The main source of bioelectric signals are those produced by excitable tissues - that is, nerve and muscle cells. There are, however, other spontaneous bioelectric signals. An important example is the electro-oculogram (EOG), which is discussed in this section. The EOG is not produced by an excitable tissue but by the static electric polarization of the eye. Through the movement of the eye it produces electric potential changes that can be measured around the eye. This phenomenon may, of course, also be detected magnetically. The EOG and its subdivision, the electronystagmogram (ENG) have wide clinical applications. These do not, however, fall directly within the scope of this book, and only the mechanism behind the generation of the EOG-signal and its measurement principle are discussed here.
There are excitable tissues in the eye and these produce bioelectric signals of the type discussed throughout the earlier chapters of the book. These include the electroretinogram (ERG), which is the electric response of the retina to light. Visual information proceeds from the retina to the central nervous system along the optic nerve. Since these bioelectric signals involve an excitable (nervous) tissue, they could, on the basis of their mechanism, have been included in Part IV (Neurological bioelectromagnetism). They are discussed here, however, in connection with other bioelectric signals originating in the eye.