Anatomical evidence of an indirect pathway for word repetition.
The classical anatomical locus of lesion for disorders of repetition in the traditional Wernicke-Lichtheim model of aphasia is the arcuate fasciculus.
This anatomo-clinical correlation, largely based on an a priori hypothesis formulated by Lichtheim (rather than by Wernicke) has been only partially supported by large-scale correlation studies in stroke patients in which damage to perisylvian cortical regions (rather than purely subcortical involvement) has often been reported.
This study (based on a multimodal-MRI study of a large sample of patients with a diagnosis of primary progressive aphasia) reports a relatively unexpected result, which may actually reconcile the mixed results from vascular aphasia. Repetition impairment in these patients correlated with atrophy in left inferior frontal, inferior parietal, and superior temporal regions, and with volumetric reduction of the connecting fiber tracts (i.e., to the indirect pathway connecting anterior and posterior language areas).
In contrast, the volume of the classical arcuate fasciculus was not related to repetition impairment. The existence of multiple pathways for repetition has been postulated on the basis of some case studies, which indicate that different mechanisms may be at stake (i.e., working memory VS sequencing abilities).
The present findings indicate the need for further investigations, and address the differential role of the indirect and direct connections in the ability to repeat – a crucial requirement for the development of phonological abilities and for reading acquisition.
Cappa S: F1000Prime Recommendation of [Forkel SJ et al., Neurology 2020]. In F1000Prime, https://f1000.com/prime/737290765
Full article: https://n.neurology.org/content/94/6/e594.full?fbclid=IwAR1ae_Licb5p4-sAX-cO8QOPyBf_uimjk243IZHDi8ZJpt5wguvgPMhL7r8