What Causes Hallucinations?

• New research implicates the subcortical systems in hallucinations.
• In a study of injured patients, scientists found that the loss of function of specific brain regions allowed the brain to hallucinate.
• The studies suggest that hallucinations have an anatomical component.

Hallucinations were once part of your normal daily experience. You don’t remember that because you were a newborn and your brain was incapable of forming the necessary neural connections that might allow you to remember such an interesting experience. These hallucinations usually disappear by age 3 or 4; about the same time that the hippocampus begins to communicate with the rest of the brain and long-term memory formation becomes possible.

Hallucinations are also a common experience, although not always recognized as such initially, for people diagnosed with synesthesia. Synesthesia is a fascinating condition. People with synesthesia often “see” music as colors when they hear it; sometimes touch sensations have a taste to them. Sensory information received by the brain is usually handled initially by a single dedicated brain region. In synesthetes, incoming sensory information appears to “leak” into adjacent brain regions. The processing of sensory information related to vision, hearing, taste, and touch is handled by structures in the back half of the brain.

Examinations by MRI of synesthetes have discovered structural and connectivity changes in their brains in the subcortical areas of the superior (for vision) and inferior (for hearing) colliculi. This study also found changes in the temporal and occipital areas, as well as the cerebellum. The cerebellum is usually involved in the control of smooth movements. This study was surprising because it implicated the subcortical systems. Hallucinations were always believed to be a function of the neocortex.

A second study used a different approach. They studied the brains of 89 patients who had suffered specific injuries that caused hallucinations. Thus, the loss of function of specific brain regions allowed the brain to hallucinate. No hallucinogenic drugs were necessary!

The scientists discovered that hallucinations occurred following injury to a variety of different brain regions, not just a single brain region. Further analysis revealed that all of the lesion locations fell within a single functionally connected network of brain structures. This brain network linked parts of the cerebellum (same as for the synesthetes) with primary auditory cortex in the right hemisphere. What was quite interesting was that the authors were able to determine that lesions that were associated with visual hallucinations were functionally linked to a brain region called the lateral geniculate. The lateral geniculate lies deep inside the hemisphere and is critical for processing visual information. This discovery makes great sense and is consistent with what is known about visual processing.

Brain lesions that were associated with auditory hallucinations were connected to the dentate nucleus in the cerebellum. This is a surprising discovery. The dentate nucleus is the largest and phylogenetically youngest of the cerebellar nuclei. Typically, the dentate nucleus talks directly to cortical areas involved with movement, not sensory processing. It is puzzling that damage to pathways into or out of the dentate nuclei of the cerebellum would produce auditory hallucinations.

The results of these studies are important because they suggest that hallucinations have an anatomical component. If parts of your brain stop talking to each other, you may experience sensory hallucinations similar to those produced by drugs. One of the best-studied hallucinogenic drugs is LSD. In the brain, LSD binds to specific serotonin receptors. Taken together, it appears that the brain will inaccurately process and interpret sensory inputs and produce hallucinations in response to vastly different types of manipulations: drugs and injuries. Obviously, there is still much to learn about the neurobiology of hallucinations.

© Gary L. Wenk, Ph.D., author of Your Brain on Food, 3rd Edition, 2019 (Oxford University Press).