Hallucinations, while widely known of and portrayed in the media, are one of science's ongoing mysteries of the human brain. Part of the problem with pinpointing the science behind and cause of hallucinations lies in semantics; the word hallucination is more like a catchall or general term that includes any kind of real perception, be it taste, sound, sight, touch, smell, temperature or loss in balance, that does not conform to reality. An example of a classic visual hallucination is the tropical oasis: clear, sparkling water surrounded by palm fronds that an unfortunate wanderer in the desert sees in the throes of psychosis and dehydration.
Many parts of the brain and body are involved in and work together to produce sensations and perceptions, and can be placed into three main groups: cells and neurons that receive the sensory information, intermediate messenger nerves that relay the signals to your brain, and the cortices and areas of the brain that put all the information together to give you a sense of the outside world. You can further subdivide these groups according to the many different jobs and information they specialize in, for instance, for feeling and touch your body has different pathways for sensations of the skin (mechanosensation), sensations inside the body and muscles (proprioception), and several for different kinds of pain (sharp pain, throbbing pain, and burning pain).
Arguably the sense people care about and rely on the most is vision, and the pathways and components that bring to life a real-time view of the world around you are no exception to the body's rule of complexity. In the retina, or back part of the eye that receives light coming in through the pupil, there are no less than five types of neurons: the photoreceptors (rods specialize in night vision, and cones perceive color and detail), horizontal, bipolar and amacrine cells (these all form networks between rods and cones to give information on what neighboring photoreceptors are seeing), and ganglion cells that take all the information through the optic nerve, and to the brain to be decoded.
Once the nerve impulses from the retina reach the brain, in addition to sending information off to the pupils to control their dilation, the hypothalamus to control sleep and wakefulness rhythms, the layered part of the midbrain known as the superior colliculus (that governs eye movements and allows you to focus on a still or moving target), visual signals are sent on a journey through successive check points in the brain, getting more and more specialized with each integration and layer, to produce vision.
In the blink of an eye (and arguably much faster), light hitting the retina is translated through the optic tract, into the lateral geniculate nucleus (where information from each eye is still separated), the optic radiation (a branching set of nerves that parcels information off to their final destinations), and the striate cortex (where sight from both eyes are combined to give depth perception). Here, on the outermost layer of your brain, just above your neck at the back of your head, is where the visual story is put together. It is also a place where hallucinations may wreak their havoc, if any part of the relay machinery or nerve health leading up to the striate cortex is altered, or if there is a problem at the cortex itself.
And given this great, complex set of working parts in the brain and body, it's not a big surprise that small mishaps can lead to unexplainable visions, music that no one else can hear, mysterious tastes, or smells on the wind. Diseases and disorders that affect the brain, like Parkinson's, epilepsy and Lewy body, alter brain chemistry and the ability of different centers to communicate and organize information properly (in the case of Parkinson's this also contributes to one of the diagnostic symptoms: inability to maintain postural control and experiencing tremors). Drugs, and the withdrawal from drugs in people with physiological addictions, alter neurotransmitter concentrations and their ability to communicate in the brain, and these either over or under-excited brain cells are thought to account for hallucinations in people using cocaine or recovering from alcohol addiction.
Damage to sensory nerves that sometimes occurs after injury or amputation, can result in the phantom limb' phenomenon, where even though there is no movement or even a hand or foot left to feel, the person may still report tingling sensations. Migraines are another more common and widespread role player in hallucinations; the aura' that occurs just before the pain of a migraine hits may be manifested in seeing zigzag lines, slowly growing dots, hearing voices or sounds, feeling a tingling sensation or separated from one's body, or having a sharp metallic taste in the mouth. Some of these hallucinations are similar for epileptic patients before the onset of seizure, and after looking at brain activity while hallucinations were taking place, unusual, rapid firing of nerves in the sensory cortex was thought to partly explain the auras.
It can be an easy thing to do to forget just how miraculous the human brain is, just how intricate and specialized all the cooperating parts are that let us take in and understand our surroundings. It's no wonder that once in a while, some toxin, disease or trauma puts a nick in the well-oiled machinery, whether at the sensing or cortical levels, and can lead to an equally impressive array of waking dreams'.