These researchers may have not found what they were looking for because the look of the brain solving math problems with a visual model vs. through algebraic symbols (x & y), looked largely the same:
From another perspective, though, it is interesting to see how much working memory is involved with algebraic problem solving. It might be also explain while young precocious math students may still prefer visual mathematical problem solving to more symbolic methods. It may not be that they aren't able to think symbolically, but rather their working memories are the limiting factor (remembering the Blessings and Burdens post?...the higher the IQ, the weaker the working memories in the early years).
Visual vs. Symbolic Math, Algebra, and fMRI
The Blessings and Burdens of High IQ