B-wave current flow was analyzed using resistance values and b-wave amplitudes measured as a function of depth within the vitreous, retina and choroid of the frog eyecup. This current arose from a proximal current source which was restricted to a narrow region bordering the inner limiting membrane, and a diffuse current sink, which had two prominent peaks, one in the distal retina and the other in the proximal retina. This source density distribution suggests that Müller cells are involved in the establishment of the b-wave response.
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~c~~o~~ef~ge~eãmr.~ g-ri atefuft o Jerome Lettvin for his criticalr eadingo f the manus~ipta nd for his illuminating discussionsc oncerningr etinal currentsa nd circuitry. I thank EdwardG ruberga nd Janice Gepnerf or their helpful commentso n the text. This researchw as supported in part by the National instituteso f Health (Grant 5 TOl EYOOO9a0n) d in part by a grant from the Bell Telephone LaboratoriesI.n c.