The dots you see in the dark are known as phosphenes. Phosphenes are the moving visual sensations of light that are perceived even when your eyes are closed and there is no external light entering the eyes. These dots or streaks of light can appear in different shapes and colors, and can move around your visual field spontaneously. Phosphenes occur within the visual system and can be triggered by a variety of factors. In this article, we will explore what phosphenes are, what causes them, and their implications for vision and health.
What Are Phosphenes?
Phosphenes are characterized as luminous phenomena that arise independently of external visual stimulation. The word “phosphene” comes from the Greek words “phos” (light) and “phainomenon” (appearance). Phosphenes can manifest as blotches, lines, circles, stars, spirals, or other abstract shapes that appear in black, gray, blue, red, and other colors. These patterns can appear still or in motion, and can last for just fractions of seconds or up to several minutes.
While phosphenes occur completely within the nervous system and require no outside light, they are perceived as if they are happening in the external visual space. That is, when you see phosphenes in the dark, they are projected in front of you as part of your visual field, rather than experienced as happening inside the brain or head. This makes phosphenes a truly unique visual experience.
Characteristics of Phosphenes
Some key characteristics of phosphenes include:
- Appear in the absence of visual stimuli
- Manifest in a variety of shapes, colors, motions
- Projected into external visual space
- Brief, lasting seconds to minutes
- More common with eyes closed
- Affect central and peripheral vision
Understanding these basic qualities helps define phosphenes and distinguish them from normal visual perception that arises from light entering the eyes.
What Triggers Phosphenes?
There are a number of internal factors that can stimulate the visual system and cause phosphenes in the absence of external light:
Applying pressure to the eyes by rubbing or pushing gently can produce phosphenes. This mechanically stimulates the retina and visual pathways, creating a sense of light. Pressure phosphenes tend to manifest as amorphous blotches or clouds of light.
Applying electrical current across the occipital lobe where the visual cortex resides can lead to phosphene perception. Researchers use trans-cranial magnetic stimulation (TMS) to generate electrical activity in the visual centers to study phosphenes. These phosphenes are often described as rays or lines radiating from the center of vision.
Lesions along the visual pathways, such as in the retina, optic nerve, thalamus, or visual cortex, can all cause phosphenes by disrupting normal signaling. Phosphenes from lesions tend to affect corresponding areas of vision since the visual field maps to specific locations in the brain.
About 60% of people experience phosphenes during the aura phase preceding a migraine headache. These manifest most commonly as zigzag lines, dots, or flashing lights. The exact causes are unclear but likely involve abnormal signaling in visual pathways.
Psychoactive drugs like LSD, mushrooms, and mescaline act on serotonin receptors and can dramatically alter visual processing, leading to vivid phosphene hallucinations. These drug-induced phosphenes are highly complex patterns that are perceived as very real.
Fever can induce phosphenes, likely due to the effects of elevated temperature on neural signaling. These tend to appear as randomly flashing spots or bursts of light. Phosphenes from fever are usually transient and disappear as temperature decreases.
Any condition that causes low blood oxygen levels, including choking, suffocation, or pulmonary disease, starves the retina and visual cortex of oxygen. This induces a range of visual effects, such as darkening, tunnel vision, and seeing luminous geometric patterns from tissue hypoxia.
Going without sleep decreases activation thresholds in the visual system so that spontaneous firing more readily occurs. This leads to heightened visual sensations when awake. Sleep deprivation phosphenes look like flickering light, halos, or colored clouds drifting across your field of vision.
The Biological Basis of Phosphenes
Phosphenes originate within the visual system itself rather than being driven by external light. But what is actually going on biologically to produce these luminous sensations emanating from within?
The retina contains millions of light-sensitive cells called photoreceptors. Under normal conditions, photoreceptors convert light information from the external world into neural signals sent to the brain. However, various stimuli can cause the photoreceptors to signal spontaneously in the absence of light due to direct mechanical, electrical, or biochemical effects. These random retinal signals are interpreted as phosphenes.
Beyond the retina, the visual cortex and associated areas also play a key role. Applying pressure, electricity, or chemicals can make neurons in the visual cortex fire action potentials more readily, leading to erratic signaling perceived as phosphenes. This hyperexcitability arises from lowering the threshold potential needed for cortical visual neurons to fire.
The thalamus acts as a gate to filter visual input before it activates the visual cortex. When this gating function is disrupted by fever, lesions, or brain states like sleep, it can lead to abnormal excitation of the cortex as normal inhibitions are reduced. This disinhibited flow of neural activity can manifest as phosphenes.
Increased connectivity and cross-talk between visual neurons driven by migraines, psychedelics, or other factors may spark erroneous signaling that feeds into phosphene perception. When sensory pathways become overly connected, activity can spread in uncontrolled ways.
Cortical Spreading Depression
In migraine, a wave of depressed neuronal activity followed by hyperexcitability spreads across the visual cortex. This cortical spreading depression generates cascades of random firing that get interpreted as luminous visual disturbances known as migraine auras.
Implications of Phosphenes
The fact that we experience anything visual at all when external light is totally absent has interesting implications:
Illusion of Vision
Phosphenes reveal that visual perception does not require light entering the eyes. Visual processing in the brain is sufficient to produce subjective experiences of seeing. This challenges intuitive notions about vision being a passive process driven by optical input.
Blurring External vs. Internal Worlds
When phosphenes mix with real visual stimuli, it becomes clear how indistinguishable the external world and the internal mind can be. Phosphenes demonstrate that the brain does not clearly differentiate between visual signals originating from the outside vs. inside but simply generates visual experiences.
Probe Visual System
Because they arise spontaneously, phosphenes let researchers non-invasively probe the functional organization of the visual system. The shape and location of phosphenes can map how the retina communicates with the brain. This provides insight into how vision works at a fundamental level.
Expanded States of Consciousness
Psychedelic phosphenes reveal states of extraordinary and mystical visual experiences representing expanded forms of consciousness. They inspire philosophical questions about the very nature of reality and perception.
Since phosphenes result from specific areas of aberrant activity in the visual system, they can serve as diagnostic tools pinpointing various neurological conditions like MS, stroke, migraine, tumors, and more. Understanding a patient’s phosphenes aids clinical analysis.
Frequently Asked Questions
Here are answers to some common questions about phosphenes:
Are phosphenes harmful?
Phosphenes themselves are not inherently harmful. They are a natural byproduct of normal visual system physiology. However, the underlying cause triggering extensive phosphenes could potentially indicate visual disorders or health issues.
Do blind people experience phosphenes?
Yes, blind individuals with intact visual pathways excluding the eyes can still perceive phosphenes since they originate in the brain. Those with damage to visual cortex would not experience phosphenes.
Can light therapy help phosphenes?
Light exposure has not been found effective for treating primary phosphene disorders. Specific treatments depend on the underlying cause, like migraine medications. But light therapy can help remedy light deprivation that worsens some conditions producing phosphenes.
Why do some people never observe phosphenes?
Roughly 80-90% of people experience phosphenes under specific conditions. Those who do not may have higher visual cortical inhibition. Unique individual neurology means phosphene susceptibility can vary.
How long do drug-induced phosphenes last?
Psychedelic phosphenes start soon after taking a dose and can persist at varying intensities for 6-12 hours depending on the substance, dose, and person. Residual effects sometimes last up to 24 hours post-ingestion.
In summary, phosphenes are intriguing luminous visual phenomena that offer a window into the inner workings of our visual system. When external light is blocked, mechanical, electrical, chemical, and biological factors can all trigger the perception of shapes and colors that are not really there. Phosphenes demonstrate that seeing does not necessarily require light entering the eyes, but can arise spontaneously in visual pathways as they strive to make sense of stochastic noise in the brain. Though often fleeting, phosphenes provide fundamental insight into the mechanics and mysteries of human vision and consciousness.