Hi to all,
Last year, I left this thread with much confusion and frustration because of conflicting research done by psychologist. Now, I have more time to deeply look at what was being done and I should come back and tidy up this thread. I will describe a model for the vagus nerve connection which was outlined before thanks to zlyja and Gawan.
First of all, I have read an article on vagal tone research which showed me that researchers use the term vagal tone with liberty and tell there is an increase in vagal tone, even when there can be other causes to explain it. For this reason, I leave out the data associated with vagal tone. I am sorry for the confusion I have created. For more information, check out this paper:http://www.ncbi.nlm.nih.gov/pubmed/11393643
Psychopathy, which is associated with reward seeking and faulty inhibition mechanism can be explained best with the enzyme called dopamine beta hydroxylase which converts dopamine into norepinephrine. This is significant because dopamine is associated with reward and the more dopamine you have, more driven you are to reach to a reward. I posted the article before, but here it is:http://www.sciencedirect.com/science/article/pii/0191886989900044
Another article was also showing psychopaths have much more dopamine responses to rewards:http://www.bmedreport.com/archives/10558
This dopaminergic system has also shown to be much more active in psychopaths, as evidenced by their enlarged striatum and very active nucleus accumbens.
And you may ask, a lot of people are goal driven, why are psycopaths like that remorseless monsters. The reason lies with norepinephrine. Since dopamine beta hydroxylase can not convert dopamine to epinephrine, a psycopath's brain swims with dopamine and lacks norepinephrine. And norepinephrine is an inhibitory signal telling us to stop when we see something negative, telling us to question ourselves etc. Brain's inhibition system is dependent on a brain region called locus coerolous which is composed of neurons that use norepinephrine to relay a signal to amygdala. If there is no epinephrine, there is no signal that says stop to a psychopath and you would get a machine with underdevelopped inhibitory system.
And how this all ties with the vagus nerve? Dopamine beta hydroxylase deficiency is associated with hypoglycemia which shows the connection with vagus nerve as outlined by zlyja and Gawan.
Gawan said aggressive individuals are hypoglycemic and blood sugar swings can damage vagus nerve. Zlyja found the article where it showed only myelinated vagus nerve is damaged when a person is hypogylcemic.http://www.ncbi.nlm.nih.gov/pubmed/15549329
This ties well with what Approaching Infinity said that psychopaths' mammalian vagus is not doing its job for engaging and social behaviour because it is myelinated and it is damaged due to hypoglycemia. And, cardiac activity deceleration observed in aggressive individuals is primarily affected by reptilian vagus. And even the functioning of that vagus is faulty. When you are in panic or any other negative emotion, vagal withdrawal occurs because you are no longer calm, and you accelerate your heart to compensate the energy for the intense experience you are living. Since psychopaths are not responsive to aversive stimuli because of their norepinephrine, there is not a complete vagal withdrawal. I suppose this was what Porges meant when he said sluggish removal of vagal brake.
I wonder the reason that dopamine beta hydroxylase is associated with hypogylcemia is this over activity of vagus nerve which causes insulin secretion as Gawan said. Very interesting.
During my research, I have come up with a few hits, but these are not associated with above mechanisms, I am sharing them if it rings any bell.
First of all 5-HIAA which is a metabolite of serotonin is found less in aggressive individuals. MAO genes which are associated with reactive aggression are required to make 5-HIAA. I don't know if there is less serotonin in these individuals or if serotonin is normal, but can not be converted to 5-HIAA because of low activity MAO genes.
The other hit is a transcription factor called tailless which is associated with the formation of forebrain development. Mice that doesn't have this gene is shown to be aggressive and decrease in maternal behaviour.http://www.ncbi.nlm.nih.gov/pubmed/12527005 http://cercor.oxfordjournals.org/content/13/9/921.full
The other hit is an enzyme called Fatty acid amide hydrolase which causes an increase in striatum and decrease in amygdala activity in psychopaths, again reward pathways are more active than inhibition pathways. This enzyme is associated with the metabolism of cannabioids which are a chemical class that is associated with social behaviour and pain. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2794920/