A recent study shows that extreme partisans don’t use rational parts of the brain when processing political information:

Is political persuasion mostly useless? Is the percentage of people
essentially immune to rational argument about political things
increasing?  […]

At Emory University, psychologist Drew Westen and his team conducted
what they believe is the first study of "the neural basis of any form
of political decision making." They did this by using brain imaging to
study people as they processed political information during the 2004
campaign.

To rape and pillage fine science with rough paraphrasing, this is
what they discovered: When 30 self-described partisans were presented
with contradictory quotes about the candidates (President Bush
supporting, then denouncing Ken Lay; Sen. John Kerry supporting, then
denouncing a Social Security overhaul), it was the portions of the
brain that process emotion, not rational thinking, that became active.
"The thinking caps went off and the feeling caps went on," is how
Westen put it to me.

Normally, Westen says, a brain faced with contradictory information
will fire up the zones where reason or rational thought happens. The 30
partisans in this study were presented with contradictory quotes from
Bush and Kerry, but also from Hank Aaron, Tom Hanks and the writer
William Styron. They processed the information about the
non-politicians with the reasoning centers of the brain. It was
politics that short-circuited them. ("This is your brain; this is your
brain on politics.")

It would be reasonable to ask whether all brains — not just
partisan ones — respond to political information emotionally. Westen
says the answer is clearly no, that research does demonstrate that
centrists or independents are more able to process rational and
non-emotional political information.

But Westen’s MRIs show that is clearly not the case with political
contradictions processed by a partisan brain. That process is almost
entirely emotional, heating up regions of the brain that govern things
like forgiveness, relief and pleasure. The reasoning zones stayed ice
cold.

Your response, I imagine, is "duh." Partisans are emotional; stop
the presses, get me rewrite. Perhaps. But I find the graphic clarity of
colorful brain scans to be sobering. It’s one thing to know that some
people get obnoxious during political arguments; it’s another thing to
see that 30 adult men who read candidates’ quotes while strapped down
in MRI machines didn’t even fire up the thinking parts of their brains.

According to Arthur Brooks, a professor at the business school at
Syracuse University, the number of partisan brains is increasing. And
they may be becoming more partisan; more precisely, they seem to hate
their opponents more.

I don’t know.  So far as I can tell, the only thing that prevents progressives from rationally responding to contradictions uttered by the evil mignons of the right are the sheer numbers of them.  One’s thought processes can only handle so much cretinous malignant nonsense before they fitz out in outrage.