Encoding of natural images by retinal ganglion cells

UnrestrictedNatural scenes have many special statistical properties that have shaped our visual system through natural evolution. It may thus be necessary to use natural images directly to examine retinal-ganglion-cells’ (RGCs) properties, rather than to extrapolate their properties from artificial stimuli. In this study, we first inspected what the most important visual property determining the responses of an RGC to natural images is. A new method was developed to estimate with natural images the sizes of the receptive-field (RF) center and surround. We showed that the center sizes estimated with our method were similar to those obtained with standard artificial stimuli. Furthermore, the temporal mean contrast of the center of the RF strongly dominated the RGC’s responses, while surround contrast mostly showed a weak and division-like (as opposed to subtractive) inhibition. We then asked whether the RGCs’ responses also depend on the local visual textures of natural images, or the luminance variation from the mean. We observed that RGCs responded asymmetrically between the transition from homogeneous backgrounds to natural images (onset), and the reverse transition (offset), even if both transitions had the same local temporal mean contrast. Furthermore, the negative of the natural images inverted this asymmetry, and their histogram equalization eliminated it. Hence, the response asymmetry arose from the asymmetric intensity distribution in the natural images. We further developed a method, spike-triggered contrast histogram (STCH), to demonstrate that a natural image with strong visual texture tended to elicit larger responses than one with weak texture. To account for these results, a nonlinear-linear model was developed. It included multiple subunits of nonlinear inputs, each covering a sub-region of the RF. Finally, we investigated whether the RGCs’ responses adapt to the spatial and temporal contrasts of natural images.; We found that RGCs displayed a variety of contrast-adaptation types across the population when responding to a step mean-contrast change of natural images, and that their contrast adaptation depended on the natural-image contrast difference. Thus, multiple biophysical mechanisms might be involved in RGCs’ contrast adaptation, and different RGC types might use a distinct subset of these mechanisms for different visual tasks