The Two Fear Channels


Hoisted from a recent email thread with the estimable Jim Davies:

“You wrote to me once that the brain has two fear channels, cognitive and reactive. Do you have a citation I can look at for an introduction to that idea?”

So I didn’t have a citation off the top of my head, though I do now – LeDoux’s 1998 book The Emotional Brain – but I did remember what I told Jim: that we have two fear channels, one fast, one slow. The fast one is primarily sensory, reactive, and can learn bad associations which are difficult to unlearn, as in PTSD (post-traumatic stress disorder); the slow one is more cognitive, deliberative, and has intellectual fear responses.

It turns out that it ain’t that simple, but I was almost right. Spoiling the lead a bit, there are two conditioned fear channels, the fast “low road” and slow “high road” and they do function more or less as I described: the low road has quick reactions to stimuli, a direct hotline from sensory processing in your thalamus to the amygdala which is a clearinghouse for emotional information; the high road involves the sensory cortex and confirms the quick reaction of the low road. The low road’s implicated in PTSD, though PTSD seems to involve broader areas of brain damage brought on by traumatic events.

Where that needs tweaking is that there’s also a third fear channel, the instructed or cognitive fear channel. This allows us to become scared if we’re told that there’s a tiger behind a door, even if we haven’t seen the fearsome beast. This one relies on an interaction between the hippocampus and the amygdala; if your hippocampus is damaged, you will likely not remember what you’re told, whereas if your amygdala is damaged, you may react appropriately to instruction, but you might not feel the appropriate emotional response to your situation (which could lead you to make poor choices).

So, anyway, that’s the gist. But, in the spirit of Check Your Work, let me show my work from my conversation with Jim.

Ok, I have an answer for you (description based on [Gazzaniga et al 2002], though I found similar information in [Lewis et al 2010]).

There are two fear channels: one involving fast sensory processing and one involving slower perceptual information. Based on the work of LeDoux [1996] these are sometimes called the “low road” (quick and dirty connection of the thalamus to the amygdala, a crude signal that a stimulus resembles a conditioned stimulus) and the “high road” (thalamus to sensory cortex to amygdala, a more refined signal which is more reliable); both of these channels help humans learn implicit conditioned fear responses to stimuli.

This “low road” and “high road” concept was what my understanding of PTSD is based on, that individuals acquire a fast low-road response to stimuli that they cannot readily suppress; I don’t have a reference for you, but I’ve heard it many times (and it’s memorably portrayed in Born on the Fourth of July when veterans in a parade react to firecrackers with flinches, and later the protagonist after his experience has the same reaction). A little research seems to indicate that PTSD may actually involve events traumatic enough to damage the amygdala or hippocampus or both, but likely involving other brain areas as well ([Bremner 2006], [Chen et al 2012]).

There’s a couple more wrinkles. Even patients with amygdala damage have unconditioned fear responses; conditioned responses seem to involve the amygdala [Phelps et al 1998]. Instructed fear (warning a subject about a loud noise that will follow a flashing light, for example) seems to involve the hippocampus as well, though patients with amygdala damage don’t show fear responses even though they may behave appropriately when instructed (e.g., not showing a galvanic skin response even though they flinch [Phelps et al 2001]). This amygdala response can influence storage of emotional memories [Ferry et al 2000]. Furthermore, there’s evidence the amygdala is even involved in perceptual processing of emotional expression [Dolan and Morris 2000].

So to sum, the primary reference that I was talking about was the “low road” (fast connection from thalamus to amygdala, implicated in fast conditioned fear responses and PTSD, though PTSD may involve trauma-induced damage to more brain areas) and “high road” (slow reliable connection from thalamus to sensory cortex to amygdala, implicated in conditioned fear responses), but there’s also a “sensory” path (conditioned fear response via the thalamus to the amygdala, with or without the sensory cortex involvement) vs “cognitive” path (instructed fear response via the hippocampus, which functions but shows reduced emotional impact in case of amygdala damage).

Hope this helps!

Bremner, J. D. (2006). Traumatic stress: effects on the brain. Dialogues in clinical neuroscience, 8(4), 445.

Chen, Y., Fu, K., Feng, C., Tang, L., Zhang, J., Huan, Y., … & Ma, C. (2012). Different regional gray matter loss in recent onset PTSD and non PTSD after a single prolonged trauma exposure. PLoS One, 7(11), e48298.

Dolan, R. J., & Morris, J. S. (2000). The functional anatomy of innate and acquired fear: Perspectives from neuroimaging. Cognitive neuroscience of emotion, 225-241.

Ferry, B., Roozendaal, B., & McGaugh, J. L. (1999). Basolateral amygdala noradrenergic influences on memory storage are mediated by an interaction between β-and α1-adrenoceptors. The Journal of Neuroscience, 19(12), 5119-5123.

Gazzaniga, M.S., Ivry, R.B., & Mangun, G.R. (2002) Cognitive Neuroscience – The Biology of the Mind (2e) W. W. Norton & Company.

LeDoux, J. (1998). The emotional brain: The mysterious underpinnings of emotional life. Simon and Schuster.

Lewis, M., Haviland-Jones, J. M., & Barrett, L. F. (Eds.). (2010). Handbook of emotions. Guilford Press.

Phelps, E. A., LaBar, K. S., Anderson, A. K., O’connor, K. J., Fulbright, R. K., & Spencer, D. D. (1998). Specifying the contributions of the human amygdala to emotional memory: A case study. Neurocase, 4(6), 527-540.

Phelps, E. A., O’Connor, K. J., Gatenby, J. C., Gore, J. C., Grillon, C., & Davis, M. (2001). Activation of the left amygdala to a cognitive representation of fear. Nature neuroscience, 4(4), 437-441.

-the Centaur
Pictured: a few of the books I looked at to answer Jim’s question.

Check Your Work


My brain’s filled with all sorts of tidbits I think I know: time is not fixed, the Earth is not flat, and the Sun doesn’t go around it on a giant chariot.

But people throughout history have believed a lot of crap – for over 2,100 years, people thought Euclidean geometry was a thing, that it was the only thing, to the point that mathematical history books are filled with an enormous amount of bullshit arguments on why parallel lines can never meet, arguments which, in a post-Einstein world in which we’ve measured the deflection of the light from the stars in the sky by the bending of space itself under the weight of the Sun, are obsolete and ridiculous. That isn’t to say tomorrow that scientists won’t find a use for a model of the world which embeds the bendy-wendy Einsteinian cosmos in a fixed Euclidean model of space and time, but it is to say that the idea that only Euclidean geometry is logically possible is dead wrong.

So, knowing that people can be wrong, and dead wrong, about things that they’re dead sure are so true they’ve mistaken them for logical tautologies, it’s worth taking out a little time, when you’re called upon to call up one of those little tidbits you think you know, to check your work.

Which is a way for apologizing for the next article on this blog, which will be a bunch of brain nerdery.

-the Centaur

Pictured: my recent efforts to revisit three things I think I know: how to construct stories, how to construct numbers, and how to construct cognitive architectures.

Skindancer in Sweden


I think I’ve mentioned this on Facebook, but not here: sometimes real life lurks beneath the surface. I read what I write, both to myself and out loud; I have beta readers and editor and publishers; I follow the reviews of my books; I follow their sales; and I pay close attention when people mention they’ve seen or read or liked my books. And then something happens which exceeds your expectations – a friend going to the ICRA conference sent me this pic of a full copy of my Skindancer trilogy in a bookstore in Sweden:


It is an English-Swedish science fiction bookstore with an extremely complete collection … but still, my trade-paperback sized volumes from a midsize publisher are up there with mass-market paperbacks from the big N publishing houses. That means someone on the other side of the world … someone with no contact with me, someone with no contact with my publisher that I know of … decided to compile a list of urban fantasy series … and mine was included.

Wow. I’m honored. And a little bit shocked.

Must write faster.

-the Centaur

“Sibling Rivalry” returning to print


Wow. After nearly 21 years, my first published short story, “Sibling Rivalry”, is returning to print. Originally an experiment to try out an idea I wanted to use for a longer novel, ALGORITHMIC MURDER, I quickly found that I’d caught a live wire with “Sibling Rivalry”, which was my first sale to The Leading Edge magazine back in 1995.

“Sibling Rivalry” was borne of frustrations I had as a graduate student in artificial intelligence (AI) watching shows like Star Trek which Captain Kirk talks a computer to death. No-one talks anyone to death outside of a Hannibal Lecter movie or a bad comic book, much less in real life, and there’s no reason to believe feeding a paradox to an AI will make it explode.

But there are ways to beat one, depending on how they’re constructed – and the more you know about them, the more potential routes there are for attack. That doesn’t mean you’ll win, of course, but … if you want to know, you’ll have to wait for the story to come out.

“Sibling Rivalry” will be the second book in Thinking Ink Press’s Snapbook line, with another awesome cover by my wife Sandi Billingsley, interior design by Betsy Miller and comments by my friends Jim Davies and Kenny Moorman, the latter of whom uses “Sibling Rivalry” to teach AI in his college courses. Wow! I’m honored.

Our preview release will be at the Beyond the Fence launch party next week, with a full release to follow.

Watch this space, fellow adventurers!

-the Centaur

Thrown off the horse and back into the saddle


I have not yet finished dealing with the aftermath of Clockwork Alchemy, and yet I already find myself dealing with the prepwork for Dragon Con! But the good news is, once again, I’m a guest (well, technically, an “attending professional”):

Anthony Francis By day, Anthony Francis is a roboticist; by night, he’s an author and comic book artist. He wrote the Dakota Frost, Skindancer urban fantasy series including Frost Moon, Blood Rock, and Liquid Fire; edited the Doorways to Extra Time anthology; and published the steampunk anthology Thirty Days Later.

Yaay! Oh wait, that means I have to do panels. Aaaa!

Watch this space.

-the Centaur