(A) Normalized spectral power, averaged across three frequency ranges (2–4, 8–16, and 64–128 Hz; from left to right), as a function of mean pupil diameter. (B) Normalized coefficient of the quadratic term (β2; see Materials and methods for details), averaged across all subjects and atlas regions. Black dots denote p < 0.05, red dots denote p < 0.01, and yellow dots denote p < 0.005 (FDR-adjusted significance threshold with q = 0.1). The gray shaded area depicts the standard error of the mean across subjects. Positive and negative values are indicative of a U-shaped and inverted U-shaped relationship, respectively. (C) Normalized β2 coefficient, averaged across all subjects, separately for four regions of interest: visual cortex, auditory cortex, somatosensory cortex, and anterior cingulate cortex (from left to right). Insets show the approximate extent of the regions of interest. Yellow dots denote p < 0.004 (FDR-adjusted significance threshold with q = 0.1). (D) Spatial distribution of the coefficient of the quadratic term (see Materials and methods for details), at center frequencies 11 Hz (left) and 19 Hz (right). Masked at the FDR- adjusted significance threshold with q = 0.1 (11 Hz: α = 0.003; 19 Hz: α = 0.0081). All p-values were obtained from two-tailed paired t-tests.