Investigating the Lateralization of Language and Cerebral Hemispheres

This essay delves into the lateralization of language within the human brain, with a specific emphasis on the left cerebral hemisphere. It explores the intricate processes involved in language comprehension and production, drawing upon research from psychologists, neuroanatomists, and other experts. The paper discusses key concepts such as Broca's and Wernicke's areas, the functional specialization of the brain's hemispheres, and the neurological pathways involved in speech and language. It examines evidence from various studies, including the Wada test and neuroimaging techniques, to illustrate the dominance of the left hemisphere in language processing. The essay also addresses syntactic and semantic processes, the roles of different brain regions in word retrieval and artificial grammar learning, and the impact of language lateralization on individuals with conditions like dyslexia. Overall, the essay provides a comprehensive overview of the neurobiological basis of language and the lateralization of language functions within the brain.

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The Lateralization of Language and Associated Processes
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

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(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

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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.
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

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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.
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.

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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

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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 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

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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.
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

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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).

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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 integrated.

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