Statistical values of the voxel-wise correlation of the formant–brain index with age split between delta band and theta band. (A) Tracking of the intelligibility index in the delta band (1–3 Hz, P <0.05, cluster corrected) indicates a strong decrease of speech tracking in the cingulate cortex and frontal areas. The black circle indicates lowest t-value extracted for C and E. (B) Tracking of the intelligibility index in the theta band (4–7 Hz, P <0.05, cluster corrected) indicates a strong decrease of speech tracking in visual areas. The black circle indicates lowest t-value extracted for D and F. (C) Correlation of formant–brain intelligibility index in the voxel with the lowest t-value extracted from A (cingulate cortex) showing a significant decrease with age (P = 9.885e−06) in the delta band. (D) Correlation of formant–brain intelligibility index in the voxel with the lowest t-value extracted from B (occipital cortex) showing a significant decrease with age (P = 0.012) also in the delta band. (E) Correlation of formant–brain intelligibility index in the voxel with the lowest t-value extracted from A (cingulate cortex) showing no significant decrease with age (P = 0.428) in the theta band. (F) Correlation of formant–brain intelligibility index in the voxel with the lowest t-value extracted from B (visual cortex) showing a significant decrease with age (P = 7.089e−05) in the theta band. Blue lines depict regression lines, and shaded areas depict SE.