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G. D. Field, J. L. Gauthier, A. Sher, M. Greschner, J. Shlens, T. A. Machado, D. E. Gunning, K. Mathieson, A. M. Litke, E. J. Chichilnisky; Functional Connectivity Between L and M Cones and Primate Retinal Ganglion Cells: Implications for Color Vision. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3475.
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To determine whether the functional connectivity between L and M cones and midget ganglion cells in peripheral macaque retina is selective or random.
Large-scale extracellular recordings were obtained from several hundred RGCs in segments of peripheral macaque retina (5-8 mm from fovea, recording area ~2 mm^2 or ~0.6 mm^2). Visual stimulation was provided by a computer display focused onto the photoreceptors. Two distinct stimuli were used to determine RGC receptive fields and chromatic properties: (1) a coarse white noise stimulus in which excitation of the L, M and S cones was modulated independently using silent substitution; (2) a fine-grained white noise stimulus in which the red, green, and blue display primaries were modulated independently. Results from the two stimulation approaches were validated against one another. The relative sensitivity of each cone to the red, green, and blue display primaries was used to unambiguously identify the spectral type (L,M,S).
Recordings were obtained from nearly complete collections of ON and OFF parasol and midget RGCs, with receptive fields that formed mosaics evenly covering the region of retina recorded. Coarse visual stimulation revealed a broad distribution of chromatic response properties in midget cells. Fine-grained white noise stimulation revealed the location and spectral type (L,M,S) of each cone feeding the receptive field center and surround. Thus, a complete functional mapping of the cone mosaic to ON and OFF midget and parasol cell mosaics was obtained. The possibility of random wiring between midget cells and L and M cones was explored using the following statistical test. Cone types were artificially permuted, and the resulting distribution of midget cell chromatic response properties was compared to those in the raw data. Permutation of cone types resulted in a small (~20%) but statistically significant narrowing in the distribution of chromatic properties of midget cells, inconsistent with random functional connectivity.
These results indicate that either connectivity between L and M cones to midget RGCs is selective, or L and M cones exhibit clumping within the cone mosaic which is not mimicked by the permutation test. Preliminary tests failed to reveal substantial cone clumping.[GDF, AS & JLG contributed equally]
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