I found some information recently about a man on Japan's death row who is said to have impaired cognitive abilities because of congenital Minamata disease.

"Matsumoto Kenji has been on death row for murder since 1993 and could face execution any moment – he has a mental disability as a result of mercury poisoning (Minamata disease), is reportedly paranoid and incoherent as a result of a mental illness he developed during his detention on death row, and his lawyers are seeking a retrial. "

http://www.geocities.jp/hyouhakudanna/punish2000.html#kmatumoto

"松本被告は胎児性水俣病からくる知的障害であり、幼少期から兄に絶対服従だった。県警は、知的障害につけ込んで「男なら自殺した兄の責任も取るべきだ」とそそのかして自白調書を取った。"

In regard to the death penalty, there is one more important connection to Kumamoto. The first man in Japan to be found innocent after first having been sentenced to death for murder was Menda Sakae, a man from what is now Asagiri-machi, near Hitoyoshi:

http://en.wikipedia.org/wiki/Sakae_Menda

http://www.amnestyusa.org/news/news-item/death-penalty-countries-continue-to-execute-people-with-mental-and-intellectual-disabilities