Which part of the brain is affected when speech is impacted by a stroke?

Brain Regions Affected When Speech is Impaired by Stroke

Speech impairment after stroke primarily results from damage to the left hemisphere language network, with the specific brain regions affected determining the type of speech deficit observed. The most critical areas include the arcuate fasciculus (white matter pathway), Broca's area, Wernicke's area, and surrounding temporal-parietal regions, though recent evidence challenges traditional assumptions about some of these structures.

Key Anatomical Regions and Their Speech Functions

White Matter Pathways (Most Critical)

• The arcuate fasciculus is the primary dorsal language pathway connecting temporal, parietal, and frontal language regions, and its damage is the strongest predictor of persistent speech production deficits 1, 2
• Arcuate fasciculus lesion load negatively influences speech production and classifies severe versus non-severe outcomes with 90% accuracy for naming and 96% accuracy for speech fluency 1, 2
• Damage to white matter directly above the insula, in the vicinity of the anterior arcuate fasciculus, solely predicts long-term speech production impairments lasting beyond 3 months post-stroke 3
• The volume of the left long segment of the arcuate fasciculus, combined with age, sex, and lesion size, improves prediction of language recovery at six months 2

Gray Matter Regions

Broca's Area (Posterior Inferior Frontal Gyrus):

• Traditionally considered critical for speech production, but recent high-quality evidence demonstrates that Broca's area damage does not contribute to long-term speech production outcome after left frontal lobe strokes 3
• Structural damage to Broca's area in the acute phase is associated with impaired repetition, but this effect is confounded by co-occurring white matter damage 1
• 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 3

Wernicke's Area (Posterior Superior Temporal Gyrus):

• Language comprehension is severely impaired with Wernicke's area damage, with deficits in word comprehension strongly correlated with blood flow in this region 4
• Recovery of word comprehension is associated with reperfusion of Wernicke's area 4
• Comprehension deficits are associated with damage to the ventral extreme capsule and posterior middle temporal gyrus 4

Temporal-Parietal Regions:

• The posterior middle temporal gyrus and underlying white matter are critical for language comprehension 5
• Damage to the posterior middle temporal lobe negatively affects aphasia therapy outcomes in chronic phases, particularly when lesions are proximal to the hippocampus 1, 6
• The inferior portion of the left supramarginal gyrus and temporal-parietal junction show tissue dysfunction (hypoperfusion and frank damage) associated with impaired repetition 1
• Recovery of naming in the hyperacute stroke period is predicted by reperfusion of the left posterior middle temporal/fusiform gyrus 6

Frontal Regions:

• The middle frontal gyrus (Brodmann's area 46) and inferior frontal gyrus (Brodmann's area 47) affect language comprehension at the sentence level 5
• Treatment-induced naming improvements correlate with fMRI activity in the middle frontal gyrus and pars opercularis 1

Type-Specific Speech Deficits by Location

Repetition Deficits

• Impaired repetition in the acute phase is associated with structural damage to the arcuate fasciculus, Broca's area, and the inferior left supramarginal gyrus 1
• Posterior temporal-parietal lesions with damage to the dorsal superior longitudinal and arcuate fasciculus produce repetition deficits 1, 2

Comprehension Deficits

• Comprehension deficits correlate with ventral extreme capsule fiber damage, not primarily arcuate fasciculus lesions 2
• The posterior middle temporal gyrus may be more important for comprehension at the word level, while other regions play a greater role at the sentence level 5

Production and Naming Deficits

• Speech production impairments after damage to the anterior arcuate fasciculus cannot be explained by disconnection of Broca's area 3
• The distributed nature of naming deficits reflects the complexity of lexical processing networks involving multiple brain regions 6

Clinical Pitfalls and Caveats

Common Misconception About Broca's Area:

• Clinicians should recognize that persistent speech production deficits after frontal stroke are due to white matter damage (anterior arcuate fasciculus), not Broca's area damage itself 3
• Speech production scores are 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 3

Right Hemisphere Contributions:

• Right hemisphere strokes can impair speech comprehension through damage to the right inferior frontal sulcus and dorsal superior longitudinal fasciculus, affecting working memory capacity needed for sentence comprehension 7
• The detrimental effect of right hemisphere strokes on language is much greater than traditionally expected and frequently observed after damage to the right inferior frontal sulcus 7

Network-Based Understanding:

• Language operations are distributed across multiple brain networks (default mode network, frontotemporo-parietal, and cingulo-opercular networks) rather than localized to single regions 1
• Interhemispheric connectivity patterns, particularly in the superior temporal gyrus, correlate with aphasia severity 1