Neural organization of speech production: A lesion-based study of error patterns in connected speech

Brielle C Stark; Alexandra Basilakos; Gregory Hickok; Chris Rorden; Leonardo Bonilha; Julius Fridriksson

Whilst disruption in the left anterior temporal lobe results in a subtler impairment of lexical-semantic access (i.e. honing into the correct semantic form), damage to more posterior left temporal and occipital regions logically impairs lexical-semantic access by disrupting the lexical-semantic system at an earlier stage of retrieval (i.e. translating visual recognition). Supportive evidence for this comes from studies of focal damage to the left middle temporal gyrus, suggesting that this damage results in profound language comprehension and semantic deficits (Hart and Gordon, 1990; Hillis and Caramazza, 1991; Kertesz et al., 1993; Dronkers et al., 2004) . We likewise showed that unrelated paraphasias made during connected speech-real words for which the target was unrelated or unknown-associated with left posterior temporal (especially middle and inferior temporal gyri) and left temporoparietal damage, indicating that disruption in areas recruited early on in the lexical-semantic system likely lead to verbal, but not phonological, paraphasias. Further, we demonstrate that damaged cortex in left posterior middle temporal gyrus area associated with semantically related and unrelated paraphasias made during both naming and connected speech, highlighting a role for this area of cortex in lexical selection. Our results, which evaluate verbal paraphasias for the first time in connected speech, support prior suppositions from single-word retrieval that the left posterior temporal lobe, in particular, has an early position in lexical-semantic access, at least during naming from visual recognition, and that the more anterior left temporal lobe, an area recruited downstream of the processes occurring in the left posterior temporal lobe, is an area whose role is more specific, likely for semantic entity knowledge.