Purkynĕ's description of pressure phosphenes and modern neurophysiological studies on the generation of phosphenes by eyeball deformation.

(a) When a subject indents one of his eyeballs in total darkness, he immediately perceives light extending slowly across the whole visual field of the indented eye. The appearance and the time course of these pressure or deformation phosphenes are described. (b) With simultaneous binocular indentation of the eyeballs a flickering patterned phosphene is observed. (c) A short history of the research on pressure phosphenes and its consequences for the theories of vision is presented. (d) Purkynĕ's observations of monocular deformation phosphenes are described. He repeatedly noted patterned light structures, which most observers only perceive with simultaneous binocular eyeball deformation. It is suggested that Purkynĕ's deviating observations were caused by amblyopia of one eye. (e) The neurophysiological basis of the monocular pressure phosphenes was investigated by means of microelectrode recordings from single optic tract fibers. The activity of single retinal ganglion cells (on-center, off-center neurons, latency class I [Y-neurons] or latency class II [X-neurons]), was recorded in anaesthetized cats. Eyeball deformation in total darkness led to an activation of the on-center ganglion cells, while the off-center ganglion cells were inhibited. The latency and strength of this activation or inhibition varied considerably between different neurons, but were fairly constant in the same neuron when the eyeball indentation was repeated after a pause of 1-3 min. The latency and strength of neuronal activation or inhibition seemed to be dependent mainly upon the neuron location relative to the point of eyeball indentation. Some on-center neurons also exhibited a short activation at "deformation off". (f) The antagonistic response type of on-center and off-center ganglion cells was also observed when the eyeball was deformed as a hydrostatic open system and the intraocular pressure was kept at 25 mm Hg basic pressure. (g) Dark adaptation up to 45 min affected the deformation responses of retinal neurons only to a small degree, if at all. This corresponds to the observation that deformation phosphenes in a human observer changed little during the course of dark adaptation. (h) We assume that the activation of on-center and inhibition of off-center ganglion cells by eyeball deformation are caused by retinal stretching, which also leads to horizontal cell stretch. Stretching the horizontal cell membrane probably generates an increase in membrane sodium conductivity and a depolarization of the membrane potential. This depolarization of the horizontal cell membrane potential is transmitted either directly or indirectly (via receptor synapses) from the horizontal to the bipolar cells.(ABSTRACT TRUNCATED AT 400 WORDS)