Geometric hallucinations (GHs), also known as simple visual hallucinations or form constants, are the perception of visual patterns without any external stimulus. These are typically not images of objects or people, but rather recurring, symmetrical shapes like lattices, checkerboards, tunnels, spirals, and cobwebs, which were first systematically categorized in the 1920s by psychologist Heinrich Klüver. The consistency of these patterns across different people and different causes suggests they arise from specific, identifiable changes in how the brain’s visual system functions. Understanding the origin of geometric hallucinations requires examining the biological processes that destabilize the normal resting state of the visual cortex.
Causes Related to Cortical Excitability
Transient geometric hallucinations frequently result from abnormal electrical activity or waves of depolarization in the visual cortex, primarily the primary visual cortex (V1) and secondary visual cortex (V2). The most common example is the migraine aura, which often manifests as a scintillating scotoma, a bright, shimmering, zigzag line that expands across the visual field. This visual disturbance is thought to be caused by Cortical Spreading Depression (CSD), a slow-moving wave of intense neural excitation followed by a period of profound suppression that propagates across the cortex at a rate of about 3 to 6 millimeters per minute. As the wave of CSD travels over the visual cortex, its shape is translated into the characteristic, expanding fortification spectrum seen in the visual field.
Geometric patterns also arise as simple visual hallucinations (SVH) during certain types of epileptic seizures. Occipital lobe epilepsy, which originates in the brain’s visual processing center, can trigger brief, vivid flashes, colored blobs, or geometric shapes. These epileptic hallucinations are usually very short in duration, typically lasting less than two minutes, which helps distinguish them from the slower-developing, longer-lasting migraine aura. The rapid, uncontrolled burst of electrical activity in a localized area of the occipital lobe directly causes the perception of these simple visual forms.
Causes Stemming from Visual Pathway Damage
Geometric patterns can also be generated when the visual system is deprived of its normal sensory input, leading to a phenomenon often called the release phenomenon. When the eyes or optic nerve are damaged, the visual cortex, lacking external stimuli, spontaneously generates simple patterns to fill the void. This is prominent in Charles Bonnet Syndrome (CBS), which occurs in people with significant vision loss due to conditions like macular degeneration or cataracts. While CBS is known for complex hallucinations of people or scenes, the initial or simpler stages often involve geometric shapes as the cortex struggles to interpret the reduced input.
Damage to deeper brain structures involved in visual processing, such as the thalamus or lower visual pathways, can also disrupt the flow of information, triggering these release hallucinations. Structural disruption, such as brain lesions or strokes affecting these areas, destabilizes the cortex and provokes geometric forms.
Causes Related to Chemical and Substance Effects
The ingestion of exogenous chemical agents that alter the balance of neurotransmitters is a well-documented cause of geometric hallucinations. Classic psychedelic substances, such as lysergic acid diethylamide (LSD) and psilocybin, act primarily by stimulating the serotonin 5-HT2A receptors, which are highly concentrated in the visual cortex. This chemical over-stimulation destabilizes the neural network, causing it to spontaneously generate the geometric patterns known as form constants. The patterns seen are predictable and dose-dependent, reflecting a consistent pharmacological effect on the visual processing circuitry.
Beyond psychedelics, a range of other chemical imbalances can also trigger these simple visual forms. High fever, certain toxins, or withdrawal from depressant drugs like alcohol (delirium tremens) can severely destabilize cortical function. These conditions create a state of high excitability in the visual cortex, similar to the electrical instability seen in a seizure or migraine aura.
The Neuroscience of Geometric Patterns
The reason that so many different causes produce the same characteristic shapes—tunnels, spirals, and lattices—lies in the fixed architecture of the primary visual cortex (V1). The V1 area is organized retinotopically, meaning that the visual field is mapped onto the cortical surface in an orderly fashion, like a distorted map. This mapping transforms concentric circles and radial lines in the visual field into straight lines on the V1 surface.
Mathematical models of brain activity propose that geometric hallucinations arise when the V1 network becomes unstable and generates standing waves of activity. Since V1 is a highly structured, interconnected sheet of neurons, when it is stimulated non-specifically by a drug or CSD, the activity naturally stabilizes into simple, symmetrical patterns. These stable patterns on the cortical surface, such as stripes or hexagonal arrays of neural firing, are then “seen” as the form constants—the tunnels, spirals, and lattices—when mapped back to the visual field.