Structural correlates of semantic and phonemic fluency ability in first and second languages

Area of Science:

• Neuroscience
• Cognitive Psychology
• Linguistics

Background:

• Semantic and phonemic fluency tasks assess speech production processes.
• Neural basis of these tasks is challenging to pinpoint with neuroimaging.
• Previous research has focused on functional correlates, but structural underpinnings are less understood.

Purpose of the Study:

• Investigate the relationship between regional gray matter density and performance on semantic versus phonemic fluency tasks.
• Examine these relationships in bilingual individuals.
• Corroborate structural findings with existing functional neuroimaging data.

Main Methods:

• Structural MRI was used to assess regional gray matter density in 59 bilingual participants.
• Participants completed both category (semantic) and letter (phonemic) fluency tasks.
• Previously published functional neuroimaging data were reanalyzed for validation.

Main Results:

• Left inferior temporal cortex gray matter density correlated with semantic fluency, consistent across first (L1) and second (L2) languages.
• Pre-supplementary motor area (pre-SMA) and bilateral head of caudate gray matter density correlated with phonemic fluency.
• The association between pre-SMA/caudate and phonemic fluency was stronger for L2 than L1, supported by functional data showing greater activation for phonemic tasks.

Conclusions:

• Left inferior temporal cortex is crucial for semantic fluency in both L1 and L2.
• Pre-SMA and caudate nuclei are structurally and functionally associated with phonemic fluency, with a more pronounced effect in L2.
• Lesions in pre-SMA and caudate may disproportionately impair phonemic fluency, particularly in bilinguals.