Broca vs. Wernicke’s aphasia – a double dissociation

“Explain the concept of a double-dissociation using the contrast between Broca’s and Wernicke’s aphasia”

An important concept in neuropsychology is the notion of double-dissociation. The purpose of this essay is to discuss this topic in relation to language disorders, specifically Broca’s and Wernicke’s aphasia. The hypothesis of double-dissociation within language suggests that the two language aphasias are separable and can occur independently of each other. This hypothesis will be addressed with both archival and more recent journal articles and experimental evidence.

Aphasia is defined as a loss or impairment of language function caused by damaged to language and association areas of the brain. There are a number of possible causes including a stroke, tumor, head trauma, toxic conditions, and degenerative disease such as Parkinson’s and Alzheimer’s. The two syndromes relevant to this essay are Broca’s and Wernicke’s aphasia.

In France in 1871 Paul Broca’s first patient was a man known as ‘Tan’ because that was the only syllable he was able to utter. Broca found that the patient appeared to have normal language comprehension and could indeed use hand gestures to communicate. Since the patient didn’t show serious cognitive impairment it suggested that damage to a specific brain region could be causing these problems. At autopsy Broca discovered there to be a large lesion in an area of the left frontal lobe, this area is now known “Broca’s area”.

Subsequent studies of Broca appeared to show that the center for speech production is localised in Broca’s area in the left prefrontal cortex. The language deficit caused by damage to Broca’s area is now called Broca’s aphasia. Broca’s aphasic’s speech is slow, laboured, halting, and lacking many function words. Speech is “telegraphic” although comprehension is generally good. The fragmented results produced with great effort give rise to the other names for Broca’s aphasia, such as “non-fluent aphasia” or ‘expressive aphasia’. We also know that Broca’s area is responsible for structure, planning and organization of speech, it is located next to the articulation centers on the motor cortex. Broca’s area is responsible for the phonemic representations used for speech production. Since in Broca’s aphasic’s comprehension is good and production is poor then these processes must be considered separable.

Subsequent to Broca’s localisation of language production, Karl Wernicke was able to determine the area of the brain responsible for language comprehension (1874). He found that many patients with intact left frontal lobe still had language problems, however they were different to those involved in Broca’s studies. Wernicke’s patients demonstrated other types of language problems, their speech is rapid, fairly well formed, but empty of meaning, neologisms are very common and comprehension is severely impaired. Wernicke found that the area of the brain damaged was located in the left parietal cortex. Wernicke’s area is therefore located in the temporal lobe (rather than the frontal lobe), between the primary auditory cortex and the angular gyrus. The semi-nonsensical babbled stream of Wernicke’s aphasia is also known as “fluent aphasia”. Wernicke’s area is responsible for connecting symbols to their referents and for the access and manipulation of words, as it is located behind the auditory projection zone.

Wernicke believed that Broca’s area was responsible for production and Wernicke’s area for comprehension. Similarly that Broca’s Area is the syntax module and Wernicke’s area is the semantics module.

Evidence that supports this theory is provided when we look at what happens when a word is said and asked to be repeated. Ludwig Lichtheim (1885) proposed a theory used for the understanding of normal course of speech, in which the process is rendered as a reflex arc. First of all, it travels from the primary auditory cortex to Wernicke’s area where meaning is accessed, then sent along pathways to Broca’s area where the planning for production takes place. The motor cortex then sends signals to the articulators and the word is produced. Modern day functional neuroimaging techniques have been used extensively to study this model, for example Posner & Raichle (1994) and their PET study of word processing. In addition the use of event-related Potentials (ERPs) and EEG technology to record brain waves temporarily linked to the processing of stimuli. Event-related brain potentials (ERPs) have recently been used to demonstrate that Broca’s and Wernicke’s aphasics show a double-dissociation in relation to ERP components that reflect either lexical/semantic processing or syntactic processing. (Friederici, Hahne, & von Cra-mon, 1998). Other techniques such as function magnetic resonance imaging (fMRI) have also been extensively used in this area.

The basic concept used in studies attempting to highlight a double dissociation within language is when a patient can perform task A but has difficulty with task B and vice versa. To show that 2 functions are found in separate areas of the brain, you need 2 cases one where A is fine, B is impaired another where B is fine, A impaired. This shows that the two functions are independent. With this in mind Caramazza & Zurif (1976) used the idea of comprehension versus production, since Broca’s area is the production module and Wernicke’s area is the comprehension module. They used a sentence or picture matching system to show that reversible sentences are difficult to understand for Broca’s aphasics. This illustrates that the components of comprehension and production are separable. Similarly, Bradley, Garrett, & Zurif (1980) applied the notion of syntax versus semantics within language and found that when presented with a lexical decision task Broca’s aphasics failed to dissociate between open and closed class words. This study appeared to show that word retrieval has two components, specifying the meaning and retrieving the sounds. This showed a double dissociation between generation of syntax and the access of grammatical elements against retrieval of content words. It was therefore argued that content and function words are therefore processed differently.

Further studies on this topic include, Consecutive ERP effects of morpho-phonology

And morpho-syntax (Karsten Steinhauer and Michael T. Ullman 2002). Steinhauer and Ullman found, unlike Broca’s aphasic individuals, those individuals most likely characterized as Wernicke’s syndrome type appear to be insensitive to the argument structure properties of verbs, even where on-line comprehension is at issue. Yet,

Their deficit does not seem to affect on-line comprehension of sentences with moved arguments. (Shapiro et al, 1993; Russo, Peach, & Shapiro, 1998).” Shapiro argued that this study therefore suggests “a double dissociation between the activation

Of argument structures and the syntactic parsing routines underlying the comprehension of sentences with moved arguments.”

These various studies suggest various double-dissociations within language, including among others, a dissociation between syntactic planning and word retrieval in speech production, a dissociation between words that carry the meaning and words that do the syntactic work. There is also dissociation between content words (which carry the meaning) and function words (which do the syntactic work) and other grammatical elements (e. g. word endings). The basic concept of the double-dissociation in question is when one patient loses ability A while retaining ability B whereas another patient loses ability B while retaining A. Broca’s and Wernicke’s aphasic’s provide a good example of such a double dissociation, where subjects can be observed in whom one function is impaired while another remains intact, and vice versa. Double dissociations are important neuropsychological evidence, as they allow us to make an estimate of localization of function separation with reasonable confidence. They also allow us to separate lexical, syntactic and semantic processing.

A subject that has been proposed in relation to aphasics is the question do aphasics communicate better than they speak? One way that this issue has been tackled is the “map task” (Paterson et al. 2000), a schematic map with route marked through it with landmark features and common reference points marked along the way. The route is then dictated and the amount that the subject’s route deviates from the original is measured as a degree of success. The results appeared to show that the aphasic’s route in general did not deviate too much from the original. This suggests that although their language is impaired their other cognitive functions remain intact; this shows a double-dissociation between language and visuo-spatial processing.

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