Note on the Spectroscopy of Subjective Light Patterns

THE ELECTRICAL stimulation of subjective (“entoptical”) light phenomena has been described as early as 1816 by Volta; 1819 by Purkinje, who excited “Galvanic light patterns” (stripes and arches) with a voltaic cell of 20 V; 1909 by Helmholtz; and later by Rohracher (1935-6), during excitation with a.c. as well as in magnetic fields; Schwarz (1938, 1940a (p. 92), 1940b, 1941); Lohmann (1940); and Motokawa (cf. Gebhard, 1953). With the exception of Purkinje, their work deals mainly with the alternating current threshold for flicker; in general, the light phenomena were ascribed to stimulation of the retinal ganglion cells by the electric current.

In an earlier paper (Knoll, 1958) it was shown that (besides flicker) a whole spectrum of subjective geometric light patterns can be excited in the brain by temporal electrodes; they correspond to patterns found by Penfield and Rasmussen (1955, p. 10) during direct electrical stimulation of the visual cortex during brain surgery. Moreover, the similarity of these patterns to subjective light phenomena produced without electrical stimulation (i.e. by mechanical, chemical, or acoustical shock or epileptiform cortical discharge) was evident. This was found in particular for the “bright dot” or “scintillation” patterns which had been described already by Purkinje (1819), and later by Ebbecke (1943), Hess (1904), Ahlenstiel (1944), and Bachem (1945) as a result of mechanical or acoustical shock, and by Ebbecke (1948) as a spontaneous phenomenon after dark adaptation. In addition, some subjects reported subjective light patterns which remained for seconds or minutes after the electric field was shut off (“after-discharge patterns”).

The 20 subjects investigated in this earlier work (Knoll, 1958) belonged to various professional and age groups. In the present paper, excitation effects experienced by subjects belonging to two more groups (24 and 22 subjects respectively) are described. They were composed of clinical patient