Archive for the 'neurophysiology' Category

22
Aug
07

what is anxiety anyway?

I’ve been trying to think about anxiety recently.  I want to argue that anxiety is part of a broader meta-level control system involved in risk assessment.  The obvious scientific response to this would be something along the lines of “well, duh.”  But I’ve had a great deal of difficulty articulating exactly what I mean to my friends.  So I’m writing this overly long post to try to work through it.  I’ll try using two different analogies, but first I’ll try to give some motivating background.

Continue reading ‘what is anxiety anyway?’

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04
Aug
07

the polyvagal theory of the autonomic nervous system

I just finished reading a paper by Stephen Porges on the Polyvagal theory.  It’s very cool.  Here’s my attempt as a non-specialist to summarize it.  The traditional view of the autonomic nervous system is that it has two opposing processes, the sympathetic and the parasympathetic systems, which speed up and slow down the heart, respectively, as well as shifting metabolic resources between organs.  The sympathetic system, which is stimulated in part by the perception of threat, raises heart rate and metabolism, and shifts energy to the brain and muscles, thus preparing the body for a fight-or-flight respond to the perceived thread.  The parasympathetic system acts through the vagus nerve to slow the heart rate and/or shift resources to the gut to aid in digestion.

Porges makes two arguments about the vagal (parasympathetic) system:

  • The vagal system isn’t just used for reallocating energy resources during rest and digestion, but it’s also part of the neural substrate for the freeze response (a different, and in fact phylogenetically older, threat response).
  • In mammals, the vagal system is split into two systems – the older dorsal system and a newer ventral system.  This newer system is also connected to facials, vocal, and neck muscles, and forms part of the social engagement system, a newer threat response system that’s only present in (social) mammals.

He argues that the autonomic system’s threat responses are organized roughly as a Brooksian subsumption hierarchy, with phylogenetically newer systems overriding older systems in when they conflict.  This is interesting if you’re interested in biologically-based AI because it gives us a much better understanding of the low-level threat-response systems and also shows that social behavior is wired in at surprisingly low levels.

Porges also makes two claims that as far as I can tell are logically independent of the poly-vagal theory, but which are nonetheless interesting.  One is that the (rare) phenomenon of voodoo death, in which people are literally scared to death, is not actually due to overactivation of the sympathetic nervous system overdriving the heart, but rather to the overactivation of the older parasympathetic (in this case, the dorsal system) slowing the heart down to the point where it can no longer supply enough oxygen for itself, much less the brain.  This apparently better matches the experimental results in a rather horrible rat experiment that was done in the 50s.  The claim is that it’s a case of a response that was adaptive for older species being carried over to mammals, in whom it is fatal because of their higher metabolic requirements.

The other claim is based on the observation that there are muscles in the inner ear, which modulate its mechanical properties so as to accentuate or attenuate certain frequencies, in particular, the frequencies associated with the human vocal tract.  He proposes that in at least some cases of developmental disorders involving lack of social engagement, it could be because of understimulation of these muscles, making it harder for children to attend to human voice, and thereby leading to understimulation of the social engagement system.  The description of the experiment in the paper is vague and I haven’t followed up to read the real papers on it, but he found that when he played computer-generated sounds that emphasized frequencies in the range of human speech, most of the children showed “noticable improvements in social behavior and communication skills following the intervention.”  Amazing.