Visual hallucinations are surprisingly common among people without mental illnesses, as neurologist Dr. Oliver Sacks discovered while researching "Hallucinations," his recently published book. In a new paper for the neurology journal Brain, Sacks details a particularly strange subset of Charles Bonnet syndrome, a condition that causes complex visual hallucinations among people with deteriorating sight: nonsensical sheet music that floats in and out of their field of vision.

People who lose their sight late in life sometimes develop Charles Bonnet syndrome. The brain's visual cortex normally devotes a great deal of processing power to analyzing input from the eyes, sending information to higher brain areas that add context and meaning to visual information. Damage to different levels of processing can interrupt and scatter the information flow, potentially causing all sorts of strange cognitive mishaps.

Charles Bonnet syndrome can occur when visual input is cut off in blinding conditions like macular degeneration or glaucoma, when the brain overcompensates with "positive pathologies of vision" that cause hallucinations in the absence of real images. These are distinct from visual floaters, which are caused by strands of clumped fluid that cast shadows on the retina.

Sacks' previous work on musical neurological syndromes led many people to reach out to him to discuss their own problems, and in the course of his correspondence he discovered at least 12 people "whose hallucinations include-and sometimes consist exclusively of-musical notation."

In the Brain paper, Sacks describes eight bizarre case studies of people who experienced musical notation hallucinations:

  • A 77-year-old woman with glaucoma whose "musical eyes" showed her "music, lines, spaces, notes, clefs - in fact written music on everything [she] looked at" in the lower half of her vision.
  • A Sanskrit scholar and pianist who started hallucinating musical notation after developing Parkinson's disease in his 60s - the music notes appeared in Sanskrit Devanagari script, though he could tell it was still western music
  • Two pianists with macular degeneration, one amateur and one professional, who saw music notes on a white background "just like a sheet of real music, though it was unreadable and unplayable.
  • A composer and music teacher with Parkinson's disease who saw "a sort of collage of music scores superimposed upon the surface" of walls, floors, and bathtubs in her peripheral vision.
  • A woman who was not a musician, but said that when she had high fevers as a child, she would hallucinate musical notation that was "angry... out of control and at times in a ball," requiring her to "mentally smooth them out and put them in harmony or order" for hours.

Text hallucinations are fairly common among people who experience Charles Bonnet syndrome, but Sacks speculates that, because most of the case studies were experienced musicians, there might be "something about musical scores that is radically different from verbal texts."

Since musical notation is much more visually complex than English text, it's likely that the visual cortex in the brain devotes a great deal of space to processing it - "one might suspect that musicians studying complex scores and notation may develop neural imprints, so to speak, which-should any tendency to hallucination later develop-are available to be activated."

It's striking that so many of these musical notation hallucinations are unreadable, says Sacks. They seem like typical sheet music at first glance, but turn out to be nonsensical when they look closely, with disjointed melody lines, time signatures, notes, staves, and clefs that lack any kind of coherent organization - symbols without meaning or context.

This quality is shared by many text hallucinations. One patient explained that the words she hallucinates "are from no known language, some have no vowels, some have too many: 'skeeeekkseegsky'."

It's unclear exactly how this dysfunction happens, but the disjointedness of these hallucinations suggests that the brain's normal processing is rewiring itself in the absence of normal stimulation.

Dr. Sacks is no stranger to visual hallucinations - he experiences a mild kind of Charles Bonnet syndrome due to the partial loss of his sight, usually with geometrical patterns like checkerboards or letters. When he started playing the piano recently, he started to hallucinate what looked vaguely like musical notation: "showers of flat signs along with the letters and runes on blank surfaces."

Sacks reassures people distracted by visual hallucinations that Charles Bonnet Syndrome isn't related to mental problems. A visit to a neurologist is recommended to rule out unidentified brain conditions, but the hallucinations typically decline with time, even if they do not completely disappear.

"For the most part," said Sacks in a recent article, "these experiences are unthreatening and, once accommodated, even mildly diverting."

For the greater purpose of scientific research, at least, conditions like Charles Bonnet syndrome "have a unique power to illuminate the processes and levels of construction in the visual system, and hallucinations of musical notation can provide a very rich field of study here."

You can watch Oliver Sacks explain more about what hallucinations reveal about our minds in this video from TED: