SummaryAlmost every neuron in the early visual system has some form of an antagonistic receptive field surround. But the function of the surround is poorly understood, especially under naturalistic stimulus conditions. Anatomical and functional characterization of the surround of retinal ganglion cells suggests that surround signals combine with the excitatory feedforward pathway upstream of an important synaptic nonlinearity between bipolar cells and postsynaptic ganglion cells. This circuit architecture suggests at least two unexplored consequences for receptive field structure. First, center and surround inputs should interact nonlinearly, even prior to the summation across visual space that forms the full receptive field center. Second, inputs to the surround may alter the sensitivity of the center to spatial contrast in a scene. We test these hypotheses using both synthetic and naturalistic visual stimuli, and find that the statistics of natural scenes promote these nonlinear interactions. This work shows that, unexpectedly, the surround modulates spatial contrast encoding based on visual context.