Fine-grained three-dimensional surface mappings of the putamen were generated using a medial surface modelling method46,47 in 1,541 healthy subjects from the IMAGEN study48 (Fig. 2c and Extended Data Fig. 10a, b). Putamen volume segmentations from either FSL (Fig. 2c and Extended Data Fig. 10a) or FreeSurfer (Extended Data Fig. 10b) were first converted to three-dimensional meshes and then co-registered to an average template for statistical analysis. The medial core distance was used as a measure of shape and was calculated as the distance from each point on the surface to the centre of the putamen. At each point along the surface of the putamen, an association test was performed using multiple linear regression in which the medial core distance at a given point on the surface was the outcome measure and the additive dosage value of the top SNP was the predictor of interest while including the same covariates that were used for volume including age, sex, age2, 4 MDS, ICV and site.
