The neural basis of aphasia, as a syndrome associated with localized damage to the cerebral cortex

Language and the ability to speak are major tools of communication of human beings therefore disturbance or loss of speech and impairment in comprehension cause severe disadvantage in daily living. Researchers study aphasia for two reasons; firstly, to improve the success of treating aphasia, and secondly, to gain further understanding of the language-brain relationship.

The main value of aphasia in neuroscientific studies is that it is a disorder that exclusively affects language ability without virtually any influence on other cognitive modules. Considering this point, aphasiology can help to explain the relationship between brain and language through those neurological mechanisms, which are implied in the language function. Language is one of the most accessible parts of human cognition.

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The phenomenon that the brain injury or trauma to the left hemisphere leads to an impairment or a loss in the language ability whereas injury on the right hemisphere can lead to a loss of ability to express emotions, thereby speech becomes flat, lifeless and mechanical, but leaves the main linguistic functions more or less intact, among other observations, led researchers to the conclusion that studying the effects of brain damage on adult language can help to clarify which neurological mechanisms are implied in the language function.

One type of language impairments caused by damage to brain areas that are responsible for language is Aphasia (MedicineNet. com, 2006) that manifests in an impairment or loss of the understanding or transmission of ideas by language in any of its forms; reading, writing, or speaking.

Because language functions are lateral to the left hemisphere in 96-99% of right-handed people and 60% of left-handed people; and approximately one half of the remaining left-handed people has mixed dominance, and one half has right hemisphere dominance, left-handed individuals usually develop aphasia after a left hemispheric stroke, but the aphasia may be milder if their have mixed or right hemisphere dominance for language function (Jacobs, 2005).

Aphasia may co-occur with other speech disorders such as disturbed language functions in writing or reading (agraphia or alexia) and also can co-occur with a disturbance of the articulation alone (dysarthria). Aphasia can arise suddenly, often as the result of a stroke or head injury, but it may develop slowly in the cases of a brain tumor or degenerative diseases. Physicians’ observations of aphasic symptoms extend back as early as 3500 B. C. (Smith Surgical Papyrus). Later in the Hippocratic Corpus (400 B. C. numerous cases of loss of speech were described along with distinguish between types of speech loss, such as between aphasia and anarthria, and loss of vocal ability. In the last two centuries Gall (1825) was probably the first to suggest the existence of anatomo-functional relationships between specific cortical areas and different human capacities. Anatomo-clinical studies carried out in Western Europe in the nineteenth century have provided an empirical basis for contemporary idea about the cerebral representation for language (Basso, 2000).

Based on clinical experiences well-known models of language processing and its neurological implementations were created by Broca (1861), Wernicke (1874), Lichtheim (1885), and Geschwind (1970). Among the many the Wernicke-Geschwind model of language (1965) has been very useful in order to make a classification of the five common aphasic syndromes, which are the Broca’s aphasia, Wernicke’s aphasia, Conduction aphasia, Global aphasia and Nominal aphasia. According to the Wernicke-Geschwind model of language (represented in Figure 1) the following seven areas of the left hemisphere mediate language-related activities (Pinel, 2003).

The primary auditory cortex (1) mediates hearing the spoken word. The primary visual cortex (2) mediates seeing the written word. The mouth and throat area of the primary motor cortex (3) mediates the motor responses of speech. Wernicke’s area (4), an area in the left temporal lobe just posterior to primary auditory cortex, mediates comprehension of spoken language. The left angular gyrus (5), the parietal lobe gyrus located on its border with the temporal lobe, translates the image of the written word into an auditory code, and passes it on to Wernicke’s area for comprehension.

Broca’s area (6), an area of the left frontal lobe just anterior to the mouth area of the primary motor cortex, stores programs of speech production and produces speech by activating the adjoining primary motor cortex. And, finally the arcuate fasciculus (7), a major tract that connects Wernicke’s area with Broca’s area, enables the Wernicke comprehension centre to activate speech programs in Broca’s area. According to this model, after a spoken word is processed in the auditory pathways and the auditory signals reach Wernicke’s area, the word’s meaning is evoked when brain structures beyond Wernicke’s area are activated.

Similarly, nonverbal meanings are converted into acoustic images in Wernicke’s area and turned into vocalizations after such images are transferred by the arcuate fasciculus into Broca’s area. Moreover, when reading aloud, the primary visual system receives the visual signal and is conducted to the angular gyrus of the left hemisphere, where it is translated into an auditory code and conducted to Wernicke’s area for comprehension. Wernicke’s area then activates, via the left arcuate fasciculus, the appropriate programs of speech in Broca’s area, and these produce speech by driving the mouth area of the primary motor cortex.

The present essay only considers Broca’s aphasia, which is also called as expressive aphasia, and Wernicke’s aphasia, which is also called as receptive aphasia. Broca’s aphasia is considered to be caused by a large frontal lobe lesion, mostly in Broca’s area. Although this type of aphasia is not a single entity, Brown (1972) characterized Broca’s aphasia by non-fluent, effortful and agrammatic speech, an incapacity for repetition with a largely (but not completely) preserved comprehension for single words.

Wernicke’s aphasia is considered to be caused by damage to left temporal lobe structures, mainly to Wernicke’s area. Brown (1972) describes the speech of these patients as fluent, abundant, well articulated and melodic but lacking of linguistic message, and often used self-created new, meaningless words. These patients’ linguistic comprehension is impaired as well as their capacity for repetition. The Wernicke-Geschwind (1965) model describes Broca’s area, Wernicke’s area, and the arcuate fasciculus as the key structures involved in language.

If this theory is right than Wernicke’s area controls all language comprehension, Broca’s area controls all language production, and the transmission of information between these areas is facilitated by the arcuate fasciculus than all patients with Broca’s aphasia have lesions in Broca’s area, all patients with Wernicke’s aphasia have lesions in Wernicke’s area, and all patients with conduction aphasia have lesions in the arcuate fasciculus.

The development of the neuroimaging techniques, which are used to define brain function, determine the severity of brain damage and predict the severity of the aphasia, is playing a major role n the review of the Wernicke-Geschwind (1965) model of cortical localization of language. New data provided by more advanced techniques, such as the positron emission tomography (PET), functional magnetic resonance imaging (fMRI) or event-related electrical potentials (ERP) did not always supported the traditional model of brain-language relationships (Basso, (2000).

In recent studies correlating the speech and language deficits of aphasic patients with the sites of their lesions was found that the traditional model does not adequately explain the data (Dronkers, Redfern, ; Knight, 1999). Petersen et al. (1988) used positron emission tomography (PET) to map the areas of the cortex that are active in the brains while engaging in language-related activities. In this method, an individual is injected with radioactive glucose into their blood stream and monitored for levels of radioactive glucose consumption throughout the brain region (Pinel, 2003).

Physicians use PET scans to determine the extremity of the disorder and also to define brain functions. In Petersen et al. ‘s study the participants were asked to stare at the screen showing either a blank display, printed nouns, or were asked to read the printed nouns aloud, or respond to the nouns with related verbs (e. g. , cake: to eat). Looking at the nouns produced bilateral activity in primary visual cortex that was not present when the participants merely stared at a blank screen.

Reading printed nouns aloud produced additional bilateral activity in primary motor cortex, primary somatosensory cortex, primary auditory cortex, and medial frontal cortex. Finally, responding to a printed noun by saying a related verb produced additional activity in the lateral prefrontal cortex of the left hemisphere just in front of Broca’s area and in the medial prefrontal cortex of both hemispheres. These results challenged the Wernicke-Geschwind (1965) model in several aspects.

Firstly, each of the 3 experimental conditions (2, 3 and 4) added activity to both hemispheres, not just to the left; secondly, none of the three conditions added activity to Wernicke’s area, Broca’s area or the angular gyrus; and finally, areas of cortex not included in the Wernicke-Geschwind (1965) model were activated (e. g. , the medial cortex). In another study Bavelier et al. (1997) used the non-invasive functional magnetic resonance imaging (fMRI) to measure the brain activity of healthy adults while they read sentences silently in order to measure the extent of cortical involvement in reading.

The fMRI is used to produce images of the increase in oxygen flow in the blood to active areas of the brain (Pinel, 2003). Functional neuroimaging are used to clarify what the traditionally referred brain areas of language processing do and define new areas that also contribute to language processing. In the Bavelier et al. (1997) study the participants were asked to read the sentences that were showed on a screen and were interposed by controlled periods, during which rings of consonants were presented.

The differences in activity during the reading and control periods served as the basis for calculating the areas of cortical activity associated with reading. Three important points emerged from this analysis. Firstly, the areas of activity were patchy; they were tiny areas of activity separated by areas of inactivity. Secondly, the patches of activity were variable; the areas of activity differed from subject to subject and even from trial to trial in the same subject. Finally, although some activity was observed in the classic Wernicke-Geschwind areas, it was wide spread over the lateral surfaces of the brain.

The widespread, spotty activity over the left cortex have been found in most of the recent cognitive neuroscientific studies and also in previous research using brain stimulation methods. The electrical stimulation procedure was used to generate and record speech deficits caused by stimulating of various specific points on the cortical surface of conscious patients during neurosurgery. Ojeman et al. (Journal of Neurosurgery, 1989) assessed the ability of electrical stimulation to disrupt the naming of common objects in 117 neurosurgery patients and found that many patients had no active sites at all in the classic Wernicke and Broca areas.

Most of the active sites were located in posterior frontal cortex, inferior parietal cortex, and superior temporal cortex. Two important findings emerged; firstly, the cortical tissue that performs a particular language function is not distributed uniformly throughout a particular area of cortex; the tissue that performs a particular language function is localized in islands of tissues that are scattered throughout a large area. Secondly, the area of cortex that participates in a particular language function varies greatly from subject to subjects.

In a recent study Dronkers et al. (1999) correlated the speech and language deficits of aphasic patients with the sites of their lesions and they found that many patients with Broca’s and Wernicke’s aphasia did not have the expected lesion and many patients had lesions in these classic areas without the predicted Broca’s or Wernicke’s aphasia. Only 85% of patients with chronic Broca’s aphasia had lesions in Broca’s area, and only 50-60% of patients with lesions in Broca’s area had a persisting Broca’s aphasia.

They also found that only 65% of patients with chronic Wernicke’s aphasia had lesions in Wernicke’s area, and approximately 30% of fluent aphasic patients with lesions in Wernicke’s area had a persisting Wernicke’s aphasia. Those patients with lesions in the arcuate fasciculus have a more complete disruption in the transmission of language information to frontal lobe speech mechanisms, such that the only speech produced is single repetitive words or syllables, a far more severe deficit than the repetition deficit predicted by the Wernicke-Geschwind (1965) theory.

A modern framework proposed by Dronkers et al. (2000) was based on recent findings (e. g. , Camarazza ; Zurif 1976, Grodzinsky 1984, 1995, Avrutin 2001) and suggests that three large systems interact closely in language perception and production. (1) Together the language areas of Broca and Wernicke, selected areas of insular cortex, and the basal ganglia form one system, the language implementation system. Through its function of analyzing incoming auditory signals it activates conceptual knowledge and also ensures phonemic and grammatical construction as well as articulatory control.

The implementation system is surrounded by a second system, the mediational system, made up of numerous separate regions in the temporal, parietal, and frontal association cortices. The mediational regions act as third-party brokers between the implementation system and the conceptual system. Finally, the conceptual system is a collection of regions distributed throughout the remainder of higher-order association cortices, which support conceptual knowledge.

Despite Wernicke’s and Geschwind’s important contribution to the field, the main criticism of the Wernicke-Geschwind (1965) model of language, is that they based the cortical localization of language on the analysis of neurological patients with diffuse and poorly defined brain damage. Recent studies using modern brain scanning techniques for mapping, which ables close evaluation of the brain-language relationship show several inconsistency with the predictions of the Wernicke-Geschwind (1965) model.

In contrast to the methods and instruments that were available to Wernicke and Geschwind, recently the localization data is drawn from studies that are carefully controlled and include only patients without previous neurological or psychiatric complications and whose handedness and language histories are also documented. These patients participate in extensive language and behavioural testing to determine their deficits and concurrently undergo structural neuroimaging to determine the precise extent of the lesion (Dronkers, 1996).

As a result, theories can now see beyond those traditional relationships and begin to use the new methods available to refine these relationships, discover new ones, and consider how they might interact to produce language. On the same line with the advantages of the modern brain scanning methods, their limitations also need to be considered when considering their results (Pinel, 2003). Both PET and fMRI have been considered as producing reasonably good spatial resolution.

PET provides good functional information, whilst fMRI provides both structural and functional information in the same image, but both of them offers poor temporal resolution. Both methodologies are based on the notion that increases in the neural activity in one particular brain region is followed by increases of the blood flow to that region. The increase in blood flow is delayed a few seconds from the initial increase in neural activity.

Because of this delay, and because of the relatively slow rise and fall of the change in blood flow, it takes time (several seconds) to collect enough information to form a single image, whereas neural events occur in milliseconds. Therefore the temporal resolution of neuroimaging studies is somewhat still limited, the images only reflect the actual neural responds with approximate accuracy. In summary, it can be said that aphasia is a disturbance of higher neuropsychological functions and can co-occur with other sets of symptoms.

The phenomenom that in aphasic patients other cognitive skills than language remain functioning is affirming that language faculty is a separate domain. Based on recent studies that employed modern brain scanning techniques for mapping, evaluated brain-language relationships and showed inconsistency with some of the predictions of the Wernicke-Geschwind model indicates that Broca’s and Wernicke’s areas are not the only important language areas. Instead, they are regarded to be part of a more rich and complex system that also involves the non-dominant hemisphere.