Flick through any neuropsychology textbook and you’ll hear about the nineteenth century pioneers Paul Broca and Carl Wernicke, who showed that language production and comprehension are subserved by two distinct brain regions, which came to be known as Broca’s and Wernicke’s area, respectively. You’ll learn too about another neurology pioneer, Norman Geschwind who described how these two regions are joined by a key connective tract – the arcuate fasciculus.
This is the “Classic Model” of the neurological basis of language function – a revolution in our understanding at the time, and hugely influential to this day. But according to a compelling new paper in Brain and Language, the Classic Model is obsolete and no longer fit for purpose. What’s more, its legacy and the continued use of its terminology is hampering progress in the field, in terms of research and medical practice.
Before publishing their paper, Pascale Tremblay and Anthony Dick last year surveyed 159 experts in the field, reached via the Neurobiology of Language Society newsletter. They asked these experts – most of them scientists, but also a few medics – whether the Classic Model is the best available theory. Only 2 per cent agreed that it was, even though the research literature is dominated by papers grounded in the model and its terminology (a literature search shows hundreds of mentions of Broca’s and Wernicke’s areas in neurobiology and neuropsychology papers in recent years).
There was also huge difference of opinion among the survey respondents about the anatomical location of Wernicke’s and Broca’s areas. “These are not innocuous terms,” write Tremblay and Dick – “they carry with them a notion of functional relevance to language, but not everyone agrees on their anatomical definition, and not everyone agrees on their function. This contributes to significant conceptual confusion …”.
Another issue is that the Classic Model tells a neat story, but it’s one that no longer fits the evidence. Modern findings show, for instance, that rather than there being one key connective tract relevant to language function, there are many, including the uncinate fasciculus, the inferior front-occipital fasciculus, the middle longitudinal fasciculus and the inferior longitudinal fasciculus. Pick up almost any neurobiology textbook and it will show two language nodes connected via a single tract, and yet “the overwhelming evidence is that multiple fiber pathways support language function in the human brain.”
And there are clearly more than two functional nodes involved – in fact we now know that language function is incredibly widely distributed through the brain, “extend[ing] far beyond ‘Broca’s’ and ‘Wernicke’s areas’,” involving areas “in the frontal, parietal, and temporal lobes, in the medial hemispheres of the brain, as well as in the basal ganglia, thalamus and cerebellum.”
Yet the continued dominance of the Classic Model means that neuropsychology and neurology students are often learning outmoded ideas, without getting up to date with the latest findings in the area. Medics too are likely to struggle to account for language-related symptoms caused by brain damage or illness in areas outside of the Classic Model, but which are relevant to language function, such as the cerebellum.
Tremblay and Dick call for a “clean break” from the Classic Model and a new approach that rejects the “language centric” perspective of the past (that saw the language system as highly specialised and clearly defined), and that embraces a more distributed perspective that recognises how much of language function is overlaid on cognitive systems that originally evolved for other purposes.
They conclude their paper: “Although the field as a whole has made tremendous progress in the past few decades, due in part to significant advances in neuroimaging and neurostimulation methods, we believe abandoning the Classic Model and the terminology of Broca’s and Wernicke’s areas would provide a catalyst for additional theoretical advancement.”
Image shows the classical Wernicke-Lichtheim-Geschwind model of the neurobiology of language, via Wikipedia