I know this is a basic question, but I can't find the answer anywhere. How can a cell that is not photoreceptive (eg LGN and retinal gangion cells) have a receptive field that is stimulated by "light" (as most textbooks explain it). Isn't it truer to say that they are stimulated by incoming neuronal inputs - in which case, how can they have a "circular surround"? Clearly I'm lost here, so all help appreciated! Thanks!
The (visual) receptive field of a neuron is the relation between the light that falls on the retina and the change in activity of that neuron, regardless of what happens in between the light and the neuron. Therefore, RGC, LGN, V1, or even higher order visual cortical neurons all have some sort of receptive fields. Not because they respond directly to patterns of light, but through cascade of signals that originate from the photoreceptors. So, yes, many LGN neurons would have center-surround receptive fields.