Hallucinations

Definition and types
Hallucinations are perceptual experiences of objects or events that do not have an external source. They can occur in various sensory modalities, such as auditory, visual, and tactile hallucinations. Auditory hallucinations, for example, might involve hearing voices that no one else hears, whereas visual hallucinations might include seeing patterns or images that are not present in the environment. Hallucinations are distinct from illusions, which are misinterpretations of actual stimuli.

Causes
Hallucinations can have various causes, including neurological disorders, mental illness, substance abuse, sleep deprivation, and sensory deprivation. The underlying mechanisms for hallucinations are not yet fully understood. However, some research suggests that they might be due to the brain's inability to update its perceptions based on new information, as observed in people with schizophrenia and schizoaffective disorders.

Neural mechanisms
The relationship between neural activity and hallucinations is complex. Researchers have found that rather than an increase in overall neuron activity, hallucinations might be associated with specific firing patterns of neurons in the brain. These altered patterns of neural activity can contribute to the subjective experience of perceiving sensory stimuli that are not present in the environment.

Hallucinations in psychosis
In individuals experiencing psychosis, hallucinations are among the most common symptoms. The sensations experienced during hallucinations have been mapped in a study of individuals with psychosis, showing that these sensations often involve multiple body parts and can be connected to various emotional experiences. In addition, auditory hallucinations, such as hearing voices, are particularly prevalent in psychotic disorders like schizophrenia.

Phantom smells
Phantom smells, or olfactory hallucinations, are another form of hallucination where individuals perceive odors that are not present in their environment. A study published in JAMA Otolaryngology – Head and Neck Surgery found that 6.5% of people aged 40 years and older reported experiencing phantom smells, suggesting that this type of hallucination might be more common than previously thought.

Evolutionary considerations
While the evolutionary origins of hallucinations are not fully understood, it is possible that the human brain's capacity for generating such experiences might be related to broader processes of perception and cognition. For example, the ability to perceive and interpret sensory information from the environment is essential for survival, and the brain's construction of mental representations of the world around us might involve mechanisms that can also give rise to hallucinations under certain conditions.