...the alphabetic forms emerging from the phosphene flare patterns of the brain. Quoting: Anonymous Coward 6853315
A sensation of light caused by excitation of the retina by mechanical or electrical means rather than by light, as when the eyeballs are pressed through closed lids.
A phosphene is a phenomenon characterized by the experience of seeing light without light actually entering the eye. The word phosphene comes from the Greek words phos (light) and phainein (to show). Phosphenes are flashes of light, often associated with optic neuritis, induced by movement or sound.
Phosphenes can be directly induced by mechanical, electrical, or magnetic stimulation of the retina or visual cortex as well as by random firing of cells in the visual system. Phosphenes have also been reported by meditators (commonly called nimitta); people who go for long periods without visual stimulation (also known as the prisoner's cinema); or those who are using psychedelic drugs.
The most common phosphenes are pressure phosphenes, caused by rubbing the closed eyes. They have been known since antiquity, and described by the Greeks. The pressure mechanically stimulates the cells of the retina. Experiences include a darkening of the visual field that moves against the rubbing, a diffuse colored patch that also moves against the rubbing, a scintillating and ever-changing and deforming light grid with occasional dark spots (like a crumpling fly-spotted flyscreen), and a sparse field of intense blue points of light.
Pressure phosphenes can persist briefly after the rubbing stops and the eyes are opened, allowing the phosphenes to be seen on the visual scene. Hermann von Helmholtz and others have published drawings of their pressure phosphenes. One example of a pressure phosphene is demonstrated by gently pressing the side of one's eye and observing a colored ring of light on the opposite side, as detailed by Isaac Newton.
In recent years, researchers have successfully developed experimental brain-computer interfaces or neuroprosthesis that stimulate phosphenes to restore vision to people blinded through accidents.
Experiments with humans have shown that when the visual cortex is stimulated above the calcarine fissure phosphenes are produced in the lower part of the visual field, and vice versa.Phosphenes have also been created by intense, changing magnetic fields
, such as with transcranial magnetic stimulation. These fields can be positioned on different parts of the head to stimulate cells in different parts of the visual system. They also can be induced by alternating currents that entrain neural oscillation as with trancranial alternating-current stimulation. In this case they appear in the peripheral visual field.
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