On the history of deformation phosphenes and the idea of internal light generated in the eye for the purpose of vision

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Abstract
Deformation phosphenes are light sensations evoked by deformation of the eyeball in total darkness. They were first reported in Western literature by Alcmaeon of Croton in the fifth century B.C. The phenomenon of deformation phosphenes was instrumental in prompting some pre-Socratic philosophers and Plato to conceive the idea that efferent light is emitted from the eye for the purpose of vision and a ‘cone of vision’ is formed by interaction with the external light. In the theories of vision this cone of vision played an important role as a signal-transmitting structure and was also used by the Greek opticians as a geometrical construction to explain optical properties of vision. The impact of the deformation phosphene experiment on the ideas of visual sensation can be followed from Greek antiquity through the period of Roman dominance and Galen's medical teaching on to medieval times and up to the late Renaissance when, based on the anatomy of the eye as illustrated by Felix Platter, the image formation on the retina was correctly described for the first time by Johannes Kepler. In the generations following, deformation phosphenes were still employed as an important argument in defence of the theories of vision. However, the idea of physical light generated by eyeball deformation was rejected with increasing frequency during the 17th and 18th centuries. The literature on this topic is discussed, comprising the contributions of the Arabic philosophers and physicians of the 9th and 10th centuries A.D., the Franciscan and Dominican philosophers of the 13th century, Nicolaus Cusanus of the 15th century, several anatomists of the 16th and 17th centuries, Kepler, Plempius, Descartes, Boyle, Newton and others. After Kepler, the mechanical interpretation of the deformation phosphene being caused by direct action of the eyeball deformation onto the retina slowly became dominant, and the idea that physical light is generated in the eye disappeared. The experimentum crucis in this matter was performed by Giovanni Battista Morgagni (1682–1771) and repeated and extended by Georg August Langguth (1711–1782). On the basis of their results, the case for physical light being generated in the eye by deformation was refuted definitively and slowly vanished thereafter from scientific literature. Deformation phosphenes were used in the 19th and 20th centuries as an instructive example of the percepts evoked by inadequate stimulation of a sense organ. J.E. Pûrkyne in particular contributed to the study of deformation phosphenes, and finally in 1978, F. Tyler devoted a careful study to the differences between monocular and binocular deformation phosphenes. Finally some remarks on the neurophysiological interpretation of deformation phosphenes, based on microelectrode recordings of the activity of single retinal ganglion cells, are added to the historical report.

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