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In the human eye, all visual information must traverse the retinal ganglion cells. The most numerous subclass, the midget retinal ganglion cells, are believed to underlie spatial pattern vision. Thus the density of their receptive fields imposes a fundamental limit on the spatial resolution of human vision. This density varies across the retina, declining rapidly with distance from the fovea. Modeling spatial vision of extended or peripheral targets thus requires a quantitative description of midget cell density throughout the visual field. Through an analysis of published data on human retinal topography of cones and ganglion cells, as well as analysis of prior formulas, we have developed a new formula for midget retinal ganglion cell density as a function of position in the monocular or binocular visual field.

We have recently shown that fixational eye movements improve discrimination of the orientation of a high spatial frequency grating masked by low-frequency noise, but do not help with a low-frequency grating masked by high-frequency noise (M. Rucci, R. Iovin, M. Poletti, & F. Santini, 2007). In this study, we explored the neural mechanisms responsible for this phenomenon. Models of parvocellular ganglion cells were stimulated by the same visual input experienced by subjects in our psychophysical experiments, i.e., the spatiotemporal signals resulting from viewing stimuli during eye movements. We show that the spatial organization of correlated activity in the model predicts the subjects' performance in the experiments. During viewing of high-frequency gratings, fixational eye movements modulated the responses of modeled neurons in a way that depended on the relative alignment of cell receptive fields. Responses covaried strongly only when receptive fields were aligned parallel to the grating's orientation. Such a dependence on the axis of receptive-field alignment did not occur during viewing of low-frequency gratings. In this case, the responses of cells on the parallel and orthogonal axes were similarly affected by eye movements. These results support a role for oculomotor synchronization of neural activity in the representation of visual information in the retina.

In behavioral tests, tammar wallabies (Macropus eugenii) are dichromats. We investigated the neural basis for this color discrimination by making patch clamp recordings from retinal ganglion cells in an in vitro preparation. Pseudo-random noise stimuli were used to probe the spectral and temporal properties of the receptive fields. Color opponent ganglion cells were excited by medium wavelength-sensitive cones and inhibited by short wavelength-sensitive cones, and were classified as M-on/S-off cells. The S-off response was delayed by 15 ms relative to the M-on response, but, otherwise, the time course of the two responses was very similar. Second-order nonlinear response components, estimated by nonlinear systems analysis, served to accentuate the color opponency. Possible synaptic mechanisms underlying the cone opponent inputs are discussed.