<P> Receptive fields have been mapped for all levels of the visual system from photoreceptors, to retinal ganglion cells, to lateral geniculate nucleus cells, to visual cortex cells, to extrastriate cortical cells . Studies based on perception do not give the full picture of the understanding of visual phenomena, so the electrophysiological tools must be used, as the retina, after all, is an outgrowth of the brain . </P> <P> Each ganglion cell or optic nerve fiber bears a receptive field, increasing with intensifying light . In the largest field, the light has to be more intense at the periphery of the field than at the center, showing that some synaptic pathways are more preferred than others . </P> <P> The organization of ganglion cells' receptive fields, composed of inputs from many rods and cones, provides a way of detecting contrast, and is used for detecting objects' edges . Each receptive field is arranged into a central disk, the "center", and a concentric ring, the "surround", each region responding oppositely to light . For example, light in the centre might increase the firing of a particular ganglion cell, whereas light in the surround would decrease the firing of that cell . </P> <P> There are two types of retinal ganglion cells: "on - center" and "off - center". An on - center cell is stimulated when the center of its receptive field is exposed to light, and is inhibited when the surround is exposed to light . Off - center cells have just the opposite reaction . On the edge between the two, in mammals, an on - off effect (i.e., discharging at switching on or off but not at a duration of either state) is present . Stimulation of the center of an on - center cell's receptive field produces depolarization and an increase in the firing of the ganglion cell, stimulation of the surround produces a hyperpolarization and a decrease in the firing of the cell, and stimulation of both the center and surround produces only a mild response (due to mutual inhibition of center and surround). An off - center cell is stimulated by activation of the surround and inhibited by stimulation of the center (see figure). </P>

The receptive field of a visual cell refers to the