In all panels, the concentration of GABA+ in hMT+ (left and right measured in separate scans and averaged) is shown for each subject along the x-axis. Psychophysical size indices are shown along the y-axis in panels (A–F), which quantify the change in motion discrimination thresholds for small-medium (s-m; A and B), small-big (s-b; C and D), and medium-big (m-b; E and F) comparisons. In (G) and (H), the magnitude of the fMRI response in hMT+ evoked by increasing stimulus size (m–b) is shown along the y-axis. Stimulus contrast was 98% in panels (A), (C), (E), and (G), and 3% in (B), (D), (F), and (H). In all panels, negative values indicate suppression; positive values indicate facilitation. The number of subjects (n), correlation coefficient (r) and significance (p) is shown for each comparison. No significant correlations between hMT+ GABA+ and size indices or fMRI suppression in hMT+ were observed. These sample sizes are sufficient to detect correlations of r ≥ 0.58 (for N = 21) to r ≥ 0.52 (N = 27) with 80% power (probability of type II error <20%) (Hulley et al., 2013).