Homogeneity and diversity of color-opponent horizontal cells in the turtle retina: Consequences for potential wavelength discrimination

Color information processing in fish and turtles starts with the transformation of the tetra-chromatic cone system into two types of color-opponent horizontal cells (C-type). Few studies reported on large variability between C-type horizontal cells of the same class, suggesting it might improve color vision. However, such variability is contradictory with the tight coupling between horizontal cells that tends to average intercellular differences. We addressed this apparent discrepancy, and studied the spectral properties of C-type horizontal cells in the turtle retina. Photoresponses were recorded in the eyecup preparation, using light stimuli of different wavelengths and intensities. The spectral properties of each cell were defined by the neutral points (wavelengths at which response polarity reversed), which were derived from sensitivity data and from large-amplitude photoresponses. For each C-type horizontal cell, a linear relationship between log stimulus intensity needed for polarity reversal and wavelength was found. With this definition, homologous C-type horizontal cells from the same retina were practically identical in their spectral properties, indicating that the averaging effects of the horizontal cell syncytium eliminated any intercellular variability. In contrast, C-type horizontal cells of the same class exhibited large inter-retina variability. We tested the potential for wavelength discrimination by applying the line element theory to the action spectra of the two chromatic (Red/Green &Yellow/Blue) horizontal cell channels, and found good agreement with behavioral data from a similar species of turtles.