Subtraction lesion analysis. The lesion overlay of patients with mild to moderate motor impairment was subtracted from the lesion overlay of patients with severe motor impairment. (a) Region 1 only, (b) Region 1+2, (c) Region 1+2+3, (d) Region 1+2+3+4 (region 4 shown). The colours indicated in the key denote frequencies, where voxels that were more often damaged in severely affected patients and spared in mild to moderately impaired patients appear toward the yellow end of the spectrum. In the enlarged images, the lentiform is outlined in white. For subtraction analysis involving Region 1 (Figure 2a), a small region in the inferior posterior putamen was identified as being lesioned 40-60% more frequently in patients with severe motor impairment than in those with moderate impairment. However, in the subtraction analysis that included Regions 1+2 and Regions 1+2+3 (Figures 2 b,c), it is seen that a region spanning from the inferior to the superior aspect of the posterior putamen was lesioned 60-80% more frequently in patients with severe motor impairment than in those with moderate impairment. No regions in the opposite hemisphere were notably highlighted as more frequently lesioned in the severely impaired group (Figure 2d).