Figure 2.

Combining information across subjects results in decreases in both the error in the estimation of centroid location (accuracy), and the point-spread of the data (precision). The centroid error (A–C) was measured by the difference between the true activation location and the centroid calculated from the top V points (averaged across spatial locations for V = 1, 2, 5, 10, 25, 50, or 100 in A; V = 25 for B,C) across different numbers of subjects (1 through 20). The point spread (D–E) was similarly measured as the average distance between the location of true activation and each of the top V spatial locations. Mean ± 2 s.e.m. (across 20,424 cortical locations) is shown by the lines and shaded backgrounds. Note that s.e.m. values are mostly small enough to be masked by the mean lines. The improvement due to combining data across subjects as a function of cortical location is shown in (B,C,E,F). There were large decreases across cortex when comparing 20 subjects to 1 in both the centroid estimation error (C) and the point-spread (F), indicating that the differing subject anatomical structure has reduced the spatial uncertainty and reduced localization error. Note also that the best absolute error values (e.g., some near 1 mm) are only valid for ideal conditions.