Knowledge about the neuroanatomy of the human brain has exponentially grown in the last decades leading to finer-grained sub-regional parcellations. The goal of this functional Magnetic Resonance Imaging (fMRI) study was to specify the involvement of the insula during visual word processing using a sub-regional parcellation approach. Specifically, we assessed: (1) the number of active voxels falling in each sub-insular cluster; (2) the signal intensity difference between word and letter strings within clusters; (3) the subject-specific cluster selectivity; (4) the lateralization between left and right clusters. We found that word compared to letter string processing was strongly sub-regional sensitive within the anterior-dorsal cluster only, and was left-lateralized. Interestingly, this sensitivity held at both group level and individual level. This study demonstrates that integrating hemodynamic activity with sub-topographic architecture can generate an enriched understanding of sub-regional functional specializations in the human brain.