Phosphenes

Abstract

A phosphene is a form of ocular vision: the sensation of seeing light when no light has entered the eye. Because the percept is generated inside the visual system rather than by an external image, phosphenes reveal where and how neural activity gives rise to seeing. They can be produced mechanically, by pressure or a blow to the eye that deforms the retina; electrically, by currents passed across the scalp; magnetically, by transcranial magnetic stimulation over the visual cortex; and by direct electrical stimulation of the retina or cortex. The site of origin differs by method, and disentangling retinal from cortical sources has been a central problem. This article traces the phenomenology and history of phosphenes, their mechanical, electrical, magnetic, and cortical elicitation, and their use in visual prostheses, with three interactive demonstrations.

Keywords: phosphenes, cortical stimulation, transcranial magnetic stimulation, visual prosthesis, ocular vision

A phosphene is a visual sensation of light that arises without light entering the eye, produced instead by some other form of stimulation of the visual system (Grüsser & Hagner, 1990). Because the light is generated within the nervous system rather than captured from the world, a phosphene is direct evidence that the sensation of brightness is a product of neural activity along the visual pathway, wherever that activity is triggered. Phosphenes are a limiting case of ocular vision in which the normal optical input is bypassed entirely, and they have served for centuries as a natural experiment on the origin of visual experience — from the pressure spots seen when the closed eye is rubbed, to the patterned percepts evoked by stimulating the visual cortex directly, which now underpin efforts to build a visual prosthesis for the blind (Tehovnik & Slocum, 2013).

What Phosphenes Are

A phosphene is defined not by how it looks but by how it is caused: it is a visual sensation — a spot, flash, patch, or pattern of light or colour — that occurs in the absence of any corresponding light entering the eye. The Medical Subject Headings vocabulary defines phosphenes as subjective visual sensations occurring with the eyes closed and in the absence of light, whether spontaneous or induced by chemical, electrical, or mechanical stimuli that cause the visual field to light up without optical input. The essential point is that the brightness is manufactured inside the visual system. Any stimulus that can drive neurons in the visual pathway to fire as they would to light — a mechanical deformation of the retina, an electrical current, a changing magnetic field, or a probe placed directly on neural tissue — can produce the sensation of light (Grüsser & Hagner, 1990).

This is why phosphenes have mattered so much to the science of vision. A normal percept confounds two things: the light in the world and the neural events it triggers. A phosphene strips away the first and leaves only the second, so it isolates the neural basis of the sensation of brightness. It also localizes that basis, because different methods of eliciting phosphenes act at different points along the pathway. A phosphene evoked by pressure on the eyeball must arise in the retina; one evoked by a magnetic pulse over the back of the head can arise in the visual cortex. Comparing phosphenes across methods therefore maps where, along the chain from photoreceptor to cortex, a given kind of visual experience is generated (Tehovnik & Slocum, 2013).

Mechanical and Pressure Phosphenes

The oldest and simplest phosphenes are mechanical: press gently on the closed eye and a glowing patch of light appears, usually in the part of the visual field opposite the point of pressure. These deformation phosphenes arise because bending or stretching the retina mechanically excites the photoreceptors and their downstream neurons, which signal light regardless of the fact that none is present. Otto-Joachim Grüsser and Michael Hagner traced the history of this phenomenon and of the broader idea of an 'inner light' generated within the eye, showing that observations of pressure and deformation phosphenes run from antiquity through the systematic mechanical studies of the eighteenth and nineteenth centuries (Grüsser & Hagner, 1990).

Mechanical phosphenes carried a philosophical charge out of proportion to their humble cause. Because the same retinal region produces a sensation of light whether stimulated by photons or by a fingertip, they demonstrate plainly that light, as experienced, is a state of the observer rather than a property of the object seen. This was a central plank of the argument, developed by George Berkeley and others, that the qualities of sensation are contributed by the perceiver. In the pressure phosphene the point is made without instruments: the light is real as experience, yet there is demonstrably no light in the world to be its cause (Grüsser & Hagner, 1990).

Electrical Phosphenes and Their Origin

Passing an electrical current through the head has been known to produce sensations of light since the earliest experiments with the voltaic pile, and modern transcranial electrical stimulation (tES), including its alternating-current form (tACS), reliably elicits phosphenes when electrodes are placed over the occipital scalp. For a time these were taken as evidence that the weak scalp current was directly modulating the visual cortex. That interpretation ran into a decisive problem: the phosphenes might not be cortical at all, but retinal, produced by current spreading to the eye.

Two lines of work established that transcranial electrical phosphenes are largely of retinal origin. Kar and Krekelberg showed that phosphenes evoked by transcranial electrical stimulation over the visual cortex track manipulations that affect the retina rather than the cortex — for example, they depend on retinal adaptation state and change with eye position in ways that a cortical source cannot explain, implying that the current reaches and excites the retina (Kar & Krekelberg, 2012). Schutter and Hortensius reached the same conclusion by a complementary route, demonstrating that phosphenes produced by transcranial alternating current stimulation behave as retinal, not cortical, phenomena (Schutter & Hortensius, 2010). The practical lesson is cautionary: a phosphene reported during a tES experiment is not a sign that the cortex is being stimulated, and it can confound studies that intend to modulate cortical activity while leaving perception untouched.

Magnetic (TMS) Phosphenes

Transcranial magnetic stimulation (TMS) delivers a brief, intense magnetic pulse through the skull that induces a current in the underlying cortex, and a pulse over the occipital pole reliably evokes phosphenes. Unlike transcranial electrical phosphenes, these appear to be genuinely cortical. Elwin Marg reviewed the early evidence that magnetic stimulation acts directly on the retina and the visual brain, framing magnetically induced phosphenes as a non-invasive probe of the visual system (Marg, 1991). The cortical locus was pinned down by mapping. Kammer showed that occipital TMS produces phosphenes and transient scotomata — brief blind patches — whose positions in the visual field are systematically related to the site of stimulation on the scalp, exactly as expected if the pulse is acting on the retinotopically organized visual cortex (Kammer, 1999).

Magnetic phosphenes have become a tool for studying visual awareness. Cowey and Walsh used TMS to elicit phosphenes in sighted observers, in a blind observer, and in a patient with blindsight, and found that the pattern of what could and could not be made to 'light up' tracked the integrity of the visual cortex rather than of the eye — informing debates about which parts of the visual system are necessary for a conscious visual sensation (Cowey & Walsh, 2000). Because a magnetic phosphene marks a moment and a place at which the cortex has been driven to produce a visual experience, its threshold and location serve as a readout of cortical excitability and organization.

Cortical Stimulation and Visual Prostheses

The most direct way to make the visual cortex produce light is to stimulate it electrically at close range. In a landmark study, Brindley and Lewin implanted an array of electrodes on the visual cortex of a blind volunteer and showed that stimulating individual electrodes produced discrete phosphenes — small spots of light — at locations in the visual field that were stable and separated, so that different electrodes lit up different points (Brindley & Lewin, 1968). This was the founding demonstration that a blind person could be given rudimentary visual sensations by patterned cortical stimulation. The positions of these phosphenes are not arbitrary: they follow the retinotopic map of the primary visual cortex, in which the central visual field is allotted disproportionately more cortical territory than the periphery — a cortical magnification that Horton and Hoyt quantified for the human striate cortex, revising the classic Holmes map (Horton & Hoyt, 1991). An electrode array therefore samples the visual field unevenly, densely near the fovea and sparsely toward the edge, exactly the mapping the `CorticalRetinotopyDemo` above makes explicit. Schmidt and colleagues extended it with intracortical microstimulation, inserting fine electrodes into the visual cortex of a blind volunteer and evoking phosphenes with far smaller currents than surface stimulation required, mapping their thresholds and positions in detail (Schmidt et al., 1996).

Modern work has made cortical phosphenes both quantitative and dynamic. Bosking and colleagues characterized how the size of a cortically evoked phosphene grows with the stimulating current and then saturates, so that beyond a certain current the phosphene stops enlarging — a constraint on how much information a single electrode can convey (Bosking et al., 2017). Beauchamp and colleagues then showed that stimulating a sequence of electrodes to trace a shape on the cortex allows sighted and blind participants to perceive the traced form, moving from isolated dots to dynamic current steering that draws letters and shapes (Beauchamp et al., 2020). In parallel, Fernández and colleagues implanted a hundred-channel intracortical microelectrode array in the occipital cortex of a blind person and evoked patterned phosphenes with it, a step toward a practical cortical visual prosthesis (Fernández et al., 2021). Across this line of work the phosphene is the elementary unit of an artificial visual display written directly onto the cortex.

Worked Example

The saturation that Bosking and colleagues observed — phosphene size growing with current and then levelling off — can be made quantitative, and doing so shows why a visual prosthesis cannot simply 'turn up the current' to convey more. Model the phosphene diameter S as a saturating (hyperbolic) function of the stimulating current I above an activation threshold I​th: S(I) = S​max · (I − I​th) ⁄ [(I − I​th) + I₅₀], where S​max is the largest attainable diameter and I₅₀ is the current above threshold that yields half of it. Take illustrative values S​max = 2.0°, I​th = 100 µA, and I₅₀ = 300 µA, in the qualitative regime the study describes.

Evaluating the model shows the saturation directly. At I = 200 µA the phosphene is 0.50° across — a quarter of the maximum; at 400 µA it reaches 1.00° (half of maximum); at 1000 µA, 1.50° (three-quarters); and even at 2000 µA it has grown only to 1.73°, still short of the 2.0° ceiling. The diameter approaches its asymptote but never exceeds it, no matter how large the current.

The consequence for coding is in the marginal effect of current, which collapses as the phosphene saturates. Near threshold, doubling the drive from 150 µA to 300 µA enlarges the phosphene from 0.29° to 0.80°, a 180% increase. Deep in saturation, the same doubling from 700 µA to 1400 µA enlarges it from 1.33° to 1.63°, a mere 22% increase — and larger phosphenes carry no finer detail. A prosthesis must therefore convey shape by which electrodes fire and in what sequence, as in dynamic current steering, rather than by how hard any one electrode is driven (Bosking et al., 2017; Beauchamp et al., 2020). The `PhospheneSizeCurrentDemo` above plots this curve as the current and parameters are varied.

Discussion

Phosphenes matter to cognitive psychology because they separate the sensation of light from light itself and localize where that sensation is made. The century of work surveyed here converges on a single organizing question — where along the visual pathway a given phosphene originates — and answers it method by method: mechanically evoked phosphenes are retinal, transcranial electrical phosphenes are largely retinal despite the cortical intent of the stimulation, and magnetic and direct cortical stimulation reach the cortex (Grüsser & Hagner, 1990; Kar & Krekelberg, 2012; Kammer, 1999). This matters beyond curiosity: an experimenter who misreads a retinal phosphene as a cortical one will draw wrong conclusions about what a stimulation method does, and a prosthesis designer must know exactly which neurons a current recruits.

The account has open seams. The mechanisms by which weak transcranial currents reach and excite the retina, and the extent to which any component of a tES phosphene is cortical, are still debated (Schutter & Hortensius, 2010). For cortical stimulation, how the discrete phosphene from a single electrode relates to the continuous percept of natural vision, and how many independent phosphenes the cortex can support before they merge, remain only partly understood (Tehovnik & Slocum, 2013). These are precisely the questions that a working visual prosthesis must answer.

Current Directions

The most active front is the cortical visual prosthesis. The demonstration that a hundred-channel intracortical array can evoke patterned phosphenes in a blind person over an extended period established that many independent electrodes can be driven safely and that their phosphenes are stable enough to form the pixels of an artificial display (Fernández et al., 2021). The parallel advance is in how those pixels are used: rather than illuminating static dots, dynamic current steering traces continuous trajectories across the cortical surface, and both sighted and blind participants perceive the traced shapes as forms, which is far closer to functional vision than a scatter of isolated spots (Beauchamp et al., 2020). The saturation of phosphene size with current is one of the design constraints this work must respect, since it caps what a single electrode can convey and pushes the coding of shape onto spatial and temporal patterns of stimulation (Bosking et al., 2017).

A second thread continues to clarify the origins of non-invasive phosphenes, sharpening the tools of stimulation research. Establishing that transcranial electrical and alternating-current phosphenes are largely retinal has reframed how tES and tACS studies interpret the sensations reported by their participants and has motivated better controls for retinal co-stimulation (Kar & Krekelberg, 2012; Schutter & Hortensius, 2010). Together these directions treat the phosphene not as a curiosity but as both an engineering primitive for restoring sight and a diagnostic for what a stimulation method actually does to the visual system (Tehovnik & Slocum, 2013).

Common Misconceptions

A phosphene means light has somehow reached the eye.

By definition a phosphene occurs without any light entering the eye; the sensation is generated by mechanical, electrical, magnetic, or direct neural stimulation of the visual system itself (Grüsser & Hagner, 1990).

Phosphenes from currents across the scalp prove the visual cortex is being stimulated.

Phosphenes from transcranial electrical and alternating-current stimulation are largely retinal in origin, produced by current reaching the eye, so their appearance is not evidence of cortical stimulation (Kar & Krekelberg, 2012; Schutter & Hortensius, 2010).

A stronger stimulating current always yields a larger, more useful phosphene.

Phosphene size saturates with current: beyond a point it stops growing, and a larger phosphene carries no finer detail, so information must be conveyed by which electrodes fire and in what sequence rather than by current alone (Bosking et al., 2017).

Glossary

Cortical magnification.

The property of the retinotopic map whereby the central visual field is represented by disproportionately more cortex than the periphery, so that a fixed cortical distance corresponds to a smaller field distance near the fovea.

Deformation phosphene.

A phosphene produced by mechanically bending or stretching the retina, for example by pressure on the eyeball; the mechanical stimulus excites retinal neurons directly.

Electrical phosphene.

A phosphene evoked by an electrical current, whether applied across the scalp (transcranial) or delivered directly to neural tissue; transcranial electrical phosphenes are largely retinal in origin.

Intracortical microstimulation.

Stimulation of the visual cortex by fine electrodes inserted into the tissue, which evokes small, discrete phosphenes at much lower currents than surface electrodes require.

Magnetic phosphene.

A phosphene evoked by transcranial magnetic stimulation, in which a magnetic pulse induces a current in the visual cortex; unlike transcranial electrical phosphenes these appear to be genuinely cortical.

Phosphene threshold.

The minimum stimulation intensity at which a phosphene is reliably reported; used as an index of the excitability of the stimulated part of the visual system.

Phosphene.

A visual sensation of light occurring without light entering the eye, produced by mechanical, electrical, magnetic, or direct neural stimulation of the visual system.

Photoreceptor.

A light-sensitive retinal neuron (a rod or cone) that normally transduces light into a neural signal; deforming or electrically exciting photoreceptors and their downstream neurons produces a phosphene without any light.

Pressure phosphene.

A deformation phosphene produced specifically by pressure on the eye, typically seen as a glowing patch in the part of the visual field opposite the point of pressure.

Primary visual cortex (V1).

The first cortical area to receive visual input, at the occipital pole; its orderly retinotopic map means that stimulating a given site evokes a phosphene at the corresponding position in the visual field.

Retinotopy.

The orderly mapping of the visual field onto the retina and the visual cortex, whereby neighbouring points in the field are represented at neighbouring locations, so that stimulating a cortical site produces a phosphene at the corresponding field position.

Transcranial alternating current stimulation (tACS).

A non-invasive method that applies a weak oscillating current across the scalp; over the occipital region it evokes flickering phosphenes that are largely of retinal origin.

Transcranial magnetic stimulation (TMS).

A non-invasive method that uses a brief magnetic pulse to induce a current in the cortex; applied over the occipital pole it evokes cortical phosphenes and transient scotomata.

Visual prosthesis.

A device that restores rudimentary vision by electrically stimulating the visual system — here the visual cortex — to evoke patterned phosphenes that serve as the elements of an artificial visual display.

Key Researchers

Michael S. Beauchamp. Neuroscientist at the University of Pennsylvania (formerly Baylor College of Medicine); he showed that dynamic current steering across electrodes on the human visual cortex can trace shapes, producing form vision from sequences of phosphenes in both sighted and blind participants. ORCID - Google Scholar - Faculty Page - Wikidata

Eduardo Fernández. Neuroengineer at the Miguel Hernández University of Elche; he led the development of intracortical visual neuroprostheses, evoking patterned phosphenes with a ninety-six-channel microelectrode array implanted in the occipital cortex of a blind volunteer. ORCID - Faculty Page

Otto-Joachim Grüsser. Neurophysiologist at the Free University of Berlin (1932–1995); he wrote the definitive history of phosphene research, tracing the idea of an inner light generated in the eye from antiquity and grounding the modern phenomenology of pressure and electrical phosphenes. Wikipedia - Wikidata

Bart Krekelberg. Vision scientist at Rutgers University–Newark; he demonstrated experimentally that phosphenes evoked by transcranial electrical stimulation over the visual cortex have a retinal rather than a cortical origin, constraining the interpretation of transcranial electrical stimulation effects. ORCID - Google Scholar - Faculty Page - Wikidata

Dennis J. L. G. Schutter. Experimental biopsychologist at Utrecht University; he provided converging evidence that phosphenes produced by transcranial alternating current stimulation arise in the retina, informing the mechanism and controls needed for transcranial alternating current studies of vision. ORCID - Google Scholar - Faculty Page

Vincent Walsh. Cognitive neuroscientist at University College London; he used transcranial magnetic stimulation to induce phosphenes in sighted, blind, and blindsighted observers, mapping the cortical origin of magnetically evoked phosphenes and their use as a probe of visual awareness. ORCID - Faculty Page

Daniel Yoshor. Neurosurgeon at the University of Pennsylvania; on the human visual-cortex stimulation studies he characterized how phosphene size scales with stimulating current and how electrode arrays over V1 can be driven to generate perceptible form. ORCID - Faculty Page

Frequently Asked Questions

What is a phosphene?
A phosphene is a visual sensation of light that occurs without any light entering the eye. It is produced by some other form of stimulation of the visual system, such as mechanical pressure on the eye, an electrical current, a magnetic pulse, or direct stimulation of neural tissue, and it shows that the sensation of brightness is generated by neural activity, not only by light (Grüsser & Hagner, 1990).

Why do lights appear when the eyes are rubbed?
Rubbing or pressing on the closed eye mechanically deforms the retina, which excites the photoreceptors and their downstream neurons as though light had struck them. The result is a pressure phosphene, a glowing patch usually seen in the part of the visual field opposite the point of pressure (Grüsser & Hagner, 1990).

Do phosphenes come from the eye or the brain?
It depends on how they are produced. Mechanical and most transcranial electrical phosphenes arise in the retina, whereas transcranial magnetic stimulation and direct cortical stimulation evoke phosphenes in the visual cortex. Identifying the true site of origin for a given method has been a central problem in the field (Kar & Krekelberg, 2012; Kammer, 1999).

Are the phosphenes from transcranial electrical stimulation cortical?
Largely not. Careful experiments show that phosphenes from transcranial electrical and alternating-current stimulation are mostly retinal, produced by current spreading to the eye rather than by modulation of the visual cortex, which is an important control in stimulation studies (Kar & Krekelberg, 2012; Schutter & Hortensius, 2010).

How does transcranial magnetic stimulation produce phosphenes?
A magnetic pulse delivered over the occipital scalp induces a brief electrical current in the underlying visual cortex, driving cortical neurons to signal light. The position of the resulting phosphene in the visual field depends on which part of the retinotopically organized cortex is stimulated (Kammer, 1999; Cowey & Walsh, 2000).

Can stimulating the brain give a blind person vision?
Electrical stimulation of the visual cortex evokes discrete phosphenes at stable, separated positions, and stimulating patterns or sequences of electrodes can produce recognizable shapes. This is the basis of cortical visual-prosthesis research, which uses phosphenes as the pixels of an artificial visual display (Brindley & Lewin, 1968; Beauchamp et al., 2020; Fernández et al., 2021).

Why does a phosphene stop growing when the current is increased?
The size of a cortically evoked phosphene increases with stimulating current and then saturates, reaching a ceiling beyond which more current does not enlarge it. Because a larger phosphene carries no finer detail, a prosthesis must convey shape through the pattern and sequence of stimulation rather than through current strength alone (Bosking et al., 2017).

What are phosphenes used for in research?
Phosphenes serve as a probe of the visual system. Their threshold and location index the excitability and organization of the cortex, magnetic phosphenes help identify which parts of the visual system are needed for a conscious visual sensation, and cortically evoked phosphenes are the working elements of visual prostheses (Cowey & Walsh, 2000; Tehovnik & Slocum, 2013).

References