Seeing colors or patterns when your eyes are closed is a common experience. Known as “phosphenes”, these are caused by activity within the visual system rather than light entering your eyes. There are a few potential causes of phosphenes:
The Pressure Phosphene
Rubbing your eyes or applying light pressure to your closed eyelids can stimulate the retina and optic nerve, producing phosphenes. The mechanical stimulation activates the photoreceptor cells in the retina, which send signals to the visual cortex creating the perception of light and color.
Neural Noise
Even when your eyes are closed and fully adapted to darkness, the visual system produces random neural activity. This background electrical noise in the nervous system can manifest as faint patterns, colors, and lights. So some phosphenes may simply be your brain interpreting meaningless visual nerve signals in the absence of real visual stimulation.
Migraine Visual Auras
People who experience migraines often report seeing shimmering spots, zigzags, or flashing lights in their visual field before the onset of a migraine headache. These visual auras are a type of phosphene triggered by abnormal brain activity associated with the migraine attack.
Hypnagogic Hallucinations
As you are falling asleep, your brain waves and consciousness start to change. In this hypnagogic state between wakefulness and sleep, many people experience vivid dream-like hallucinations. These can include phosphenes in the form of colors, geometric patterns, faces, or motion.
Psychedelic Drugs
Psychedelic substances like LSD, magic mushrooms, and DMT profoundly alter perception and mental state. They can induce vivid visual hallucinations even with eyes closed. These drug-induced phosphenes arise from the psychedelics stimulating parts of the visual system and altering communication between brain regions.
Charles Bonnet Syndrome
People who have lost their sight due to eye damage or disorders may experience complex visualized hallucinations like faces, animals, patterns, or landscapes. This condition is called Charles Bonnet Syndrome and can cause phosphenes in those with visual impairment.
Spreading Depression
In rare cases, a neurological phenomenon called spreading depression can cause phosphenes. This involves a wave of abnormal brain cell activity progressing across the visual cortex, which can produce transient visual sensations.
Occipital Lobe Injury or Stimulation
Since the occipital lobe at the back of the brain contains the primary visual cortex, injury or electrical stimulation of this area can elicit phosphenes. Strokes, tumors, epilepsy, or transcranial magnetic stimulation applied here may cause phosphenes in the absence of visual input.
Common Features of Phosphenes
While the precise nature of phosphenes varies, there are some general characteristics that are commonly reported:
- Brief, transient appearance
- Specks, flashes, zigzags, grids, or geometric patterns
- Shimmering lights, waves, or pulsations
- Diffuse glows or hazes of light
- Less structured clouds or blots of light and color
The color, shape, complexity, duration, and frequency of phosphenes all depend on the underlying cause and individual differences in the visual system.
Benign Nature of Most Phosphenes
For the majority of people, phosphenes are a harmless phenomenon and a normal consequence of the inherent excitability of the visual system. However, some causes like migraines and hallucinations associated with vision loss require medical advice.
Persistent, frequent, or intense phosphenes with no obvious explanation should be discussed with an optometrist or neurologist. But in general, phosphenes from eye rubbing, hypnagogia, and random neural activity are benign and temporary visual effects.
Mechanism Behind Phosphenes
Visual information normally follows a set path through the eye and brain:
- Light enters the eye and stimulates retinal photoreceptors
- Signals pass via the optic nerve to the lateral geniculate nucleus
- Information proceeds to the visual cortex for processing
- Higher cortical areas analyze visual features and properties
Phosphenes involve this visual system becoming activated from within, rather than by external light. Sources like mechanical pressure on the retina, electrical noise, and spreading depression can trigger neurons to fire along the visual pathway to the cortex.
Adaptation and Neural Excitability
Visual neurons exhibit adaptation – they decrease their activity and sensitivity when exposed to unchanging stimulation. In the absence of visual input when your eyes are closed, the visual system becomes hypersensitive due to adaptation.
This neural excitability makes it easier for random electrical noise or small mechanical disturbances to trigger phosphenes by activating photoreceptors and downstream visual areas.
Location of Phosphenes
The apparent location of phosphenes provides clues about their origin:
- Retinal phosphenes appear in precise locations corresponding to the retina.
- Optic nerve phosphenes tend to occupy the entire visual field.
- Cortical phosphenes have vague, imprecise locations.
Retinal and optic nerve phosphenes maintain a retinotopic map consistent with the structure of visual inputs. Cortical phosphenes do not retain spatial mapping due to extensive neural processing in higher visual areas.
Differences from Hallucinations
Phosphenes are distinct from visual hallucinations which occur in medical conditions like schizophrenia, delirium, and Charles Bonnet syndrome:
| Phosphenes | Visual Hallucinations |
|---|---|
| Brief, transient, intermittent | May be prolonged, consistent, recurring |
| Simple patterns and colors | Often complex scenes involving people/objects |
| Aware they aren’t real | May seem convincingly real |
| Not a symptom of mental illness | Often reflect psychiatric or neurological disorder |
While related phenomena, phosphenes represent benign noise and excitation within the early visual system rather than psychiatric hallucinations.
Historical Reports
Phosphenes have fascinated thinkers for centuries, with scientists recording their effects long before understanding their cause:
- Aristotle described pressing his eye and seeing moving colors.
- Newton, Descartes, Boyle, and Goethe all self-experimented with phosphenes.
- 19th century neurologists studied phosphenes from electrical stimulation.
- 20th century psychedelic research investigated drug-induced phosphenes.
Throughout history, scientists and philosophers recognized phosphenes as an intriguing manifestation of inherent visual system sensitivities and excitability.
Conclusion
In summary, phosphenes are a common visual phenomenon with a variety of potential causes all related to activation of the visual system independent of external light stimulation. While sometimes associated with medical conditions, simple phosphenes from pressure, neural noise or hypnagogia are harmless manifestations of our innate visual excitability.
