Tara Smith began getting migraine headaches when she was just 12, and as she got older, she started experiencing visual auras before some of the attacks.

“They start out as little sparkles or rainbow-colored zigzags that would move from left to right and grow larger,” says Smith, now 49, a mother of three and professor at Kent State University in Ohio. These patterns are superimposed on whatever Smith is looking at, causing the object or scene to blur.

The first time this happened, in 2014, she lost verbal abilities, too, so doctors hospitalized her to make sure she had not had a stroke. Fortunately, CT and MRI scans ruled out the possibility that something more serious was going on.

These days, Smith still gets migraine with aura about once a year, with typical (aura-free) migraine attacks occasionally in between.

“The visual auras are mostly annoying because I have to pause what I'm doing,” she says. “It's a feeling of almost being drunk, and it lasts about half an hour.” The only option is to wait it out, she adds, because her migraine medication does not kick in quickly enough to stop the illusions.

Most of us have experienced optical illusions—images that trick your vision, causing something to appear as something else or making you see something differently from what it actually is. (Classic examples include the drawings of M.C. Escher and trompe l’oeil paintings.)

Visual illusions are different, though. In general, they are related to the brain's misperception of a stimulus present in the external environment—something that may appear distorted in size, shape, or other characteristics or may seem to move when it's not, explains Steven L. Galetta, MD, FAAN, professor and chair of neurology at NYU Langone Health Center. Sometimes, this can feel like looking at a funhouse mirror.

Visual hallucinations, on the other hand, involve perceiving something or someone not actually in the environment. Both forms of visual disturbances can accompany neurologic conditions, such as migraine, epilepsy, dementia, stroke, traumatic brain injury, and Parkinson's disease, experts say.

Sometimes, the distinctions between these phenomena aren't crystal clear. In a symptom called palinopsia, for example, people perceive visual images as lingering or reappearing after an actual object has left their field of vision; this can happen because of illusions or hallucinations. And sometimes people experience illusions and hallucinations in relation to an ophthalmologic condition, such as a retinal detachment.

Complicating matters, “if you misinterpret a physical object you see in reality, there may be a hallucination overlying an illusion,” says Alexander Solomon, MD, a neuro-ophthalmologist at the Pacific Neuroscience Institute in Santa Monica, CA. “Your brain is always trying to achieve a balance in determining if something is real or not.” But sometimes it misses the mark.

Sources of Illusions and Hallucinations

“There are so many different things that can cause visual illusions, and there are lots of different types of visual illusions,” says Heather E. Moss, MD, PhD, FAAN, a professor of ophthalmology and neurology and neurological sciences at Stanford University. “Things can happen in the eye and brain that can distort what we see in the world.”

For example, research has found that kinetopsia, an illusion in which people perceive stationary objects as moving, and object misidentification illusions, in which people perceive certain objects as something else, are the most common types of visual illusions in Parkinson's disease. People with stroke or migraine might experience other visual illusions, such as the world appearing tilted or flipped, which has to do with the brain's balance center, Dr. Moss says.

The underlying mechanisms behind visual illusions depend largely on what causes them. When it comes to migraine with visual aura, it may be that light activates the visual cortex in the brain and irritates the structures responsible for processing vision, Dr. Galetta says. With other neurologic conditions, anything that damages the visual cortex—such as Lewy body dementia, stroke, a brain tumor, or Parkinson's—can be associated with visual illusions or hallucinations, he adds. With some types of seizures, inappropriate activation of certain visual pathways in the brain may trigger illusions, Dr. Solomon says.

Regardless of the cause, the disturbances are “the brain's attempt to make sense of the world by putting together snippets from vision and trying to predict what's in the environment or going to be in the environment,” says Eric Eggenberger, DO, MS, FAAN, a professor and consultant in the department of ophthalmology, neurology, and neurosurgery at the Mayo Clinic in Florida.

In fact, “a lot of illusions can be created based on past experiences,” says Aileen A. Antonio, MD, FAAN, a neuro-ophthalmologist and associate program director for the neurology residency at Trinity Health Grand Rapids in Michigan. “Our minds are trained to put patterns together.”

By contrast, visual hallucinations can occur when an injury occurs anywhere along the visual system, from the thalamus to the visual cortex in the brain's occipital lobe, Dr. Galetta says. “Sometimes the brain plays tricks because you're not getting the proper signals from your vision,” he explains. “The mind tries to fill in gaps, and it may not do it very well.”

In 2022, a retired journalist in New York City with glaucoma and severe myopia saw a neurologist because she had started seeing things that weren't actually there. She saw flowers instead of people's faces, and food appeared to rise from the bowl as she ate.

In Charles Bonnet syndrome, the brain can create spontaneous visual hallucinations that are associated with poor vision, such as advanced macular degeneration. “If a person loses a degree of vision from any cause, the brain will fill in the pieces of the missing visual elements,” Dr. Eggenberger says.

People who lose some degree of their vision and have hallucinations often worry that they are developing dementia, says Sachin Kedar, MD, a professor of neurology and ophthalmology at the Emory University School of Medicine in Atlanta. In these instances, it's important to discuss these concerns with a neurologist because reassurance can help.

Some medications can cause visual illusions, including clomiphene citrate (Clomid, a fertility drug), topiramate (Topamax, used to treat seizures and help prevent migraine headaches), some antidepressants (particularly serotonin-based drugs and trazodone), dopaminergic medications (like levodopa, which is used to treat Parkinson's disease), anticholinergic drugs (used to treat asthma, overactive bladder, and other medical conditions), and high doses of anti-seizure drugs.

“Digoxin, a cardiac medication, can lend a yellow or green tint to what someone is seeing, and erectile dysfunction drugs such as Viagra can cause a blue tint,” Dr. Moss adds.

Seeking Help

If you experience visual illusions or hallucinations for the first time, see a neurologist. “Anything that is new, getting worse, or interfering with daily life is cause for concern and investigation,” Dr. Moss says.

“Sometimes it's hard to talk about these—there's a social stigma where people worry that people will think they're crazy,” Dr. Solomon adds. “But it's important to tell your neurologist about them.” It may help to bring a family member or friend with you for support.

During the exam and discussion of your medical history, the cause of the disturbances may become apparent; if not, the neurologist may order a brain scan to rule out a structural cause, such as a tumor, Dr. Galetta says.

Once the underlying cause is identified, an appropriate course of action can be determined. If a retinal detachment has caused illusions, for example, surgery may stop them. Dehydration can cause visual disturbances as part of delirium in older adults, Dr. Antonio points out, but if both conditions are treated, those disturbances likely will go away.

If a medication is the culprit, your doctor may switch you to a different drug or lower the existing dose, Dr. Galetta says. Should the illusions persist and interfere with your quality of life, your neurologist may prescribe a mild neuroleptic (antipsychotic), such as quetiapine (Seroquel) or olanzapine (Zyprexa), or an antiseizure medication.

And sometimes, just an ounce of reassurance can make a difference. That was the case for the retired journalist, now 87: Once she learned that the visual disturbances she experienced weren't due to anything scary, her anxiety about them diminished—and so did the perception of seeing things that weren't there. But the perception of food rising from her bowl has persisted. Still, she says, “I'm used to it, so it doesn't particularly bother me.”