The wikipedia article about receptive fields of visual system tells us the following:
The receptive field is often identified as the region of the retina where the action of light alters the firing of the neuron. In retinal ganglion cells (see below), this area of the retina would encompass all the photoreceptors, i.e., all the rods and cones in the retina.
Then it shows some pictures
I understand the part where it says that the neurons firing increases as we put light on the center of the receptive field (for ON center type) and etc.
What I'm not sure about is the location of this circle; where is this circle that we put light on?
What I think might be the answer is that, as the quote says, the circle is the area of retina where we have the rod/cone shaped photoreceptors.
So this means that, for every ganglion cell, the photoreceptors connected to it are always shaped like a circle?