and speech without an input from the right hemisphere. Language is a competitive process that cannot be effectively handled by one region of the brain alone. Therefore, the input from the right hemisphere is critical in ensuring that all processes related to language have been well-perceived and integrates.

Conclusion

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