Figure 1. Circuit basis of the surround and implications for receptive field (RF) structure.

(A,B) Two models for how the surround is integrated with the center to form the full RF: The linear-nonlinear model shown in (A) linearly combines the center and surround before treatment with an output nonlinearity that occurs after spatial integration. The model in (B) combines center and surround signals within each nonlinear subunit, and treats the combined center-surround signal with a private nonlinearity. In other words, individual subunits in (B) have their own RF surrounds. (C) Retinal circuit schematic. Bipolar cells make excitatory synapses on to retinal ganglion cells (RGCs). The nonlinear synaptic transfer from bipolar cell to RGC means that bipolar cells act as nonlinear subunits within the RF center of the RGC. The surround is generated by horizontal and/or amacrine cells, both of which can influence bipolar cell responses upstream of the synaptic nonlinearity. Surrounds that hyperpolarize bipolar cells shift the synapse into a more rectified state (blue portion of synaptic nonlinearity). Surrounds that depolarize bipolar cells shift the synapse into a locally linear state (red portion of curve). (D) This circuit organization suggested a working model of how inputs to the surround could change RF structure in the center. Shown is a schematic illustrating this hypothesis for an Off-center RGC. A weak or hyperpolarizing surround will be associated with rectified subunit output nonlinearities, and thus nonlinear spatial integration and sensitivity to spatial contrast (e.g. a split-field grating stimulus). A depolarizing surround will be associated with more linear subunit outputs and the RGC should integrate across visual space approximately linearly.