Interaction of low frequency electric fields with the nervous system: The retina as a model system

The retina provides an example of effects, the visually perceived 'phosphenes', being generated in nervous tissue by external electric or magnetic fields of low frequency and intensity. What is known about the cellular mechanisms by which the phosphenes are generated is reviewed, whether they provide useful information for setting limits on the magnitude of induced electric fields to which nervous tissue can be safely exposed is assessed, and some difficulties in translating these values of internal fields into safe values of external electric or magnetic fields are considered.