Closing/Covering the Eyes during the first Verse: Berachot 13b
Motion of the eyes during the first Verse: Mesoret HaShas Berachot 13b
Source for requiring Mental Focus: Berachot 13b
How much of Shema requires Mental Focus: Berachot 13b, 16a
Mental Focus involved in the first Verse, specifically in the word "Echad": Berachot 13b
Reciting Shema while Reading/Editing a Torah Scroll: Berachot 13a; Rosh HaShanah 28b
Dozing during the First Verse, and during the Rest: Berachot 13b