Where is Wernicke's area?
Although the location of Wernicke's area is often presented in images and texts as definitive, there is some controversy about the exact location of the region. Typically, however, Wernicke's area is considered to reside in the cortex of the left cerebral hemisphere, surrounding a large groove called the lateral sulcus or Sylvian fissure, near the junction between the parietal and temporal lobes.
What is Wernicke's area and what does it do?
In the second half of the 19th century, neuroscientists were trying to come to grips with a new perspective on the brain that suggested that the two cerebral hemispheres were not completely equivalent in terms of function. The most convincing evidence to support this perspective at that point had been offered up by the famous physician Paul Broca, who had identified a number of cases where damage to the left hemisphere produced deficits in language---whereas damage to the right hemisphere was much less likely to do so. These observations coincided with Broca's identification of what would come to be known as Broca's area---a brain region typically found in the left hemisphere that is thought to be important to the production of speech (see this article for more about Broca's area).
This idea that one hemisphere could be more responsible for a behavior than the other---and in this case that the left hemisphere was dominant when it came to language---was mostly foreign to neuroscientists before Broca (although not completely unheard of as it had previously been proposed by the physician Marc Dax). Many were hesitant to accept it.
Left hemispheric dominance for language got some additional support, however, from the German physician Carl Wernicke in 1874. Wernicke reported that damage to a certain region in the left hemisphere often resulted in a speech deficit where patients were able to produce speech sounds that resembled fluent language, but actually were meaningless. These patients would string together incongruous syllables, neologisms, similar-sounding words substituted for one another, and so on, to produce speech that made little sense. Patients with this disorder, which would come to be known as Wernicke's aphasia, usually also suffer from a deficiency in their ability to understand language. You can see an example of Wernicke's aphasia in the video above.
Wernicke's aphasia contrasted with the syndrome Broca had observed after damage to Broca's area (that syndrome is known as Broca's aphasia). Patients with Broca's aphasia generally have difficulty producing the sounds necessary for speech. Often a patient with Broca's aphasia knows what he or she wants to say, but can't get the words out. Comprehension of language generally remains intact. You can see an example of Broca's aphasia in the video to the right.
Because Wernicke's area seemed to play an important role in language comprehension and the production of language that was intelligible, Wernicke proposed a model for language that involved both his area and Broca's area. Wernicke's area, according to this model, generates plans for meaningful speech. Broca's area, on the other hand, is responsible for taking these plans and generating the movements (e.g. of the tongue, mouth, etc.) required to turn them into vocalizations. To do so, Broca's area sends information about intended speech to the motor cortex, which then signals the muscles involved in speech production to create the vocalizations. Thus, according to this view, Wernicke's area makes sure that language makes sense, while Broca's area helps bring about the muscle movements necessary to actually produce the sounds.This model was later expanded upon by neurologist Norman Geschwind, and it eventually became known as the Wernicke-Geschwind model.
It is now thought, however, that this model is too simplistic. Language is a complex behavior made possible by a list of individual functions---ranging from the retrieval of particular phonemes to the adding of intonation and rhythm---that each likely involves widespread networks; it cannot simply be boiled down to a connection between two brain regions. Additionally, later studies have found that the functions of Broca's and Wernicke's areas are not as circumscribed as once thought. For example, Wernicke's area seems to play a role in speech production and Broca's area contributes to language comprehension. And damage to what is considered Wernicke's area does not always disrupt comprehension, which suggests Wernicke's area is just one component in a larger network involved in understanding language.
Wernicke's area is thus not as anatomically well defined nor functionally well understood as many textbooks would lead you to think. It is thought to be important to language, but researchers are still trying to work out exactly what its role is. It's likely that it functions as part of a larger network, which---when fully understood---might allow us to appreciate the network as the important functional unit for language, rather than focusing so much on the individual brain regions that make up the network.
References (in addition to linked text above):
Binder, JR. The Wernicke area: Modern evidence and a reinterpretation. Neurology. 2015; 85(24): 2170-2175.
Breedlove SM, Watson NV. Biological Psychology. 7th ed. Sunderland, MA: Sinauer Associates, Inc.; 2013.