A Historical and Neurolinguistic Perspective on Language and the Brain

History has shown that the dedication of scientists and medical doctors to researching brain anatomy has led to phenomenal findings related to the human brain, its connection with the functioning of particular parts of the body, and its abilities to perform everyday tasks and behaviours (Gleitman et al., 2011).

One of the most fascinating functions of the human brain is language, used by humans to express their thoughts, feelings, and intentions. While communication is an ability used by all animals, human language is unique in its networked complexity (Wickens, 2009). Language studies are conducted in many fields of science, such as linguistics, cognitive science, and neuropsychology. However, the field strictly interested in neural connections, and thereby in the brain regions responsible for the acquisition, comprehension, and production of language, is neurolinguistics (Strelau & Doliński, 2008).

The discovery of neuroimaging techniques, such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), and their subsequent improvements, play an important role in understanding which brain region is active during a specific task, such as reading or speaking. Nowadays, these techniques contribute significantly to the constant development of neurolinguistics. However, as research on brain anatomy and function began in the 19th century, the first very important discoveries related to brain and language connections occurred through lesion studies and autopsies (Harley, 2014).

The two most prominent neurologists in the 19th century, who devoted themselves to researching the effects of brain damage and its relationship with language, were Paul Broca and Carl Wernicke.

The first brain autopsy performed by Broca on his mute patient led him to conclude that damage to the back of the frontal lobe in the left hemisphere was responsible for his patient’s speech deficit. It is worth underlining that spoken and written language was comprehended properly by Broca’s patient. The link between the frontal lobe and language was confirmed by eight other studies by Broca. Similar damage to the right hemisphere had no impact on understanding or producing speech. Autopsies performed by Broca revealed that the region responsible for language production was localised in front of the motor cortex in the left hemisphere only. This brain region is named after Broca (Broca’s area), and the language condition caused by damage in this area is called Broca’s aphasia (Wickens, 2009).

Shortly after Broca’s discoveries were announced, Wernicke introduced his findings regarding a different brain region responsible for language disturbances which occur with another type of aphasia (Wernicke’s aphasia). Patients with an impairment in comprehending language and producing fluent language that makes little sense (Wickens, 2009) were found to have damage to the temporal gyrus, which is part of the temporal lobe. This area of damage, found by Wernicke, as well as Broca’s area, is located in the left hemisphere and is currently known as Wernicke’s area. Patients diagnosed with Wernicke’s aphasia have no capability to understand more complex forms of speech (Wickens, 2009).

Wernicke proposed a model describing the organisation of language function in the brain. The scientist suggested that a pathway for comprehending and producing speech begins with sound images (Harley, 2014) that are stored in the superior temporal gyrus in the posterior section (Wernicke’s area), and the encoded information is sent to the frontal lobe (Broca’s area) via the arcuate fasciculus. Based on new findings, Geschwind (1972) elaborated Wernicke’s model with a few more details regarding the flow of language information in the brain regions described by Wernicke. Thus, an upgraded model was introduced to neurolinguistics and has been named the Wernicke-Geschwind model (Harley, 2014).

Relying only on simple schemes will not provide a clear understanding of the relationship between the brain and language functions, as neuroimaging studies have proved the various functionalities of language and the brain areas associated with it (Harley, 2014; Kolb & Whishaw, 2009; Poeppel & Hickok, 2004). For instance, the right hemisphere was found to be associated with a condition known as deep dyslexia (a disruption of reading processes). Researchers suggest the right hemisphere is involved with speech production and patterns of stress and intonation in language (Lindell, 2006).

In truth, patients with brain damage form an invaluable tool for learning the mechanisms of speech, although there are limitations to these studies regarding the functionality of brain regions in healthy humans, because the research is based only on the consequences of brain damage. It must be underlined that damage in one brain area can greatly impact neurophysiological changes in other brain areas. Also, by studying brain functioning in its pathological state, it is impossible to extend the results of analyses to understand a properly functioning brain (Strelau & Doliński, 2008).

These limitations can be reduced through research using neuroimaging techniques to detect activity in a particular brain area during language task performance. According to Strelau and Doliński (2008), numerous studies using these methods have provided rich, but sometimes equivocal data, which has slightly altered the view of the brain’s control of language functions. Pathological functioning is not limited to areas that have been directly damaged, for example, by a brain infarct. Around the damaged region, a large area of functional nerve tissue (penumbra) may be exposed to a functional shadow.

Research using neuroimaging techniques, where the participant performs a variety of linguistic tasks, is used to analyse the structure of language processing, with PET and fMRI being the most adequate techniques. Using fMRI, Binder et al. (1997) found that speech perception activates the upper temporal lobe. During speech perception, activation also occurs in the motor cortex in the frontal lobe, which is involved in speech production (Wilson, Saygin, Sereno & Iacoboni, 2004). The results of these studies have shown that the areas responsible for speech perception and speech expression are closely related, and the division between frontal and posterior parts of the brain is relative.

However, there is no consensus on which structure decodes the meaning of a particular word. Crinion, Lambon-Ralph, Warburton, Howard and Wise (2003), as well as Wise and Price (2005), showed in their studies that the middle and lower temporal lobe and the temporal-occipital junction are responsible for decoding. The authors agree that decisive roles are played by posterior brain areas, which also corresponds to clinical trials. Moreover, activation of lower frontal lobe areas was observed when participants made decisions relating to the meaning of individual words (Strelau & Doliński, 2008). Wagner, Pare-Blagoev, Clark and Poldrack (2001) revealed in their study that this area is not unified in its functionality; the front part of this region is associated with semantic processes, and the back part is related to phonological processes. Research using the modern method of voxel-based lesion-symptom mapping (VLSM) has allowed scientists to refine the definition of the relationship between impaired function and brain damage, which has proven to be dependent on areas in the frontal, temporal, and lower-parietal lobes of the left hemisphere. However, these regions do not coincide with the classic Broca’s and Wernicke’s areas.

According to Strelau and Doliński (2008), some study results led to the conclusion that the most frequent brain activation occurs in the frontal lobe. Activation occurred in larger areas than the locations where the damage was situated. Grammatical analysis of speech activated frontal and upper temporal lobes. Furthermore, one study showed that damage to Brodmann area 22 leads to disruptions in understanding the grammatical structure of a sentence (Gazzaniga, Ivry & Mangun, 1998).

Despite various controversies, it is clear from much research that Broca’s area cannot be considered the only region where syntactic processes take place. According to Kaan and Swaab (2002), these processes depend on a wide neuronal network, which includes the temporal cortex, the posterior parietal lobe, and the frontal lobes, and also occur in both the left and right hemispheres.

Expression of speech includes motor components associated with language motor programmes and the articulatory apparatus, for which appropriate parts of the motor and somatosensory cortex are responsible. However, according to Petersen and Fiez (1993), speech is a complicated process consisting of many elements such as the formulation of thoughts, the selection of appropriate words and their placement in the right sequence, the conversion of words to phonological code, and then articulation. Of course, verbal activation is influenced by Broca’s area, as well as the motor and somatosensory cortex (Strelau & Doliński, 2008).

The above examples of neuroimaging study results show that the activation pattern for language cannot be associated as previously understood, for example, that only Broca’s area is responsible for speech expression and Wernicke’s area for speech comprehension. Regarding the latest brain imaging techniques, aspects of language processing involve complex neuronal systems, whereby a whole array of structures belonging to both the frontal and posterior parts of the brain is implicated (Wise & Price, 2005).

Brain lesions to the arcuate fasciculus, posterior parietal and temporal regions, left auditory cortex, insula, and supramarginal gyrus (Wickens, 2009) have been found to be responsible for conduction aphasia, which is diagnosed by naming deficits and impaired ability to repeat non-meaningful single words and word strings (Wickens, 2009). Damage to regions which mediate phonological processes in the temporal lobe is associated with transcortical sensory aphasia, a condition in which language comprehension, naming, reading, and writing are impaired and where semantic irrelevances in speech occur (Wickens, 2009). Transcortical motor aphasia is described as a condition with disruption in dysprosodic speech and transient mutism. This type of aphasia has been associated with a lesion of the connection between Broca’s area and the supplementary motor area, the medial frontal lobe, and regions anterolateral to the left hemisphere’s frontal horn (Wickens, 2009). A generalised deficit in comprehension, repetition, naming, and speech production (Wickens, 2009) are symptoms of global aphasia. Damage to the basal ganglia and thalamus, white matter, and the left perisylvian region is responsible for this condition (Wickens, 2009).

According to Ullman, Corkin, Coppola, Hickok, Growdon, Koroshetz et al. (1997), storing and generating grammatical rules are strictly associated with the lexicon, which is a part of language. The Declarative/Procedural (D/P) model by Ullman et al. (1997) describes a lexicon associated with a temporal-parietal declarative memory system and shows a relationship between a frontal, basal ganglia procedural system and the generation of grammatical rules. In their study, the researchers found posterior aphasia to be a condition which occurs when a patient has difficulty finding a word. This type of aphasia has been found in Alzheimer’s disease patients, and the researchers also found them to have impairments in memory, more precisely with worse recall of irregular verbs than regular verbs. During the analysis of patients diagnosed with Parkinson’s disease, the researchers discovered problems with suppressed motor activity and grammatical rule use. The association of excessive grammatical rule use with the basal ganglia, according to Ullman et al. (1997), is greatly reflected in Huntington’s Disease (HD), a disease caused by the loss of neurons in that brain area. Lesion to the basal ganglia led to excess motor activity in patients suffering from HD. In turn, Parkinson’s disease can be diagnosed in people for whom a lesion to the basal ganglia has caused suppression of motor activity. In this particular example, suppression also occurs for grammatical rule use.

Harley (2014) argued that the anatomy of the brain contains areas that are more or less important for language functions than others. In his argument, Harley underlined that the brain is developed with multiple routes involved in language production and comprehension. In addition to the above finding, a few more should be adduced. Harley (2014) listed several examples of the complexity of brain regions, its network, and its relationship with language functions. First, language comprehension has been concluded to have an association with the temporal gyrus (Dronkers, Wilkins, van Valin, Redfern, & Jaeger, 2004). Second, studies on auditory comprehension (Hickok & Poeppel, 2004) require the involvement of both sides of the temporal gyrus in speech perception. Hickok and Poeppel’s (2007) framework of language functional anatomy shows a model in which a ventral stream processes speech signals for comprehension, and a dorsal stream maps acoustic speech signals to frontal lobe articulatory networks. The model assumes the ventral stream is largely bilaterally organised—although there are important computational differences between the left- and right-hemisphere systems—and that the dorsal stream is strongly left-hemisphere dominant.

The evidence that specific language functions are localised to distinct brain regions has been presented. The lateralisation of language functions has been studied by many scientists, beginning in the 19th century with early neurologists like Paul Broca and Carl Wernicke, through the 20th century with the use of modern techniques by researchers like Geschwind, Ullman, and Poeppel.

The findings of several studies have revealed that regions in the left hemisphere are mostly associated with language comprehension and production. However, the right hemisphere has also been found to be associated with particular language functions, though the left hemisphere determines a more complex neural network for the phenomena of language generation. The brain regions in the left hemisphere responsible for comprehension and production of language are the frontal lobe, which contains Broca’s area, and the temporal and parietal lobes, where Wernicke’s area is located. Also, the occipital lobe has been found by some researchers to play a role in language functions.

A lesion to a specific area of the brain responsible for language processes causes a condition called aphasia. Different types of aphasia can be distinguished with regard to the symptoms following brain damage.

As mentioned at the beginning, in its processes, the brain is a very complex network of neurons connected with each other in various ways. Nowadays, researchers test new hypotheses regarding language functions and their location in the brain through injured brain analysis as well as by using modern neuroimaging techniques (Rumsey & Ernst, 2009).