The fMRI results shown come from a self-described pilot study with only 3 subjects per group [1] [2].

> Results must be interpreted with caution due to the small number of participants and a fixed-effects analysis. However, the results are intriguing and statistically significant despite the small group size. They suggest that further fMRI investigations of children exposed to MeHg and PCBs are warranted and that this technique deserves to be used in neurotoxicity studies in general.

[1] http://dx.doi.org/10.1016/j.neuro.2011.04.001 (the study)

[2] http://dx.doi.org/10.1016/ S147 4-4422(13)70278-3 (a review containing a revised image based on that study -- the one that is included in the Atlantic piece)