September 01, 2004

Snakes Lurking in the Brain

Arne Öhman and Susan Mineka, A Malicious Serpent: Snakes as Prototypical Stimulus for an Evolved Module of Fear, Current Directions in Psychological Science, 12:1 2003

Why are so many people afraid of snakes? Even in snake-rich Australia only 18 people died over a 10-year period, during which time traffic deaths were more than 1,220% more common. If fear is learned we ought to fear cars far more.

The paper by Öhman and Mineka suggests that we have an ancient, subcortical module evolved to detect and warn us for snakes.

The evidence seems fairly plausible. Monkeys reared in labs can easily acquire snake phobia from other monkeys, while they wonät acquire flower phobias. Snake fear in human can occur without conscious awareness, and we tend to link snakes to aversive stimuli.

But how do we encode a snake detector? Genes only encode proteins, and while a suitable genetic program of switches, receptors and morphogen production could build any neural network, it likely has to be pretty big to encode a specialized snake detector. Even if the stimulus is only a long thin object with a sinous movement pattern it seems to require a pretty complex module. Similar modules might exist for insects (many radial legs) and faces (two dots and a line).

Given the fairly short neural pathway from the retina to the amygdala to process fear stimuli there doesn't seem to be that many places the module can hide. Is it in the superior colliculus, posterior thalamic pulvinar nucleus or one of the amygdala nuclei? My guess would be one of the smaller amygdala nuclei, but I wouldn't bet any money. A rewarding approach might be to look at gene expression patterns in these systems among species that have a snake fear tendency (like primates) and species that do not (are there any mammals that do not fear snakes?). There should be one pattern common to the snake fearing species in these systems that does not exist in non-fearing species or outside systems.

If we find the module, what should we do with it? Keep it as a reminder of our evolutionary history, or see it as an atavism making us fear beauty?

Posted by Anders at September 1, 2004 12:25 AM

I think I read this paper when it was published, and a few others they wrote on the topic - if it's the one I'm thinking of, the notion seemed plausible enough on the surface (I'm terrified of spiders, to an irrational degree), but their argument was actually borderline ridiculous in places. Didn't they argue that this was built into us in our almost ancient past, circa the dinosaurs? And yet (and I'm working from memory here), I recall practically on page 1 that they reported that some captive primates failed to exhibit the fear response. I worried terribly about the complete lack of learning in their model!

Posted by: Robin L. Zebrowski at September 2, 2004 07:53 AM

The dinosaur idea was credited to Carl Sagan, and probably not too serious.

The model is about a built in propensity to fear snakes, but it doesn't have to be active at birth - most people learn to fear snakes as they see their parent shouting and running around during a picnic when they see those funny moving sticks. So the learning aspect is likely strong, since it is needed to connect the built-in (and perhaps non-learning) snake detector module with the fear reaction.

Phobias are fairly amenable to removal though training and extinction. But this removal seems to be about the introduction of a new inhibitory connection (from frontal executive functions?) than the removal of the old stimulus-fear association.

Posted by: Anders at September 2, 2004 07:46 PM

Sorry, I'm just now getting back to respond. Your comments are well-taken! I'm probably mis-remembering the paper, since it left a bad impression on me. I recalled them actually arguing for the ancient evolutionary beginning to the module, and not just off-handedly referencing Sagan on it. I'm deferring to you since you've read it in the last 6 months, unlike me! I guess I just remember thinking that they didn't adequately show at all that there was such a disposition, given that learning actually does play so large a role in most fears of those things. (And given that, unless I'm wrong again, which is possible, there are species of monkey that actually *eat* snakes.)

I went into the article thinking the idea was quite clever, but never found it worth pursuing your larger question of where the module might be located simply because I didn't think they had adequately demonstrated any such module existing.

Thanks a bunch for posting this; it's really put the whole argument back on my mind!

Posted by: Robin at September 6, 2004 09:12 AM