Auras are caused by a slow-moving wave of electrical disruption that rolls across the brain’s surface, temporarily shutting down normal nerve cell activity as it goes. This wave, called cortical spreading depression, is the primary mechanism behind migraine auras and explains why symptoms like zigzag lines or tingling sensations build gradually rather than appearing all at once. About 25% of people with migraines experience auras, and the phenomenon can also occur before epileptic seizures, though the underlying brain activity differs.
The Electrical Wave Behind Auras
Cortical spreading depression is a wave of intense nerve cell firing followed by a period of electrical silence that spreads across the outer layer of the brain. It moves at roughly 2.5 to 3 millimeters per minute, which is slow enough that you can actually track its progress through your symptoms. If it crosses the visual processing area at the back of your brain, you’ll see the disturbance expand across your field of vision over several minutes. That slow crawl is the hallmark of an aura and one of the key features that distinguishes it from something more sudden, like a stroke.
At the cellular level, the wave begins when voltage-sensitive calcium channels open on nerve cells, triggering a flood of glutamate, the brain’s main excitatory chemical messenger. This creates a chain reaction: neighboring cells fire intensely, release more glutamate, and pass the wave along. Once each patch of brain tissue fires, it goes quiet for several minutes, unable to respond normally. That initial burst of activity produces “positive” symptoms like flashing lights or tingling, while the silence that follows produces “negative” symptoms like blind spots or numbness.
Common Triggers That Set It Off
The wave doesn’t start spontaneously in most cases. It needs a push, and that push comes from anything that tips the brain’s chemical balance toward hyperexcitability. Common external triggers include bright or flickering lights, loud noises, strong odors, and temperature swings. Internal triggers are just as potent: emotional stress, anxiety, poor sleep, skipped meals, and hormonal shifts can all lower the threshold for cortical spreading depression to ignite.
Sleep deprivation and low blood sugar deserve special attention because researchers have identified a clear biological link. When the brain runs low on stored energy (glycogen), levels of potassium and glutamate rise in the spaces between nerve cells. Both of these make the brain more susceptible to the spreading wave. This is why many people notice auras after a night of poor sleep, during a fasting period, or when they’ve gone too long without eating.
What Visual Auras Look and Feel Like
Visual auras are the most common type. The classic version is called a fortification spectrum, named because its zigzag pattern resembles the walls of a medieval fortress. It typically starts as a small bright spot or hole of light near the center of your vision, then expands outward into a C-shaped or crescent arc with shimmering zigzag edges. The whole episode builds over about five minutes and usually resolves within an hour.
Not everyone sees the same thing. Some people experience bright flashing spots, geometric shapes, or sparkles of light. Others develop a scotoma, a patch of lost vision that may appear as a gray or dark area. These visual disturbances affect both eyes (since they originate in the brain, not the eye itself), though people often mistakenly think only one eye is involved.
Sensory and Other Aura Types
When the spreading wave crosses areas of the brain that process touch, it produces sensory auras. The most typical pattern is tingling or numbness that starts in the fingers of one hand and slowly creeps up the arm, sometimes reaching the face and lips on the same side. People describe the sensation as tingling, pins and needles, burning, or a feeling of thickness in the skin. The slow march of tingling from hand to face, taking several minutes, mirrors the wave’s physical movement across the brain’s sensory strip.
Sensory auras usually follow visual auras rather than replacing them. Less commonly, auras can affect speech, causing temporary difficulty finding words or slurring. Motor auras, which cause temporary weakness on one side of the body, are rare and associated with a specific genetic subtype of migraine.
Genetics and Inherited Risk
Your susceptibility to auras has a strong genetic component. The clearest evidence comes from familial hemiplegic migraine, a severe subtype where auras include temporary paralysis on one side of the body. Researchers have identified mutations in four specific genes (CACNA1A, ATP1A2, SCN1A, and PRRT2) that cause this condition. All four genes control how ions move in and out of nerve cells, directly affecting how easily cortical spreading depression can be triggered.
Even in typical migraine with aura, family history matters. If one of your parents experiences auras, your risk is significantly higher. The male-to-female ratio for migraine with aura is about 1 to 2, meaning women are twice as likely to be affected. This is a narrower gap than migraine without aura, where women outnumber men roughly 7 to 1, suggesting that the aura mechanism has a somewhat different genetic and hormonal profile.
Auras in Epilepsy
Auras also occur before epileptic seizures, particularly those originating in the temporal lobe. In this context, an aura is actually the beginning of a seizure, a focal aware seizure where abnormal electrical activity starts in one part of the brain while the person remains conscious. The type of aura depends on which brain region is involved.
Temporal lobe auras are strikingly different from migraine auras. Instead of visual disturbances, they typically produce a rising sensation in the stomach, sudden unexplained fear or anxiety, déjà vu (the eerie feeling you’ve lived this moment before), or its opposite, jamais vu, where familiar surroundings suddenly feel foreign. Some people smell or taste things that aren’t there. These episodes are usually brief, lasting seconds rather than minutes, and may or may not progress to a seizure with loss of awareness. The most common underlying cause is scarring in a structure deep in the temporal lobe called the hippocampus.
How Auras Differ From a Stroke
One of the most important distinctions to understand is the difference between a migraine aura and a transient ischemic attack (TIA), sometimes called a mini-stroke. Both cause temporary neurological symptoms, but they behave very differently.
Migraine auras build gradually over five or more minutes, with symptoms that spread or shift. They often involve positive phenomena like flashing lights or tingling before progressing to negative symptoms like vision loss or numbness. A headache typically follows within an hour. TIA symptoms, by contrast, hit their peak within one minute or less, tend to be purely negative (sudden vision loss, sudden numbness, sudden weakness), and don’t gradually spread from one body area to another. TIA symptoms are also not typically followed by a headache within the first hour.
If you’ve had similar episodes before that fit the migraine aura pattern, a new episode is very likely another aura. A first-ever episode with sudden onset is more concerning and typically warrants brain imaging to rule out a vascular cause.
Auras and Cardiovascular Risk
People who experience migraine with aura carry a modestly higher risk of cardiovascular events compared to those who have migraines without aura or no migraines at all. In a large study of women aged 45 and older published in JAMA, the adjusted rate of major cardiovascular events was 3.36 per 1,000 person-years for women with migraine with aura, compared to 2.11 per 1,000 person-years for women without aura. That’s a real difference in relative terms, though the absolute risk remains low for most individuals.
This association is strongest in women who smoke or use estrogen-containing birth control, both of which independently affect blood vessel function. The combination of migraine with aura, smoking, and hormonal contraceptives multiplies risk in a way that each factor alone does not. This is why healthcare providers often recommend alternative contraception for women who experience auras.