I should have mentioned that hallucinations are also frequently related to cerebral cortex problems such as epilepsy and migraines. About 1/3 of migraine sufferers see auras preceding the onset of the headache.
In addition, people frequently report having hallucinations as they are falling off to sleep, known as "hypnopompic hallucinations." They can be frightening, particularly if they are accompanied by sleep paralysis (as they often are). These are often images of faces floating over the person.
Tuesday, July 5, 2016
Why do we sometimes see things that are not there?
The phenomenon of “seeing things” comes in two varieties: illusions and hallucinations. If you see something but misinterpret what you are seeing, it is an illusion. If you see something that is unrelated to visual stimuli, then that is a hallucination.
Visual illusions are caused by misinterpreting visual stimuli. Since humans are visual animals, we make sense of our world primarily based on what we see. We are constantly taking in images and trying to fit them into the world we construct in our mind. Every time we see something new, we tend to compare it to things that we already know to make sense out of it. So, for example, visible heat waves rising in the distance might be interpreted as water in classic illusion known as a "mirage." Since faces are so important to our lives we tend to see faces even in inanimate objects, which is another classic type of illusion called "pareidolia." (Strictly speaking, pareidolia does not merely mean seeing faces, but seeing the familiar in the unfamiliar - but we usually use it for seeing faces.) There are many websites that give cool examples of pareidolia.
The reason behind seeing illusions is pretty well understood. People will try to make sense of the world in accordance to learned patterns, and if something does not fit into those patterns, we try to make it fit. For some cool examples and an explanation of how our mind tricks us, click here.
Hallucinations are less well understood. Hallucinations can happen at any time, but most frequently are caused by physiological conditions brought about by stress, exhaustion, drug intake, starvation, repeated rhythmic activity, etc. Hallucinations are typically individual phenomena, although there are many interesting cases of shared hallucinations related to "mass hysteria."
Visual hallucinations are a fertile field for investigation, and scientists still debate what exactly is going on during hallucinations. The famous neurologists Oliver Sacks dedicated much of his life to understanding hallucinations, and his work was still in progress when he died.
That being said, scientists are pretty sure that hallucinations are related to dreams. The physiological impetus that leads to hallucination seems to allow the brain to dream while you are still awake. In other words, you see the dream at the same time that you can interact with the ordinary world. Most people who are hallucinating can tell which are the hallucinatory images and which are not, although in cases of severe schizophrenia or under the influence of certain drugs (such as the daturas), the person is unable to distinguish. It is not unusual for people who recover from an episode of such hallucination to have little or no memory of it, much as people rarely have memories of their dreams.
In fact, it seems that forgetting dreams is necessary for maintaining a healthy sense of reality. If people are consistently awoken during their dreams and made to write down what they dreamed, they can start to show symptoms of schizophrenia. Schizophrenics often can remember their own dreams, and can't always tell whether their memories were of things that actually happened.
Finally, let me take a minute to implore you not to experiment with any of the daturas. They are extraordinarily poisonous, and people who have used them "recreationally" report the experience as extremely unpleasant.
Why do farts stink?
I am answering this unsavory question mostly because the student who asked it asked me to answer it, and he was a student who demonstrated an admirable spirit of inquiry throughout the year.
People have a symbiotic relationship with numerous bacteria that live in our intestines. In fact, there are more bacteria cells in our bodies than human cells. Many of these bacteria live in our intestines and help is to break down food. There are many gasses produced as a byproduct of these processes, and farting (flatulence) is our main way of expelling these gasses. It has long been debated which of these gasses were most responsible for the smell, but recent research has shown that it is caused by hydrogen sulfide and other volatile sulfur compounds. This is why eating high-sulfur foods such as cabbage or eggs can result in stinky flatulence.
There is also the question of why people find sulfur compounds so malodorous. Flatulence does not have much sulfur in it, but we are able to smell it quite easily. Most likely our sensitivity to volatile sulfur compounds is adaptive because it helps us to avoid eating rotten meat and eggs.
There are a couple of other interesting compounds that contributes a little to the smell of flatulence, but quite a bit to the smell of feces: skatole and indole, both of which are produced by the bacterial breakdown of tryptophan in the intestines. In small amounts these compounds have a flowery fragrance, and they are found in many flowers – most notably jasmine and orange blossoms. In larger amounts they smell like feces. They tend to be most prominent in the feces of carnivores, and that is one of the reasons carnivore feces generally smells worse than that of herbivores. (Another reason is that many carnivores have glands that add distinctive aromas to their feces to help them mark their territories.)
One of the most interesting characteristics of skatole and indole is that they make smells linger. This is why it is so hard to get the stink off of your shoes when you step on dog poop. It is also the reason that people who make perfume add these chemicals to their products. If you make your own fragrance from pure essential oils, it will be strong at first, then will quickly fade. Commercial perfumes can last all day, largely due to the addition of skatole and indole.
I once speculated that animals that harbor a large amount of skatole- and indole-producing bacteria do so because it is adaptive; it would make the territory-marking characteristic of their feces last longer. I cannot find any research to back up my hunch, and there seems to be some animals (Homo sapiens, for example) that produce large amounts of skatole but who are not known to mark their territories this way (although we may have in our past).