As all groups comprise neurotypical adults, we hypothesized equal variance between populations in order to control for differences in group size (Penny & Holmes, 2003). Common brain response irrespective of expertise was investigated using a minimum statistic conjunction (Nichols et al., 2005) between the three groups. Brain response specific of each group was assessed by masking exclusively the effect of this group by a global null conjunction (P < 0.05 uncorrected) of the other two groups; for instance, the contrast between Acheulean and Oldowan in Naïve is exclusively masked by a conjunction of the same contrast in Trained and Expert subjects. Our procedure used exclusive masking instead of interactions, which were

not significant at the threshold used, to favour the effects within the group of interest over GSI-IX solubility dmso the reversed effects in the Metformin other groups, which are included in the statistics of interactions (Culham, 2006). All contrasts were thresholded at P < 0.05 FDR-corrected with an extent threshold of 20 voxels. Anatomical localization was performed using a brain atlas (Duvernoy, 1999) and, in particular for inferior frontal and parietal clusters, functional localization made use of distribution analysis of the activated voxels on the basis of probabilistic

cytoarchitectonic maps (Eickhoff et al., 2007) implemented in SPM (Eickhoff et al., 2005). For the sake of consistency, only anatomical labels are used in the tables. Thus, clusters attributed to Brodmann area (BA) 44 were labelled ‘pars opercularis’ (Amunts et al., 1999), those attributed to BA45 were labelled ‘pars triangularis’ (Amunts et al., 1999), and those attributed to BA6 were labelled ‘precentral gyrus’ (Geyer, 2003). In the parietal cortex, clusters attributed to areas PF and PG (Caspers et al., 2006) were labelled ‘inferior parietal lobule’, and those attributed to hIP1 and hIP2 (Choi et al., 2006) were labelled ‘anterior intraparietal sulcus’. While recognizing that functional localization and anatomical landmarks may not strictly overlap in individuals, these conventions were adopted in the interest of coherence in the presentation

of results. Statistical maps were rendered Immune system on FreeSurfer’s fsaverage pial surface with 50 inflation steps (http://surfer.nmr.mgh.harvard.edu). In order to assess the effect of Group, local activity in clusters of interest was further characterized using the SPM extension toolbox MarsBar (http://marsbar.sourceforge.net/) to extract percentage signal change in 5-mm radius volumes centred on the maximum of each cluster, then analysed with spss. Across all subject groups, the contrast of Toolmaking conditions with Control yielded activations is a series of cortical regions, including a large cluster extending from the primary visual and lateral occipital cortices to the inferior temporal cortices, intraparietal sulci, inferior parietal cortices and postcentral gyrii bilaterally.