Validation of Task-Based fMRI and Bootstrap-Derived Index for Language Lateralisation in Healthy Subjects

Preservation of language functions is a central priority in neurosurgical planning, particularly for patients with drug-resistant epilepsy undergoing resective surgery. Given the variability in hemispheric language representation and the limitations of traditional lateralisation methods, this study developed and validated four functional MRI (fMRI) language tasks -Verbal Fluency, Silent Picture Naming, Auditory Sentence Comprehension, and Verb Generation - in 15 healthy, right-handed native Italian speakers. These tasks were designed to engage expressive and receptive language circuits across frontal and temporal regions. To address the methodological constraints of conventional laterality quantification, the threshold-independent bootstrap approach by Wilke and Lidzba (2007) was applied to compute a robust Lateralisation Index (LI). Verb Generation demonstrated the strongest lateralisation in both frontal (Mean LI = 0.56, SD = 0.26) and temporal (Mean LI = 0.43, SD = 0.22) regions. Verbal fluency also showed strong left-lateralisation in the frontal ROI (Mean LI = 0.46, SD = 0.20) but minimal in the temporal ROI (Mean LI = 0.05, SD = 0.22). Silent Picture Naming elicited moderate frontal lateralisation (Mean LI = 0.37, SD = 0.26) and low temporal lateralisation (Mean LI = 0.15, SD = 0.25). Auditory Sentence Comprehension had the lowest lateralisation, with frontal (Mean LI = 0.28, SD = 0.34) and temporal (Mean LI = 0.22, SD = 0.21) ROIs showing more bilateral activation and higher inter-individual variability. The bootstrap LI method provided stable and reliable estimates of hemispheric dominance, reducing susceptibility to thresholding bias and varied activations. These results support the clinical utility of the tasks and the bootstrap-based LI computation for preoperative language mapping in epilepsy patients, offering a non-invasive, quantifiable, and individualised framework for assessing language lateralisation