Extended Data Figure 5. Variability of brain-wide projection signal strength.

To examine animal-to-animal variability in projection patterns, 12 sources with two spatially overlapping injection experiments were identified from the full data set of tracer injections shown in Fig. 3. a, Rows show segmented projection volumes normalized to the injection volume (log₁₀-transformed) in the 295 ipsilateral target regions for each of 2 individual overlapping tracer injections per source region indicated (above and below solid black line). The colour map is as shown in Fig. 3. b, Maximal density projections of whole brain signals from each of the two spatially overlapping injection experiments per source region visibly demonstrate consistency of brain-wide connections. Scatter plots of all ipsilateral and contralateral target structure values above a minimum threshold in both members of the pair (log₁₀ = −3.5; non-blue values from a) show significant correlations between each pair of injections across a four orders of magnitude range of projection strengths. Values in the scatter plots are Pearson’s correlation coefficients (r). Note that in some cases (for example, PTLp) axon pathways appear to be labelled in only one of the pair. This could be due to random differences in the proportion of corticospinal projecting neurons labelled in a particular injection within the same source area. Signal in large annotated white matter tracts are computationally removed from the connectivity matrix, and thus not included in the scatter plots. c, Detected fluorescent signals from each of two injections into the same location of primary somatosensory cortex registered and overlaid with the average template brain (grey). Lower 2 rows, 2D raw images from each injection experiment at different anterior-posterior levels. The centres of these injection sites are in the far left panel and their major targets are in the right panels. See Supplementary Table 1 for the corresponding full name and acronym for each region.