(A) BOLD amplitudes evoked with a double-sided stimulus were linearly proportional to that evoked with the corresponding single-sided stimulus of the same contrast. There were 5 runs per data point for the 6-s condition and 9 runs for the 1-s condition. For each of the visual areas V1-V3, a linear function provides a good fit the data points [6 s: R2≥0.90; 1 s: R2≥0.85], and the intercepts are not significantly different from zero [nested model comparison for each visual area; 6 s: F(1,3)<0.37, p>0.59; 1 s: F(1,3)<0.27, p>0.64]. (B) Equivalently, contrast had no significant effect on the ratio of BOLD amplitude of a double-sided condition to that of the corresponding single-sided condition [nested model comparison for each visual area, 6 s: F(1,3)<0.53, p>0.52; 1 s: F(1,3)<0.16, p>0.72]. Performing a one-way ANOVA on the BOLD ratios obtained from individual runs also failed to find any significant effect of contrast in any of the visual areas [6 s: F(4,20)<0.37, p>0.8; 1 s: F(4,40)<1.76 p>0.15]. Assuming that the underlying neural responses to the double-sided stimuli is twice those of the corresponding single-sided stimuli, these results imply that the relationship between neural and fMRI BOLD responses follows power law: B = kZγ , where γ can be determined from the ratio of BOLD amplitude evoked by the double-sided stimuli to that evoked by the single-sided ones: γ = log2(B2,i/B1,i). The estimated values of γ from the BOLD ratios are 0.75 (V1), 0.54 (V2), 0.53 (V3) from the 6 s presentation and 0.55 (V1), 0.48 (V2), 0.44 (V3) from the 1 s presentation. These values are very close to those estimated using the stitching procedure (Figure 4C).