This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The goal of this project is determine how the receptive field surround is established at the level of the ganglion cell and how the surround contributes to color-coding by identifying the cone inputs to the surround. The presynaptic hypothesis predicts that the creation of the cone bipolar center-surround organization is the critical locus for cone opponency. We showed for the first time in a mammalian retina that cone bipolar cells have robust center-surround organization comparable to their ganglion cell counterparts. We then undertook a series of experiments to attempt to selectively block ganglion cell surrounds. We found, in parasol cells, that either cobalt or the gap-junction blocker, carbenoxolone, selectively reduces surround antagonism. We are now using these drugs to assess the receptive field surround contribution to color-opponency in a number of ganglion cell classes. In a second series of experiments, we developed a new stimulus protocol to measure the strength and sign of L, M and S-cone inputs to the receptive field surrounds of midget and parasol ganglion cell types. The specific goal was to assess how cone signal weighting may be altered during transmission from outer to inner retinal circuitry. We showed that there are no gain changes from the horizontal-bipolar cell level to the ganglion cell surround and concluded that cone type-specific changes in synaptic gain are not a likely mechanism for generating color-opponent signals.
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