Table 2. Seed regions and connections included. Seed regions were centered at the MNI coordinates given in parentheses.

	Seed		Target	Relative frequency of individual tracts accounting for mean tract
More Difficult (magnitude-related) Mental Arithmetic	LH IPS [−45 −38 45]	↔	LH pIPS [−30 −71 49]	33/33
	LH IPS [−45 −38 45]	↔	LH IFG (BA 45) [−53 19 26]	33/33
	LH IPS [−45 −38 45]	↔	LH IFG (BA 44) [−49 11 26]	33/33
	LH IPS [−45 −38 45]	↔	LH IFG (BA 47) [−36 45 −15]	31/33
	LH IPS [−45 −38 45]	↔	LH SMA [ −9 19 45]	32/33
	LH pIPS [−30 −71 49]**	↔	LH FEF [−26 7 55]**	31/33
	LH VNF [−38 −71 −15]	↔	LH IPS [−45 −38 45]	29/33
Easier (verbally mediated) Mental Arithmetic	LH AG [−53 −64 23]	↔	LH SMG [−64 −24 23]	28/33
	LH AG [−53 −64 23]	↔	LH MTG [−60 −60 8]	33/33
	LH AG [−53 −64 23]	↔	LH STG [−56 −23 11]	31/33
	LH AG [−53 −64 23]	↔	LH insula [−38 4 −4]	33/33
	LH AG [−53 −64 23]	↔	LH RC [ −9 −53 30]*	29/33
	LH AG [−53 −64 23]	↔	LH medial FG [ −9 45 11]*	31/33
Interhemispheric tracking	LH IPS [−45 −38 45]	↔	LH pIPS [−30 −71 49]	33/33
	LH IPS [−45 −38 45]	↔	RH IPS [ 41 −49 45]	30/33
	RH IPS [ 41 −49 45]	↔	RH PSPL [19 −68 56]	33/33

AG: angular gyrus; BA: Brodmann Area; FEF: frontal eye fields; IFG: inferior frontal gyrus; IPS: intraparietal sulcus; LH: left hemisphere; medial FG: medial frontal gyrus; MTG: middle temporal gyrus; pIPS: posterior intraparietal sulcus; PSPL: posterior superior parietal lobule; SMA: supplementary motor area; SMG: supramarginal gyrus; STG; superior temporal gyrus; RC: retrosplenial cortex; RH: right hemisphere; VNF: visual number form area.

^*Seed points proposed but not exactly specified by the TCM are given in italics. Second, the seed points not proposed by the TCM but nevertheless theoretically motivated are given in italics and marked by an asterisk.

^**Please note that the fiber pathways involved travel from the FEF first via the LH IPS before arching in association fibres to the pIPS (thereby making the connection between FEF and pIPS almost identical to the connection between FEF and IPS).

Number of individual tracts accounting for mean tract:

Evaluation of how consistently the mean-courses of the tracts are reflected by the individual tracts of the participants. This way, it is possible to indicate, in how many participants their individual tracts are more or less identical to the mean tracts. However, for those participants whose individual tracts does not correspond to the mean course of the respective tract, it is important to note that the respective PIBI value can nevertheless be large (simply reflecting that the algorithm can tell for sure that the respective tract passes via a different pathway in these participants).

Nevertheless, this means that the mean tracts identified by our probabilistic fibertracking accounted for at least 28 out of 33 individual tracts.