When the Eyes Play Tricks on the Brain: Understanding Charles Bonnet Syndrome
When people with vision loss begin seeing things that aren’t really there, it can be frightening or confusing. The good news is that this experience – called Charles Bonnet Syndrome (CBS) – is not a sign of mental illness or dementia. It happens when the brain, deprived of normal visual input, starts to create its own images. In this article, we explore what CBS is, how it differs from other causes of visual hallucinations, and what modern brain-imaging studies reveal about this fascinating connection between the eyes and the mind.
What Is Charles Bonnet Syndrome?
CBS occurs in people who have significant vision loss from conditions such as macular degeneration, glaucoma, diabetic eye disease, retinal detachment, or even after eye surgery. When the eyes stop sending normal signals, the visual part of the brain stays active and begins to ‘fill in the blanks’ by creating its own images. These images can be people, animals, patterns, textures, or colors, and the person usually knows they aren’t real. In short: when vision fades, the brain’s imagination turns on.
Why It Happens — The Cortical Release Theory
Our brains rely on constant input from the eyes. When that input weakens or stops, the visual cortex (the brain’s sight center) doesn’t shut down—it becomes overactive. Scientists call this ‘deafferentation’—a word meaning loss of incoming sensory signals. Functional MRI (fMRI) and PET-scan studies have shown that during hallucinations, the same brain regions light up that normally respond to real images. For example, when someone with CBS ‘sees’ faces, the fusiform face area in the brain becomes active—even though no face is actually present. This strongly supports the cortical release theory: when normal visual signals are lost, the brain ‘releases’ stored visual memories. It’s similar to phantom limb pain, where the brain feels sensations from a limb that’s no longer there.
What CBS Is Not — The Differential Diagnoses
Because CBS involves vivid imagery, it can easily be mistaken for other conditions. Here’s how it differs:
| Condition | How It Differs from CBS |
| Psychosis or schizophrenia | Usually includes hearing voices or fixed beliefs; insight is poor. CBS patients remain clear-minded and know what they see isn’t real. |
| Lewy Body or Parkinson’s dementia | Visual hallucinations may occur with confusion, movement changes, and fluctuating attention. |
| Migraine aura | Causes flashing lights or zig-zag lines that move across vision, not formed people or scenes. |
| Occipital or brainstem stroke | Produces permanent blind spots or one-sided vision loss, often with other neurological signs. |
| Medication or substance effects | Hallucinations involve multiple senses or occur soon after starting a new drug. |
What Brain Imaging Reveals
Modern neuroimaging has given us remarkable insight into CBS:
- fMRI scans show spontaneous activity in the visual cortex even with eyes closed.
- PET scans confirm increased blood flow in areas tied to the type of image seen—faces, shapes, or places.
- EEG and TMS studies show that these visual areas become more excitable when deprived of input, and calming them can reduce hallucinations.
Together, these findings confirm that CBS is a neurological response, not a psychological disorder.
How It’s Diagnosed
There’s no single test. Doctors usually:
- Confirm there’s vision loss from an eye condition.
- Ensure the person’s thinking and memory are clear.
- Rule out stroke, infection, medications, or psychiatric illness.
Once other causes are excluded, reassurance is often all that’s needed.
Living with CBS
Although the images can be startling, they’re harmless. Many people find they fade over time, especially when the environment is bright or visually engaging. Helpful tips include keeping rooms well-lit, changing focus or blinking when images appear, staying socially and mentally active, and talking openly with an eye-care provider.
The Takeaway
Charles Bonnet Syndrome reminds us that seeing is as much about the brain as the eyes. When the eyes fall silent, the visual brain continues to speak – sometimes in vivid detail. Recognizing CBS helps patients and families replace fear with understanding and ensures no one mistakes this fascinating brain phenomenon for something it’s not.
Need Support?
If you or someone you know is experiencing unexplained visual images after vision loss, you’re not alone. Our team at A-Ha Vision Neuro-Optometric Rehabilitation Clinic helps individuals understand and manage visual-perceptual changes through education, assessment, and customized care. Oakville, Ontario • 905-582-4800 • info@ahavision.ca
