THE JOURNAL OF COMPARATIVE NEUROLOGY 358~134 (1995) The Organization of the Turtle Inner Retina. I. ON- and OFF-Center Pathways JOSEF AMMERMULLER AND HELGA KOLB zyxw Department of Neurobiology, University of Oldenburg, D-26111Oldenburg, Germany (J.A.), and John Moran Eye Center, University of Utah, School of Medicine, Salt Lake City, Utah 84132 (H.K.) ABSTRACT Intracellular recordings and dye injections of Lucifer yellow, horseradish peroxidase, or Neurobiotin were made in bipolar, amacrine, and ganglion cells of the zyxw Pseudemys turtle retina. By using a standard light-stimulation protocol in a sample of 375 labeled neurons, we were able to identify morphological and physiologicalcharacteristics of 11 types of bipolar cell, 37 types of amacrine cell, and 24 types of ganglion cell. To make sense of these data, we have chosen to group the 72 essentially different neuron types into traditional, functionally significant pathways. In this paper we look at the neuronal types in the inner plexiform layer (IPL) in terms of their contribution to generalized luminosity responses such zyxw as sustained ON- or OFF-center and transient ON-OFF ganglion cells; in the companion paper (J. Ammermuller, J.F. Muller, and H. Kolb, 1995, J . Comp. Neurol. 358:35-62) we look at them in terms of their involvement in color opponency and directional selectivity. A functional organization of the turtle IPL into OFF sublaminae (strata 1 and 2) and ON sublaminae (strata 3, 4, and 5), as has been described for other vertebrate retinas, was quite clear for two varieties of OFF-center bipolar cells (B4 and B5) and for all four types of sustained ON-center bipolar cell (Bl, B2, B6, and B7). Thus, we found no sustained ON-center bipolar cell terminating in strata 1 and 2. We did, however, see three varieties of sustained OFF-center bipolar cells (B3, B9, and B10) having axon terminals in strata 3-5 (the ON sublamina) in addition to their terminations in stratum 1 or 2 (the OFF sublamina). Monostratified sustained ON- and OFF-center amacrine and ganglion cells rigidly obeyed the border of ON and OFF sublaminae. However, multistratified and diffuse sustained amacrine and ganglion cells could be either ON-center or OFF-center, and they did not strictly obey the border: such ON-center cells always had processes in one of the ON sublaminae (strata 3-5), and the equivalent OFF-center cells always had processes in one of the OFF sublaminae (strata 1 and 2). Monostratified transient amacrine and ganglion cells were concentrated in the middle of the IPL (around stratum 3), whereas bi-, tri-, or multistratified transient amacrine or ganglion cells always had processes in both the ON and the OFF sublaminae. Our findings in turtle retina confirm the general “rule” of an IPL subdivided into ON- and OFF-center sublaminae, a rule that ensures that ON- and OFF-center monostratified sustained amacrine and ganglion cells are driven by ON- and OFF-center bipolar cell types, respectively.The bipolar, amacrine,and ganghon cellsthat appear to violatethe bisublaminarrule in turtle inner retina are those with multistratified morphologies. We discuss our findings in terms of structural principles underlying functional receptive field organizaton and physiological response characteristics of the multitude of cell types we have encountered in this species. The findings of this paper together with those in the following paper present the most comprehensive morphophysiologid descriptions of neurons contributing to the IPL of any vertebrate retina to date. Indexing terms: bipolar cells, amacrine cells, ganglion cells, receptive field, inner plexiform layer D 1995 Wiley-Liss, Inc. Extracellular recordings from turtle ganglion cells have shown that most of them have “complex” receptive fields rather than the “simple” concentrically organized fields that are more typical of cat retina (Kuffler, 1953; Enroth- cugell and Robson, 1966). In turtle, the majority of gan- Accept& November 20,1994. Address reprint requests to J. Ammermuller, Department of Neurobiol- zy ogy, University of Oldenburg, Postfach 2503, D-26111 Oldenburg, Germany. o 1995 WILEY-LISS, INC.