Yes, there are studies which show a correlation between methylmercury exposure and attention deficits.
One of the 'cornerstone' mercury studies was performed in the Faroe Islands by Grandjean, et al 1999, (Methylmercury exposure biomarkers as indicators of neurotoxicity in children aged 7 years
) where they followed a cohort of 917 Faroese children from birth until the age of 7, which reported
Outcome variables were the results of neuropsychologic examination earned out in 1993-1994 at age 7 years. After adjustment for covariates, the cord-blood concentration showed the clearest associations with deficits in language, attention, and memory."
They further suggest that:
the greatest susceptibility to methylmercury neurotoxicity occurs during late gestation, while early postnatal vulnerability is less, and they suggest that the time-dependent susceptibility may vary for different brain functions
As a comparative note to the Ha, et al, 2009 study given above, it is interesting to note that the Faroese children had at birth a geometric mean Hg cord blood concentration of 22.9 μg/l, and at aged 7 years 8.82 μg/l, notably higher than the Korean children studied.