August 30, 2010

The neuroanatomy of language

Posted in Language and Myth tagged , , at 11:24 pm by Jeremy

What parts of the brain are responsible for language?  Most people up to speed on the subject would argue for Broca’s area and Wernicke’s area.  But it’s really the prefrontal cortex and its symbolizing capability that’s responsible for our language capability.  Here’s a section of my book draft, Finding the Li: Towards a Democracy of Consciousness, that explains in more detail.

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The neuroanatomy of language

Considering the crucial importance of the pfc in enabling symbolic thought, it has been relatively ignored until recently as a major anatomical component of our capability for language.  Traditionally, when researchers studied the anatomical evolution of language, they focused attention not just on the brain’s capacity but also on our descended larynx, which was thought to be a unique feature of the human vocal tract.  However, recent studies have shown that a number of other species, including dogs barking, lower their larynx during vocalization, and some mammals even have a permanently descended larynx.  An even more powerful argument against the descended larynx as a prerequisite of language is that infants born deaf can learn American Sign Language with as much speed and fluency as hearing children learn spoken language.  There’s seems little doubt that the human larynx co-evolved with our language capacity to enable our fine, subtle distinctions in speech sounds, but it doesn’t seem to have been required for language development.[1] In the words of Merlin Donald, “it is the brain, not the vocal cords, that matters most.”[2]

Broca discovered a crucial area relating to language in the late 19th century

Even within the brain itself, the pfc hasn’t had much press in relation to language.  In the late nineteenth century, two European physicians named Paul Broca and Carl Wernicke focused attention on two different regions in the left hemisphere of the cerebral cortex – now named appropriately enough Broca’s area and Wernicke’s area – as the parts of the brain that control language.  They made their discoveries primarily through observing patients who had suffered physical damage to their brains in these regions and had lost their ability to speak normally (known as aphasia.)  For over a hundred years, it has become generally accepted that these two areas are the “language centers” of the brain.[3] Equally importantly, both of these areas were noticed to be on the left side of the brain, and in recent decades neuroanatomical research has shown that the left hemisphere is generally the one most used for sequential processing, for creating “a narrative and explanation for our actions,” for acting as our “interpreter.”[4]

However, although Broca’s and Wernicke’s areas have long been viewed as unique to humans, recent research has shown them also to be active in other primates.  In one study, for example, the brains of three chimpanzees were scanned as they gestured and called to a person requesting food that was out their reach.  As they did so, the chimps showed activation in the brain region that corresponds to Broca’s area in humans.[5] Terrence Deacon believes that, rather than view these areas as “language centers” controlling our ability to speak, we should rather think of language as using a network of different processes in the brain.  Broca’s area is adjacent to the part of the brain that controls our mouth, tongue and larynx; and Wernicke’s area is adjacent to our auditory cortex.  Therefore, these areas likely evolved as key nodes in the language network of the brain, which would explain the aphasia resulting from damage to them.  “Broca’s and Wernicke’s areas,” Deacon explains, “represent what might be visualized as bottlenecks for information flow during language processing; weak links in a chain of processes.”[6] Neuroscientist Jean-Pierre Changeux agrees, arguing that “efficient communication of contextualized knowledge involves the concerted activity of many more cortical areas than the ‘language areas’ identified by Broca and Wernicke.”[7]

Deacon also warns against reading too much into left hemisphere specialization, known as lateralization.  He sees lateralization as “probably a consequence and not a cause or even precondition for language evolution,” pointing out that several other mammals, including other primates, also show lateralization, and that even in humans, nearly 10 percent of people are “not left-lateralized in this way.”  Lateralization, in his view, “is more an adaptation of the brain to language than an adaptation of the brain for language.”[8]

So, if it’s not the larynx, not Broca’s and Wernicke’s areas, and not lateralization, is there anything about the human anatomy that makes it uniquely capable of creating language?  No prizes for guessing that the answer may be the pfc.  As Deacon puts it, “two of the most central features of the human language adaptation” are “the ability to speak and the ability to learn symbolic associations.”[9] We’ve already noted that skilled vocalizations are a helpful, but not a necessary, part of our language capability.  So that leaves “the symbol-learning problem,” which “can be traced to the expansion of the prefrontal cortical region, and the preeminence of its projections in competition for synapses throughout the brain.”[10] Changeux once again agrees, noting that “propositions and structured speech can be seen as evolutionary phenomena accompanying the expansion of the prefrontal cortex,”[11] as does celebrated neuroscientist Joaquin Fuster who writes that “given the role of prefrontal networks in cognitive functions, it is reasonable to infer that the development of those networks underlies the development of highly integrative cognitive functions, such as language.”[12]

If the pfc was, in fact, the central driver of the emergence of language, what light (if any) does that shed on those raging debates about when and at what rate language evolved, and whether there is something that can be called a “language instinct”?  In order to answer that, we need to understand a little more about the social context in which language emerged.

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[1] For a full review of this issue, see Fitch, W. T. (2005). “The evolution of language: a comparative review.” Biology and Philosophy, 20, 193-230.

[2] Donald, M. (1991). Origins of the Modern Mind: Three Stages in the Evolution of Culture and Cognition, Cambridge, Mass.: Harvard University Press, 39.

[3] See Donald op. cit., 45-94, for a full discussion of the history of anatomical theories of human language.

[4] Gazzaniga, M. S. (2009). “Humans: the party animal.” Dædalus(Summer 2009), 21-34.

[5] Taglialatela, J. P., Russell, J. L., Schaeffer, J. A., and Hopkins, W. D. (2008). “Communicative Signaling Activates ‘Broca’s’ Homolog in Chimpanzees.” Current Biology, 18, 343-348.

[6] Deacon, op. cit., 288.

[7] Changeux, J.-P. (2002). The Physiology of Truth: Neuroscience and Human Knowledge, M. B. DeBevoise, translator, Cambridge, Mass.: Harvard University Press, 123.

[8] Deacon, op. cit., 310, 315.  Italics in original.

[9] Ibid., 220.

[10] Ibid.

[11] Changeux, op. cit., 123-4.

[12] Fuster, J. M. (2002). “Frontal lobe and cognitive development.” Journal of Neurocytology, 31(December 2002), 373-385.

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