What is the location of the center-surround receptive fields of retinal ganglion cells? I have read wikipedia article about receptive fields of visual system and it states 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, all the rods and cones from one eye.




I understand the part where it says that the neurons firing increases as we put light on the center of the receptive (for the ON center type) and etc.

What I'm not sure about is the location of this circle, in other words, where is this circle that we put light on?

What I think is that the circle is the area of retina where we have the rod and cone shaped photoreceptors. Does this means that for every ganglion cell the photoreceptors connected to it are always shaped like a circle ?
A nice illustrative image of how the photoreceptors connect to the retinal ganglion cell (RGC), and thereby facilitate the center-surround structure is provided in figure 1 below. It shows a cross section through the retina.


Fig. 1. Photoreceptor organization and connections to the retinal ganglion cell. source: McGill University   

The receptive fields (RFs) are determined by the number of photoreceptors connected to the RFs. RFs are smaller in the central high acuity area of the retina (the fovea) and increase in size more eccentrically. As a result, visual acuity is highest in the center visual field, and degrades more eccentrically in the field of view.