Editing Receptive field (section)

===Retinal ganglion cells===
[[Image:Receptive field.png|thumb|right|300px|On center and off center [[retinal ganglion cell]]s respond oppositely to light in the center and surround of their receptive fields. A strong response means high frequency firing, a weak response is firing at a low frequency, and no response means no action potential is fired.]]
[[Image:Red on centre green off centre.png|thumb|300px|right|A computer emulation of "edge detection" using retinal receptive fields. On-centre and off-centre stimulation is shown in red and green respectively.]]

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.

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 [[Edge detection|detecting objects' edges]].<ref>{{Cite book|title=Biological psychology|last=Higgs, Suzanne|others=Cooper, Alison (Senior lecturer in neurobiology), Lee, Jonathan (Neuroscientist), Harris, Mike (Mike G.)|isbn=9780857022622|location=Los Angeles|oclc=898753111|date = 2014-12-19}}</ref>{{Rp|188}} 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.

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 suppression|surround]] produces a ''[[hyperpolarization (biology)|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).

Photoreceptors that are part of the receptive fields of more than one ganglion cell are able to excite or inhibit [[postsynaptic neuron]]s because they release the [[neurotransmitter]] [[glutamate]] at their [[synapse]]s, which can act to depolarize or to hyperpolarize a cell, depending on whether there is a metabotropic or ionotropic receptor on that cell.

The ''center-surround receptive field organization'' allows ganglion cells to transmit information not merely about whether photoreceptor cells are exposed to light, but also about the differences in firing rates of cells in the center and surround. This allows them to transmit information about contrast. The size of the receptive field governs the [[spatial frequency]] of the information: small receptive fields are stimulated by high spatial frequencies, fine detail; large receptive fields are stimulated by low spatial frequencies, coarse detail. Retinal ganglion cell receptive fields convey information about discontinuities in the distribution of light falling on the retina; these often specify the edges of objects.  In dark adaptation, the peripheral opposite activity zone becomes inactive, but, since it is a diminishing of inhibition between center and periphery, the active field can actually increase, allowing more area for summation.