Board Thread:Questions and Answers/@comment-5239435-20130422175359/@comment-5551986-20130424095959

Pretty sure the 6th Sense is Sensing Reiatsu. Tosen was extremely good at sensing, He lost his sight, he still had hearing, touch, smell and taste, this making him have [had] six sense in total. If that theory is correct, Aizen is VERY good at hiding his Reiatsu. Its a mixed theory. Yamamoto could sense his Zanpakuto (We are not 100% sure if that WAS actually Aizen though) so it would seen it doesn't stop that 6th sense.