Hallucinations occur when the brain creates sensory experiences without any external stimuli, making a person see, hear, feel, or even smell things that aren’t actually present. This phenomenon involves complex interactions within various brain regions and neural circuits, disrupting the normal way the brain processes reality.
At the core of hallucinations is the brain’s predictive processing system. Normally, the brain constantly makes predictions about incoming sensory information and compares these predictions with actual sensory input. When there is a mismatch, called a prediction error, the brain updates its model of the world to stay accurate. During hallucinations, this balance between prediction and sensory input becomes disrupted. For example, in visual hallucinations induced by rapid flashing lights, the rhythmic stimulation can disturb the natural balance of excitation and inhibition in neurons, causing the brain to overemphasize prediction errors. This leads to the brain generating vivid, colorful, or geometric images that have no real external cause.
Another important player is the thalamus, a deep brain structure that acts as a relay station for sensory information. The thalamus sends signals to the cortex, the brain’s outer layer responsible for perception and higher cognitive functions. When thalamo-cortical communication becomes excessive or rhythmic in an unusual way, it can interfere with how the cortex processes sensory data, contributing to hallucinations. This top-down influence from the thalamus can cause the brain to misinterpret or fabricate sensory experiences.
Different types of hallucinations may involve distinct brain areas. For example, visual hallucinations often relate to activity in the occipital lobe, the part of the brain responsible for processing visual information. Damage or abnormal activity in this region can produce phenomena like seeing tunnels or lights, which are common in near-death experiences. Auditory hallucinations, such as hearing voices, often involve the temporal lobes, especially the left temporal-parietal junction, which processes sounds and language. Emotional and memory-related aspects of hallucinations can engage the limbic system, including the hippocampus and amygdala, which are critical for emotion and memory formation.
In some cases, hallucinations arise from altered brain chemistry. Neurotransmitters like dopamine and serotonin play crucial roles in regulating perception and cognition. An imbalance in these chemicals can cause neurons to fire inappropriately, leading to false sensory experiences. This is seen in conditions like schizophrenia, where excessive dopamine activity is linked to hallucinations.
The brain’s frontal lobes, which are involved in reasoning, decision-making, and reality testing, also influence hallucinations. When frontal lobe function is impaired, the brain’s ability to distinguish between real and imagined experiences weakens, making hallucinations more vivid and convincing.
In summary, hallucinations emerge from a breakdown in the brain’s normal sensory processing and predictive coding systems. Disruptions in the balance of neural excitation and inhibition, abnormal thalamo-cortical signaling, altered activity in sensory and limbic brain regions, and chemical imbalances all contribute to the brain fabricating sensory experiences without external input. This complex interplay results in the vivid and sometimes compelling hallucinations that people experience.
