Conduction Aphasia

Aphasia describes a disturbance of language function resulting from brain injury, typically an ischemic stroke. Traditionally, clinicians characterize aphasia as either a Broca aphasia—a so-called motor or expressive aphasia, with decreased verbal fluency—or Wernicke aphasia—a so-called sensory or receptive aphasia, with decreased comprehension—depending on symptoms and the location of the underlying brain lesion. Conduction aphasia is a much rarer aphasia wherein both expression and comprehension are relatively preserved, but the patient demonstrates phonological sequencing errors, especially when repeating polysyllabic sentences. Affected individuals often struggle to repeat nonwords, are prone to phonemic paraphasic errors and neologisms, have difficulty naming objects, and may also show some features of Wernicke aphasia, albeit usually mild.  Carl Wernicke first proposed in the mid-1870s that a disconnection between the 2 language systems in the para-Sylvian dominant hemisphere (Broca's area in the frontal lobe and Wernicke's area in the posterior region of the superior temporal gyrus) leads to this unique condition. During the first half of the 20th century, Ludwig Lichtheim and Norman Geschwind expanded on Wernicke's work.  In the Lichtheim-Geschwind model, conduction aphasia arises from a lesion in the arcuate fasciculus, a white matter fiber tract that connects the two language centers.  More recent work suggests that the classical model is insufficient. Functional MRI (fMRI) and diffusion tensor imaging (DTI) studies have identified gray matter lesions, now termed the area Spt, between the superior-posterior temporal and inferior parietal lobe in patients demonstrating classical conduction aphasia. Due to these findings, current aphasia models now emphasize that language processing occurs in interdependent dual "dorsal" and "ventral" pathways. The dorsal pathway mediates phonological information, and the ventral pathway mediates semantic information. The area Spt may serve as a gray matter hub. According to this model, conduction aphasia likely represents a parietotemporal disconnection syndrome impacting cortical area Spt and its associated white matter circuits of the frontal lobe, as opposed to being purely a disconnection of the arcuate fasciculus.