Fig. 2.

Classical and Extra-Classical Receptive Fields in the LGN. (A) The classical receptive field (CRF) comprises a central on or off region and a surrounding ring having the opposite sign. For on-center cells, light in the center excites the cell and light in the surround inhibits the cell; the reverse is true for off-center cells. Firing rate is approximately linearly determined by weighting the light in the center and surround regions. (B) The CRF can be modeled as the sum of two Gaussians, shown in section through the center of the field, a narrower excitatory region shown in red and a broader inhibitory one shown in blue for the example on cell here. The sum of the two is in black, and forms the well-known Mexican Hat profile. (C) The same difference of Gaussians is shown in a full two dimensional plot where color ranges from deep red for excitatory, through white for indifferent, and deep blue for inhibitory. Since the inhibitory field is not as strong as the excitatory field, it does not reach into deep blues, but remains at lighter ones. (D) The ECRF is an as-yet poorly defined region that is larger than the CRF, and is shown here in hatched gray. The reader should note that the ECRF may also extend through the area of visual space in which the CRF resides. Stimuli in the ECRF modulate the response to stimuli in the CRF, but without being able to directly generate spikes. Current thought holds that the ECRF provides contrast-dependent gain control on CRF sensitivity.