Left anterior temporal lobe is necessary for efficient lateralised processing of spoken word identity

In the healthy human brain, the processing of spoken words is strongly left-lateralised, while the processing of complex non-linguistic sounds recruits brain regions bilaterally. Here we asked whether the left anterior temporal lobe, strongly implicated in semantic processing, is critical to this special treatment of linguistic stimuli. Nine patients with semantic dementia (SD) and fourteen age-matched controls underwent magnetoencephalography and structural MRI. Voxel based morphometry demonstrated the stereotypical pattern of SD: severe grey matter loss restricted to the left anterior temporal lobe. During magnetoencephalography, participants listened to word sets in which identity and meaning were ambiguous until utterance completion, for example played vs plate. Whereas left-hemispheric responses were similar across groups, patients demonstrated increased right hemisphere activity 174-294ms after stimulus disambiguation. Source reconstructions confirmed recruitment of right-sided analogues of language regions in SD: atrophy of left anterior temporal lobe was associated with increased activity in right temporal pole, middle temporal gyrus, inferior frontal gyrus and supramarginal gyrus. Moreover only healthy controls had differential responses to words versus non-words in right auditory cortex and planum temporale. Overall, the results indicate that anterior temporal lobe is necessary for normal and efficient processing of word identity in the rest of the language network.