Pure Word Deafness (PWD) is an extremely rare disorder, usually caused by bilateral temporal damage. In a small but sizeable number of cases, it follows unilateral, left temporal lesions. In these subjects, it is attributed to strategically placed cortical/subcortical damage, disconnecting Wernicke’s area from the primary auditory area in the left hemisphere and from its right-hemisphere homologue. Reversal of the Right-Ear Advantage (REA) in dichotic listening tasks is considered as the functional signature of such disconnection.
In two unreported cases, PWD followed an isolated left temporal lesion, that disrupted input speech processing but spared the processing of environmental noise and music. Both subjects showed a marked REA reversal. Tractography and spherical deconvolution algorithms showed incomplete damage to the primary auditory cortex, and to interhemispheric connections between primary and associative auditory cortices. Therefore, an essentially complete functional disconnection resulted from an incomplete anatomical disconnection. Possible mechanisms underlying PWD in these and other unilateral cases are discussed.
In addition, both patients benefited from lip reading during conversation. This phenomenon, frequently reported on in PWD, reflects audio-visual integration, that allows PWD patients to use visual information to integrate degraded auditory speech input. The same mechanism is thought to underlie the McGurk effect, in which healthy subjects exposed to contrasting visual and auditory linguistic information (eg, seeing /ba/ and hearing /ga/) perceive a non-existing sound (eg, /da/). Recent neuroimaging studies led to suggest that this phenomenon is mediated by the Middle Longitudinal Fasciculus (MdLF). Both patients presented with a strong McGurk effect (much stronger than observed in healthy volunteers). Yet, MdLF was damaged in both cases (severely in one, moderately in the other). This observation suggests that MdLF may not be critical for audiovisual integration.
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