Electroretinographic (ERG) and extracellular potassium activity measurements were carried out in superfused eyecup preparations of several amphibians . Light-evoked changes in extracellular K+ activity were characterized on the bases of depth profile analysis and latency measurements and through the application of pharmacological agents that have selective actions on the retinal network. Three different extracellular potassium modulations evoked at light onset were identified and characterized according to their phenomenological and pharmacological properties. These modulations include two separable sources of light-evoked increases in extracellular K+: (a) a proximal source that is largely post-bipolar in origin, and (b) a distal source that is primarily or exclusively of depolarizing bipolar cell origin . The pharmacological properties of the distal extracellular potassium increase closely parallel those of the b-wave . A distal light-evoked decrease in extracellular potassium appears to be associated with the slow P111 potential, based on a combination of simultaneous intracellular Muller cell recordings and extracellular ERG and potassium activity measurements before and during pharmacological isolation of the photoreceptor responses. The extracellular potassium activity increases are discussed with respect to the Miller cell theory of b-wave generation .

Electroretinogram (ERG) and extracellular potassium activity (Kõ measurements were carried out in isolated superfused rabbit eyecup preparations under control conditions and during the application of pharmacological agents that selectively modify the light-responsive retinal network. Light-evoked Ko changes in the rabbit (E-type) retina resemble those previously described in amphibian (1-type) retinas. Different components of the lightevoked Ko changes can be distinguished on the bases of retinal depth, V vs . log I properties, and their responses to pharmacological agents . We find two separable sources of light-evoked increases in extracellular K+: a proximal source and a distal source . The properties of the distal light-evoked Ko increase are consistent with the hypothesis that it initiates a K+-mediated current through Muller cells that is detected as the primary voltage of the electroretinographic b-wave . These experiments also support previous studies indicating that both the corneal-positive component of c-wave and the corneal-negative slow P1I1 potential result from K+-mediated influences on, respectively, the retinal pigment epithelium and Muller cells.