The slope values were calculated in such a way that zero corresponds to complete rectification
whereas a value of unity corresponds to linear summation of s1s1 and s2s2. Details of these quantifications can be found in Supplemental Experimental Procedures. To obtain the nonlinearities for the subunit model (insets in Figures 3A–3C), we calculated the ganglion cell response as a weighted sum of two inputs. The two inputs were selleck screening library generated from the respective stimulus components s1s1 and s2s2 by the same nonlinear function N(si)N(si). This function is parameterized as a power law for preferred stimuli with potentially incomplete rectification of nonpreferred stimuli. We determined the parameters of the nonlinear function for individual iso-response curves by a maximum-likelihood fit. To investigate the effect of subunit size on rectification in the iso-response curves for stimuli arranged in a checkerboard fashion (Figure 4C), we modeled a ganglion cell with 600 μm receptive field diameter, composed of circular subunits with varying sizes. Each subunit integrated the visual signal linearly and transmitted the result through a threshold-quadratic nonlinearity with incomplete rectification to the ganglion cell. The ganglion cell’s response was computed as a weighted sum over all subunit inputs, with weights determined
by a Gaussian curve, centered Selleck PCI-32765 on the midpoint of the ganglion cell receptive field. These responses were used to compute the slope of the iso-response curve in the same way as for the experimentally measured data. To quantitatively test the hypothesized circuit for homogeneity detectors based
on local inhibition (Figure 7C), we set up a model with two subunits that correspond to the inputs from each half of the receptive field. Each subunit comprises a bipolar cell and an amacrine cell. The bipolar cell transmits the contrast signal of its respective receptive field half as excitatory input to the homogeneity detector through a threshold-quadratic synaptic Terminal deoxynucleotidyl transferase nonlinearity. The amacrine cell receives the same excitatory input from the bipolar cell and provides inhibition through another threshold-quadratic nonlinearity. In addition, the amacrine cell signal is low-pass filtered to account for the temporal delay. From the integrated input to the homogeneity detector, we calculated iso-rate and iso-latency curves (Figures 7D and 7E). Details of the models are provided in Supplemental Experimental Procedures. We thank A. Borst for comments on the manuscript and the members of the Gollisch Lab for helpful discussions. This work was supported by the Max Planck Society, the German Initiative of Excellence, the International Human Frontier Science Program Organization, the German Ministry for Education and Research through the Bernstein Center for Computational Neuroscience Munich, and the Deutsche Forschungsgemeinschaft (DFG) through the Collaborative Research Center 889.