How do gory images lead to nausea?
Yesterday my friend visited shock sites (a website). He said that he got nauseous sensation within seconds after viewing the gory images. However, I don't understand how seeing such sights can lead to nausea? Eyes are present at the top of the head and the digestive tract is present below the mouth. They aren't physically connected.
The connection between eyes and food pipe had to do nothing with the motion sickness because the visual inputs and outputs to these sensory information are processed in the brain itself, So there is not need for eyes and food pipe to be connected together in order to cause nausea.
Vasovagal response or vasovagal response or vasovagal attack (also called neurocardiogenic syncope) is a malaise mediated by the vagus nerve. When it leads to syncope or "fainting", it is called a vasovagal syncope, which is the most common type of fainting.
(from http://en.wikipedia.org/wiki/Vasovagal_response)
This case is most commonly seen in adults and adolescents. There are several syndromes which comes under the vasovagal syncope which consists of symptoms like loss of consiusness,lightheadness,nausea,sweating,tinnitus,confusion,altered heart rate,fuzzy thoughts,cloud like spots in vision etc.
Vasovagal syncope occurs in response to a trigger, with a corresponding malfunction in the parts of the nervous system that regulate heart rate and blood pressure. When heart rate slows, blood pressure drops, and the resulting lack of blood to the brain causes fainting and confusion.
(from http://en.wikipedia.org/wiki/Vasovagal_response)
Several pshychiatric disorders can cause vasovagal response like,
http://www.nwocc.com/Pt%20Education/neuro_syncope.pdf
http://en.wikipedia.org/wiki/Vasovagal_response
When it comes to seeing red, the faint feelings could be tied to a phenomenon called blood-injury phobia. According to WebMD, this condition evolved as a way for humans to cope with threats:
"The idea is that back in time, when someone was coming at someone else with a sharp stick or rock, a kind of genetic variation allowed certain people to faint in response," explains Tyler C. Ralston, PsyD, a clinical psychologist in Honolulu, who treats people with blood-injury phobias. Warriors who fainted looked dead and were passed over during battle. The blood pressure drop also might have helped those who were wounded avoid bleeding to death. Survivors then passed on the "fainting" gene.
(from: http://www.popsugar.com/fitness/Why-Sight-Blood-Makes-You-Feel-Faint-3061135)
"Overcoming Obstacles in CBT" written by Yvonne Tone and Michael McDonough states a condition about a patient having similar psychological problem described in the question. Here the patient's panic is associated with "gory images" of injury or illness, and his responses included intense nausea, pale skin, sweat and sometime fainting. He also reports avoiding gory scenes on TV/filim and medical programmes. The problem is identified as co-morbid panic/agoraphobia and blood injury phobia.
http://books.google.co.in/books?id=1uIYqkKgKRYC&pg=PA44&dq=nausea+and+gory+images&hl=en&sa=X&ei=qLZ2VJPMJ420uAS8qoHIBA&ved=0CB0Q6AEwAA#v=onepage&q=nausea%20and%20gory%20images&f=false
The connection is almost certainly psychological. For me, I can watch Grey's Anatomy, Spartacus, or any number of bloody shows while eating, but while researching microsurgery I came across an image of a hand cut in half and it bothered me for an entire day. Therefore, for me at least (and likely others) nausea from gore is context dependent and therefore likely psychological.
Then again, there may be some biological basis. From https://en.wikipedia.org/wiki/Disgust#Neural_basis :
Functional MRI experiments have revealed that the anterior insula in the brain is particularly active when experiencing disgust, when being exposed to offensive tastes, and when viewing facial expressions of disgust.
That Wikipedia article links to this Letter to Nature: http://www.nature.com/nature/journal/v389/n6650/full/389495a0.html which provides more detail.
Also, the part of the brain that actually controls vomiting is called the Area postrema.
So... in short, my answer is "The insular cortex of the brain is responsible, though there is a large psychological component."
