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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

enter image description here

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?

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The center-surround structure is created by the horizontal connectivity of the horizontal cells (Periman et al., 2012) that receive input from multiple photoreceptors (Fig. 1).

![center-surround
Fig. 1. Retinal patch showing the connectivity between a horizontal cell and photoreceptors. Source: Harvey Mudd College

The horizontal cell passes on the congregate output to the bipolar cell, which in turn drives the retinal ganglion cells (RGCs) that send their axons to the optic nerve and send the signal on its way to the brain.

The photoreceptors are connected in two concentric circles to the horizontal cell. When light stimulation of the inner patch (center) evokes an excitatory response in the RGC, and the outer circle (surround) inhibits this response, the receptive field is said to have an ON-center configuration. In the reverse scenario, it is OFF-center (Fig. 1). This center-surround configuration is one of the first visual processing steps in the visual system and enhances contrast in the perceived image.

ON and OFF center
Fig. 2. ON and OFF center-surround receptive fields. Source: Brain Connection

Reference
- Periman et al., Webvision. The Organization of the Retina and Visual System. Utah

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