Field potential induced by injection of potassium ion into the frog retina: a test of current interpretations of the electroretinographic (ERG) b-wave.

Using excised frog retinas, the field potential induced by mechanical intraretinal injection of K+ Reinger's was mapped by a penetrating electrode. The high K+ Ringer's contained 75 mM K+ and the amount per injection was of a sphere about 40-50 micron in diameter. The K+ injection pipette was so made that it also served for recording potentials at the site of K+ injection. The study revealed that: (1) a maximum negativity is produced at the depth of K+ injection; and that (2) the negativity subsides in both directions, proximally and distally; but that (3) when recording from the retinal surfaces, the vitreal side becomes positive relative to the receptor side, irrespective of the retinal depth of K+ injection, excepting for the innermost layer of some 20 micron in thickness where some uncertainty remains. The results indicated that the "neutral zone", as defined as a zone where a high K+ produces no potential differences between the opposite retinal surfaces, is localized very close to the inner limiting membrane. Recent interpretations of the b-wave on the Müller cell hypothesis are discussed on the basis of our results.