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Magnetophosphene

Magnetophosphene

A perceived flash of light in the absence of a light source, provoked by magnetic stimulation of the eye or the visual cortex.

A magnetophosphenes is a type of phosphene, which is a perceived flash of light in the absence of a light source. Phosphenes can be induced by pressure on the eyelids. Magnetophosphenes are provoked by magnetic stimulation of the eye or the visual cortex. Induction of magnetophosphenes was reported in 1896 by Arsene D’Arsonval when human subjects were place inside a magnetic field generated by wire coils. D’Arsene’s report showed that electrical functioning of the human brain could be altered non-invasively by applying magnetic pulses. In 1910, evoked magnetophosphenes were attributed to the direct stimulation of low-level visual areas of the brain by Silvanus Phillips Thompson. Stimulation of the primary motor cortex (M1), eliciting movements in the contralateral arm was first stimulated in 1985 by Barker, Jalinous and Freeston using a simple round coil.

The international guidelines protecting the general public and workers from adverse effects from exposure to electromagnetic fields are based on the perception of magnetophosphenes at less than 300 Hz. Magnetophosphene perception increases with increasing levels of magnetic stimulation resulting from induced electric fields that modulate retinal neuron signaling.

Transcranial magnetic stimulation (TMS)

Devices used for Transcranial Magnetic Stimulation (TMS) generate magnetic fields by rapidly alternating current passing through the TMS coil. Electrical current is delivered to the coil by connection to a capacitator. The coil is placed on the scalp of an awake participant and a magnetic field of about 1-2 Tesla for approximately 100-200 microseconds is applied. An electrical current is induced in the neural tissue. TMS induced excitation of the occipital cortex induces phosphene perception in the contralateral visual field. The phosphene threshold is the stimulus intensity when a subject perceives the presence of phosphenes after stimulus 50% of the time. Phosphene perception requires the presence of an intact primary visual cortex (V1).

Conditions such as sensory deprivation, hypoglycaemia, fever, drug intoxication, psychotic episodes and epilepsy are thought to increase or decrease the incidence of phosphene perception. TMS is used to research and treat a neurological conditions including migraine, Parkingson’s disease and tinnitus, as well as psychiatric conditions such as depression and schizophrenia. Repetitive TMS (rTMS) is being researched for treatment of major depressive disorder, schizophrenia, cognitive disorders, obsessive-compulsive disorder and posttraumatic stress disorder.

An investigation using electroencephalogram (EEG) and the tracking of conscious perception with increasing strength of magnetic stimulus showed that increase in magnetic stimulation resulted in an increase of information integration at the brain-scale. The use a similar method to non-invasively characterise residual consciousness in minimally conscious patients was proposed by the authors.

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