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Wernicke's area: are we still looking for it? Was it ever lost

posted 1 Feb 2012, 08:24 by SCL Laboratory   [ updated 23 Feb 2012, 08:23 ]

Where is speech perception in the brain? In 1881 Karl Wernicke described patients with lesions in the left temporal lobe and a problem in understanding spoken language. Since this discovery, people have commonly used “Wernicke’s area” as a term to describe the neural location of speech perception. The notion of what brains areas might comprise Wernicke’s area evolved over the next century, with a consensus emerging that Wernicke’s area, and thus speech perception, was associated with to the left posterior superior temporal sulcus, at the back end of the temporal lobe (e.g. Bogen and Bogen, 1976, in their bracingly honest Wernicke’s area – where is it? A paper which essentially everyone should read ).

 

 

Coming into this area in the age of functional imaging, it was surprising to me that in contrast to the widespread view that Wernicke’s area (and thus speech perception) was in the left posterior superior temporal sulcus, we generally found the peaks of activity in the anterior temporal lobes (Mummery et al, 1999, Scott et al, 2000), often lateralized to the left. In the 2000 paper, we explicitly compared this pattern of response to the anterior ‘what’ stream of processing that the primate auditory neurophysiologists and neuroanatomists were starting to describe as being important in the processing of conspecific vocalisations by monkeys. Maybe human speech, while different from monkey calls in many ways, is processed in similar cortical fields initially?

 

However, this wasn’t the whole story of speech in the temporal lobes, and it was becoming clear that posterior auditory fields were also important in processing speech, just not necessarily with respect to the sense of the speech. Several labs described auditory areas behind primary auditory cortex, and lying nearer to the middle of the brain, which responded when people articulated – even if they were silently mouthing (so it wasn’t just activated by people hearing their voices when they speak). This ‘how’ pathway has been linked to a sensori-motor representation of sound, maybe a way of encoding whether a sound is ‘doable’ (Warren et al, 2005 ). Further down in, at the back end of the temporal lobes, nearer to the areas previously linked to “Wernicke’s area” the neural response was activated by representing heard speech when doing some cognitive work on it, e.g. holding a word in mind while thinking of synonyms. All of these areas could been seen to be activated by speech (as delineated by the territory of brain areas that have been associated with speech perception, Figure1), and careful analysis revealed that they were performing distinctly different kinds of processing on the speech. This has been linked to different streams of processing (e.g. Scott and Johnsrude, 2003). The left temporal lobe may be central to speech perception, but when we perceive speech our brains do more than one thing with the speech we hear. Wernicke’s area, therefore, covers a multitude of perceptual processes and anatomical areas. It’s a moot point whether it’s a meaningful anatomical term at all.


 

Figure 1: general outline of left hemisphere brain areas associated with speech production and perception

It’s very rewarding to me, 12 years after my original paper, that Josef Rauscheckers’ new meta analysis of functional imaging studies has confirmed this dominance of the anterior ‘what’ stream in the processing of speech for meaning, especially since he was the first to describe an auditory ‘what’ pathway with respect to monkey call processing. But does it this finding relocate Wernicke’s area? Yes, if we say that Wernicke’s area is central to understanding speech, and yes if we previously thought it was in the back of the temporal lobe. However, in a review that we wrote in 2009, Josef also went back to Wernicke’s original writings, and he determined that Wernicke did not specify the posterior temporal lobe as a focus for speech perception problems, instead implicating the length of the left STG in speech processing (Figure 2). We may only need to relocate Wernicke’s area because we misplaced it in the first place.

 

Figure 2: Wernicke's original figure, showing left superior temporal gyrus associated with speech perception deficits 

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