Geometric hallucinations are visual experiences characterized by the perception of repeating, simple patterns that do not represent recognizable objects, people, or scenes. These phenomena can include grids, checkerboards, tunnels, spirals, and cobwebs, often appearing in bright, saturated colors. In the 1920s, these recurring images were categorized into four groups known as “form constants” due to their consistent geometry regardless of the cause. Understanding what triggers these patterns requires examining the brain’s visual processing architecture, as the form of the hallucination is determined by the visual system itself.
The Underlying Neural Mechanism
The source of geometric hallucinations lies predominantly in the primary visual cortex, known as V1, which is the brain’s initial processing center for visual information. The visual field is mapped onto the surface of V1 in a highly organized way called the retinotopic map. This map uses a log-polar transformation, meaning that straight lines in the visual cortex correspond to spirals or concentric circles in the actual field of vision.
The cortex is composed of columns of neurons, and the geometry of these interconnected neural networks determines the shape of any pattern that emerges from spontaneous activity. When the balance between excitatory and inhibitory signals in V1 is disrupted, the neural network can become unstable and enter an oscillatory state. This instability causes waves of electrical activity to spread across the cortical surface in predictable patterns like stripes or hexagons.
These cortical patterns are then translated back into the visual field via the fixed retinotopic map. For example, a striped pattern of neural activity in V1 is perceived as a tunnel or spiral because of the log-polar coordinate system. The brain’s intrinsic wiring for processing edges and contours imposes the geometric structure on any internally generated visual signal.
Triggers Stemming from Brain Activity Changes
One common cause of these geometric patterns is the visual aura that often precedes a migraine headache. This phenomenon is typically a slow-moving wave of electrical activity and blood flow change, called cortical spreading depression, that sweeps across the visual cortex. The resulting hallucination, known as a scintillating scotoma, is often described as a jagged, zigzag line that may shimmer or pulsate and gradually expands over several minutes.
Epilepsy and Delirium
Geometric hallucinations can also be a symptom of certain types of epilepsy, particularly those originating in the occipital lobe. Visual seizures typically manifest as brief, intense flashes of light, or small, brightly colored circular patterns or spheres. These epileptic visual phenomena are distinct from migraine auras because they are usually very short, lasting only seconds to a few minutes, and often have a rapid onset.
The sudden, intense bursts of electrical activity in epilepsy create a more localized and transient pattern of geometric shapes. Generalized changes in brain state, such as those caused by high fever or acute delirium, can also temporarily disrupt the inhibitory balance in V1, leading to similar transient geometric flashes.
Causes Related to Vision Loss and Sensory Input
A separate category of triggers involves the reduction of sensory input to the visual cortex, a phenomenon most clearly seen in Charles Bonnet Syndrome (CBS). This syndrome occurs in people who have experienced significant vision impairment from conditions like macular degeneration or cataracts. The lack of incoming visual data essentially leaves the visual cortex “idle.”
According to the “perceptual release” or “deafferentation” theory, the visual cortex begins to generate its own spontaneous activity to compensate for the missing input. This internally generated activity can result in both simple geometric patterns, like tiled grids or brickwork, and more complex, formed images of people or scenes.
The visual hallucinations in CBS are purely visual and do not involve other senses, and the affected person usually understands that the images are not real. The simple geometric patterns are thought to arise from the earliest visual processing areas, while the more complex hallucinations engage higher visual association areas. The onset of CBS hallucinations often follows a sudden worsening of vision, and the patterns may eventually lessen in frequency as the brain adapts to the lower level of visual input.
Substance and Medication Triggers
Geometric hallucinations are famously associated with the use of psychedelic substances like lysergic acid diethylamide (LSD), psilocybin, and mescaline. These compounds are known as serotonergic hallucinogens because their primary mechanism involves activating the 5-HT2A serotonin receptors in the brain, particularly those found in the visual cortex. This activation increases the excitability of cortical neurons, disrupting the normal excitatory-inhibitory equilibrium.
This pharmacologically induced over-excitation pushes the neural network into the unstable, oscillatory state that generates the geometric form constants, such as tunnels and spirals. The profound and vivid nature of these drug-induced patterns is a direct consequence of the widespread, intense activation of the visual processing centers. Other medications, particularly high-dose anticholinergics, which block the neurotransmitter acetylcholine, can also cause visual hallucinations, though they are more commonly associated with a state of generalized delirium.
Withdrawal States
Geometric patterns can also appear during periods of chemical withdrawal, such as delirium tremens (DTs) associated with severe alcohol cessation. Chronic alcohol use suppresses the nervous system by enhancing the inhibitory neurotransmitter GABA. Abrupt removal of alcohol causes a sudden, dramatic loss of this inhibitory effect, leading to a state of hyperexcitability and overactivity in the brain. This hyper-excitable state, driven by an imbalance of neurotransmitters, results in the visual and auditory hallucinations characteristic of DTs, which can include the simple geometric forms generated by the overactive visual cortex.