What is an on-response in a retinal ganglion cell?

In the context of the physiology of the retina, an on-response in a retinal ganglion cell refers to a specific pattern of neural activity triggered by light stimulation.12

When a spot of light falls on the center of an "on-center" receptive field, it provokes a discharge in the ganglion cell or its associated optic nerve fiber. In mammals, this response typically manifests as an increase in the cell's background discharge rate, which is otherwise present in darkness. Conversely, when light is switched off in these regions, the cell often exhibits an inhibition of that background discharge.12

This response is part of a center-periphery organization where the central and annular regions of the receptive field possess opposite properties. Because these cells respond to increases or decreases in local light intensity, they function primarily as detectors of local contrast rather than simple light intensity meters.13