Scholz et al performed diffusion tensor imaging on 48 adults randomly placed in either a juggling or control group. By the end of the 6-week training each of the adults in the juggling group could perform 2 cycles of the 3 ball cascade, which is somewhat but not overly impressive. As compared to their pre-scanning fractional anisotropy, a somewhat loose measure of myelination, fiber density, and axon diameter, the juggling group had a percent increase of ~ 5.5 +/- 1.5 % immediately following the training, and a percent increase of ~ 4 +/- 1 % four weeks later. The control group had no real increase following training, which makes sense because they didn’t do anything!
The increase four weeks post-training indicates that although the effects of the training diminish somewhat over time, they should last for at least a little while. Perhaps further studies could continue to perform diffusion tensor imaging on the adults to see when the percent increases due to training are extinguished, if ever. A back of the envelope calculation based on a linear trend would suggest that after ~ 15 weeks following training the increased would be gone. But the reality may be wildly different.
Scholz J et al. 2009 Training induces changes in white-matter architecture. Nature Neuroscience 12 1370-1371. doi:10.1038/nn.2412