Sunday, January 3, 2016

Wernicke area

Binder JR.  The Wernicke area; modern evidence and interpretation. Neurology; 2015;85:2170-75.
Blinder updates the concept of Wernicke area.  It currently is defined as pSTG and SMG, or posterior part of Brodman area 22 and area 40. 
Wernicke area is crucial for speech production, through its primary function of phonemic retrieval, but is not at all involved in speech comprehension, a reversal of the traditional notion.  Neuropsychologically, phonemic retrieval is demonstrated by visual rhyming judgment paradigms of testing.  This notion is supported by studies using fMRI, PET and MEEG studies as well as electrical stimulation studies.  Also in logopenic variant primary progressive aphasia, functional imaging shows involvement of the posterior STG and SMG.  
Re speech comprehension, lesions to Wernicke's area may cause conduction aphasia much more than comprehension disorders.  The latter are more likely to be related to lesions in medial temporal gyrus, angular gyrus, anterior STG,and areas in frontal lobes.  Comprehension is a two stage process, with the initial stage involving phonemic perception, which involves high level auditory areas in STG and superior temporal sulcus in both hemispheres.  These areas are anterior to Wernicke area. Bilateral lesions of these areas cause pure word deafness, which is rare. 
The second stage of speech comprehension,which involves a semantic network, includes a broader network of structures including AG, MTG, ventral temporal lobe, medial parietal, medial prefrontal,  and inferior lateral prefrontal. 
Lesions of Wernicke area would be expected to effect all aspects of speech production,including repetition, naming, reading aloud.  Speech repetition and reading aloud require input from the more anterior phoneme perception system (in the case of repetition) or letter/word perception system into the phoneme retrieval area (Wernicke area).  Spontaneous speech requires input from semantic areas into phoneme retrieval system.  A pure Wernicke area lesion would cause anomia or phonemic paraphasias.  This is seen, again, in lvppa and conduction aphasia,  although to a greater extent in lvppa.  A lesion of semantic areas would cause transcortical sensory aphasia.  Wernicke's aphasia would be caused by a lesion of phonologic retrieval (Wernicke area) plus semantic areas that are far larger.