Magneto- and electrophosphenes: A comparative study

The purpose of this study was to compare the threshold values for magnetophosphenes and electrophosphenes under identical experimental conditions. Such comparisons between the phosphene types would increase our knowledge of the mechanism of the interaction between magnetic fields and electric current, respectively, and excitable tissue. The phosphenes were generated in the frequency range 10–45 Hz at moderate magnetic flux densities up to 40 mT (400 G)] and electric currents up to 0.3 mA, respectively. The first part of the study was devoted to the problem of how electrode location and consequent current directions influence the threshold values of electrophosphenes. In the second part a comparison was made of the threshold values for electrophosphenes and magnetophosphenes under identical experimental conditions apart from the stimulation method. With electric-current stimulation in different directions no great differences were obtained with regard to the mean value for the threshold values within the frequency range 10–30 Hz. However, from 30 Hz upwards a significant difference developed between the threshold values for some of the curves. When generating electrophosphenes and magnetophosphenes we found significant differences in the threshold values between approximately 25 and 45 Hz. Both types of phosphenes had a concurring sensitivity maximum at 20 Hz. The deviations between the curves may be due, among other factors, to the generation of different current paths in electrical and magnetic stimulation, respectively.