Homunculus (Omnibus) Vol. 1-2 (manga)

Nakoshi Susumu, age 34, is living out of his car. Between spending his days with the homeless and his nights in his vehicle, he has little to his name--but not so little that he'll agree to be the subject of a scientific experiment. An unnerving medical student is stalking him and offering to pay Nakoshi a significant sum to test trepanation: the ability to draw out a sixth sense by drilling into the skull.

Nakoshi refuses. When Nakoshi's beloved car is towed, though, he finally agrees to cash and the operating table. At first, the experiment seems to bear no fruit, and Nakoshi's life is unchanged.

That is, until Nakoshi realizes his vision has warped in his left eye...showing him a twisted homunculus inside a normal human.