As far as I know, there's no evidence that our modern brain is fundamentally different from that of the ancient Greeks. Why should we expect it to work so differently, then? I've heard of neuroplasticity ("the brain's ability to reorganise itself by forming new neural connections throughout life"), and everyone knows that the internet is shortening our attention span, but what Jaynes suggests... is that back then everyone was schizophrenic.

Even assuming that a society where everyone is hallucinating all the time could be functional (I have my doubts), schizophrenia is a disease of the brain, not the mind. You can't cure schizophrenics by training them in the use of metaphors.

I hate to be that guy, but I think the phrase "extraordinary claims require extraordinary evidence" is relevant here. If I remember correctly (it's been a while since I read the book and I didn't finish it) the whole basis for his argument is a certain interpretation of consciousness applied to literary analysis of ancient poems. Kind of shaky for a hypothesis that has such profound implications, if you ask me.