Abstract

Broca’s area in the posterior half of the left inferior frontal gyrus has long been thought to be critical for speech production. The current view is that long-term speech production outcome in patients with Broca’s area damage is best explained by the combination of damage to Broca’s area and neighbouring regions including the underlying white matter, which was also damaged in Paul Broca’s two historic cases. Here, we dissociate the effect of damage to Broca’s area from the effect of damage to surrounding areas by studying long-term speech production outcome in 134 stroke survivors with relatively circumscribed left frontal lobe lesions that spared posterior speech production areas in lateral inferior parietal and superior temporal association cortices. Collectively, these patients had varying degrees of damage to one or more of nine atlas-based grey or white matter regions: Brodmann areas 44 and 45 (together known as Broca’s area), ventral premotor cortex, primary motor cortex, insula, putamen, the anterior segment of the arcuate fasciculus, uncinate fasciculus and frontal aslant tract. Spoken picture description scores from the Comprehensive Aphasia Test were used as the outcome measure. Multiple regression analyses allowed us to tease apart the contribution of other variables influencing speech production abilities such as total lesion volume and time post-stroke. We found that, in our sample of patients with left frontal damage, long-term speech production impairments (lasting beyond 3 months post-stroke) were solely predicted by the degree of damage to white matter, directly above the insula, in the vicinity of the anterior part of the arcuate fasciculus, with no contribution from the degree of damage to Broca’s area (as confirmed with Bayesian statistics). The effect of white matter damage cannot be explained by a disconnection of Broca’s area, because speech production scores were worse after damage to the anterior arcuate fasciculus with relative sparing of Broca’s area than after damage to Broca’s area with relative sparing of the anterior arcuate fasciculus. Our findings provide evidence for three novel conclusions: (i) Broca’s area damage does not contribute to long-term speech production outcome after left frontal lobe strokes; (ii) persistent speech production impairments after damage to the anterior arcuate fasciculus cannot be explained by a disconnection of Broca’s area; and (iii) the prior association between persistent speech production impairments and Broca’s area damage can be explained by co-occurring white matter damage, above the insula, in the vicinity of the anterior part of the arcuate fasciculus.

Highlights

  • For over 150 years, clinical aphasiology and behavioural neurology have been fundamentally influenced by Paul Broca’s finding that stroke survivors with severe and persistent speech production impairments had damage to the third convolution of the left frontal lobe (Broca, 1861a, b, 1865)

  • Our analyses strongly imply that damage to arcuate fasciculus (aAF), not BA44, is critical for explaining long-lasting impairments in speech production abilities. To illustrate this finding further, we identified three groups of patients who differed in the degree of damage to BA44 versus aAF

  • We briefly review prior findings regarding the role of BA44 and BA45 in normal speech production and the evidence we have provided that the prior association between Broca’s area damage and persistent speech production impairments can be accounted for by co-occurring damage to white matter, above the insula, in the vicinity of aAF

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Summary

Introduction

For over 150 years, clinical aphasiology and behavioural neurology have been fundamentally influenced by Paul Broca’s finding that stroke survivors with severe and persistent speech production impairments had damage to the third convolution of the left frontal lobe (Broca, 1861a, b, 1865) Since this part of the brain has been known as Broca’s area. Broca’s descriptions focused on the parts of the lesions that were visible to him (primarily at the level of the cortex) without evaluating the potential contribution of neighbouring damage, for example, to the underlying white matter and surrounding cortical areas It was not until 2007 that the full extent of the lesions incurred by Broca’s two famous cases (Broca, 1861a, b) was revealed in an MRI study showing damage to multiple subcortical grey and white matter regions (Dronkers et al, 2007)

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