The most common modality referred to when people speak of hallucinations. These include the phenomena of seeing things which are not present or visual perception which does not reconcile with the consensus reality.
There are many different causes, which have been classed as psychophysiologic (a disturbance of brain structure), psychobiochemical (a disturbance of neurotransmitters), and psychological (e.g. meaningful experiences consciousness).
Numerous disorders can involve visual hallucinations, ranging from psychotic disorders to dementia to migraine, but experiencing visual hallucinations does not in itself mean there is necessarily a disorder. Visual hallucinations are associated with organic dos orders of the brain and with drug and alcohol related illness.
Auditory hallucinations (also known as Paracusia), particularly of one or more talking voices, are particularly associated with psychotic disorders such as schizophrenia or mania, and hold special significance in diagnosing these conditions, although many people not suffering from diagnosable mental illness may sometimes hear voices as well.
Auditory hallucinations of non-organic origin are most often met with in paranoid schizophrenia.
Their visual counterpart in that disease is the non-reality-based feeling of being looked or stared at.
Other types of auditory hallucination include exploding head syndrome and musical ear syndrome, and may occur during sleep paralysis. In the latter, people will hear music playing in their mind, usually songs they are familiar with.
Recent reports have also mentioned that it is also possible to get musical hallucinations from listening to music for long periods of time. This can be caused by: lesions on the brain stem (often resulting from a stroke); also, tumors, encephalitis, or abscesses.
Other reasons include hearing loss and epileptic activity. Auditory hallucinations are also a result of attempting wake-initiation of lucid dreams.
Phantosmia is the phenomenon of smelling odors that aren’t really present. The most common odors are unpleasant smells such as rotting flesh, vomit, urine, feces, smoke, or others.
Phantosmia often results from damage to the nervous tissue in the olfactory system. The damage can be caused by viral infection, brain tumor, trauma, surgery, and possibly exposure to toxins or drugs.
Phantosmia can also be induced by epilepsy affecting the olfactory cortex and is also thought to possibly have psychiatric origins. Phantosmia is different from parosmia, in which a smell is actually present, but perceived differently from its usual smell.
Olfactory hallucinations have also been reported in migraine, although the frequency of such hallucinations is unclear.
Other types of hallucinations create the sensation of tactile sensory input, simulating various types of pressure to the skin or other organs. This type of hallucination is often associated with substance use, such as someone who feels bugs crawling on them (known as formication) after a prolonged period of cocaine or amphetamine use.
This type of hallucination focuses typically on food and is common to individuals presenting persecutory perceptions along with the experience of epileptic aura.
General Somatic Sensations
General Somatic Sensations of a hallucinatory nature is experienced when an individual feels that his body is being mutilated i.e. twisted, torn, or disembowelled. Other reported cases are invasion by animals in the person’s internal organs such as snakes in the stomach or frogs in the rectum.
The general feeling that one’s flesh is decomposing is also classified under this type of hallucination.