Tuesday, December 15, 2015

Wernicke's area ubi es?

Binder JR. The Wernicke area: Modern evidence and a reinterpretation.
Neurology. 2015 Nov 13. pii: 10.1212/WNL.0000000000002219. [Epub ahead of print]


The term "Wernicke's area" is most often used as an anatomical label for the gyri forming the lower posterior left sylvian fissure. Although traditionally this region was held to support language comprehension, modern imaging and neuropsychological studies converge on the conclusion that this region plays a much larger role in speech production. This evidence is briefly reviewed, and a simple schematic model of posterior cortical language processing is described. 

From the article:

In an influential 1976 article called “Wernicke's region: Where is it?” Bogen and Bogen 1 defined the Wernicke area (commonly known as Wernicke's area) unequivocally as “the area where a lesion will cause language comprehension deficit.” They reviewed the large literature on this topic, emphasizing how anatomical evidence up to that time had variously implicated the left posterior superior temporal gyrus (pSTG), supramarginal gyrus (SMG), middle temporal gyrus (MTG), angular gyrus (AG), and inferior temporal gyrus in language comprehension. Like many authors before them, Bogen and Bogen concluded that language comprehension is not highly localized, but involves large regions of the left temporal and inferior parietal lobe.
In contrast, the widely accepted definition of the Wernicke area has in recent decades become an anatomical rather than a functional one. Rather than “the area where language comprehension occurs,” the Wernicke area has come to be synonymous with the left pSTG and SMG, i.e., the cortices that surround the left posterior sylvian fissure...

But does this de facto anatomically defined Wernicke area truly have the functional properties traditionally ascribed to it? As this brief review will make clear, there is now compelling evidence, and a general consensus among language researchers, that the region currently labeled the Wernicke area plays little or no role in language comprehension. This new understanding should motivate a revision of standard teaching at medical schools and neurology residency programs. Practical benefits will include a better understanding of the vascular and degenerative fluent aphasia syndromes, and improved understanding and application of clinical brain mapping data...

Although the end product of speech production is a series of muscle movements, the brain mechanisms involved in speech production should not be seen as limited to motor commands that move muscles. Before such commands can be sent, the speaker must momentarily activate knowledge about the sequence of consonant and vowel speech sounds (phonemes) that form the word to be spoken. This mental stage prior to articulation is known as phonologic retrieval. Its existence can be demonstrated by the fact that one knows that the word “snow” rhymes with “blow” but not with “plow” without needing to say these words aloud. In the jargon of language scientists, this knowledge reflects activation of a phonologic “representation” or mental image of the sounds comprising the words. Partial disruption of this phonologic retrieval process causes a speech production impairment called phonemic paraphasia, in which the phonemes of the spoken word are chosen incorrectly or are incorrectly ordered. Phonemic paraphasia is a cardinal feature of both Wernicke aphasia and conduction aphasia. Although these are fluent aphasias because there is no slowing of overall word output, the paraphasic component is nevertheless a deficit of speech production, not speech comprehension. Thus Wernicke aphasia, though often thought of as a syndrome affecting comprehension, also includes a prominent speech production impairment.

Functional neuroimaging methods, including functional MRI, PET, and magnetoencephalography, have provided compelling and consistent evidence that the Wernicke area is involved in phonologic retrieval in healthy participants...

As compelling as the evidence in favor of a role for the Wernicke area in speech production is the evidence against a role in speech comprehension. By definition, patients with conduction aphasia and patients with lvPPA have relatively intact word comprehension; therefore, if the lesions associated with these syndromes are centered in the Wernicke area, it follows that lesions in the Wernicke area do not as a rule impair comprehension...

Speech comprehension is best viewed as involving at least 2 distinct processing stages. The first of these is a sensory process that analyzes the auditory input for phoneme content, independent of word meaning. A wide range of evidence suggests that this “phoneme perception” process involves high-level auditory areas in the STG and adjacent superior temporal sulcus in both hemispheres. The STG regions responsible for this process are anterior to those involved in speech production, and anterior to the classical Wernicke area...

The second distinct processing stage in speech comprehension is the retrieval of semantic information, or meaning, associated with the input. A meta-analysis of 120 neuroimaging studies on this topic identified a large network of brain regions involved in semantic processing, including AG, MTG, ventral temporal lobe, medial parietal cortex, medial prefrontal cortex, and inferior lateral prefrontal regions. These results closely mirror the previously mentioned lesion data. Together, the data converge on the conclusion that many cortical regions support speech comprehension, whereas the classical Wernicke area is one of the few brain regions that does not.

If the posterior perisylvian region now labeled the Wernicke area does not support the main function traditionally ascribed to it (i.e., speech comprehension), one possible course of action is to apply the Wernicke area label instead to those regions that do support speech comprehension. The main problem with this approach is that speech comprehension is a highly distributed function, involving a bihemispheric phoneme perception system and a widely distributed semantic network. To refer to all of these regions as the Wernicke area seems to sacrifice any utility that the term might have, and furthermore these other brain networks were never the focus of Wernicke's claims. Given the pervasive application of the Wernicke area label to the posterior perisylvian region, which seems unlikely to change, and the fact that damage in this location produces one component of Wernicke aphasia (i.e., paraphasic production), a wiser course might be to retain the label while keeping in mind the true function of this brain region.

Courtesy of a colleague

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