Figure 4: Outline of a base brain and evaluation of variability using the average distance between corresponding landmark points in chosen base brains and all other brain images. (a). Three-dimensional model of the average brain. (b). Euclidean distance is used to measure positional differences of landmarks. The standard brain (SB) was assumed to be bilaterally symmetric, so 15 landmarks in each of the individual samples ( 𝑛 = 1 2 from 6 moths assuming bilateral symmetry, L is the left and R the right hemisphere) and the standard brain were used for analysis. When assigning the standard brain as the base brain image, the average positional error for landmarks was minimized, resulting in an error of 38.4 ± 10.7 μm. There was no significant difference among groups according to the Tukey multiple comparison test.