Surround suppression and RF expansion.

<p>(A) A plot illustrating that cortical neurons show surround suppression and expansion of CRF size at low contrast (reprinted by permission from Macmillan Publishers Ltd: Nature Neuroscience, <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003191#pcbi-1003191-g001" target="_blank">Figure 1a</a> from <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003191#pcbi.1003191-Sceniak1" target="_blank">[37]</a>). (B) The size tuning curve of a simulated sparse coding model neuron at various contrast levels (“c” stands for contrast, with lighter curves representing lower contrast). The model neuron exhibits two characteristic behaviors reported in the electrophysiology literature: suppression with increasing stimulus size and an increase in the optimal stimulus size with lower contrast. The maximum of each tuning curve is marked by an arrow. (C) Physiologically measured distribution of surround suppression index (SI) in cat V1 (data replotted from <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003191#pcbi.1003191-Walker1" target="_blank">[36]</a>, <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003191#pcbi-1003191-g002" target="_blank">Figure 2A</a>), illustrating that most cells do not exhibit significant surround suppression and the SI distribution is relatively uniform among suppressive cells. (D) The SI distribution for the model cells, illustrating the same qualitative properties as the distribution in (C). (E) Distribution of the SI difference (SI) between low and high contrast levels in macaque V1 (reprinted by permission from Macmillan Publishers Ltd: Nature Neuroscience, <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003191#pcbi-1003191-g006" target="_blank">Figure 6b</a> from <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003191#pcbi.1003191-Sceniak1" target="_blank">[37]</a>). The mean difference is 0.06, demonstrating that on average the SI for a cell is contrast invariant. (F) The distribution of SI for the sparse coding model cells. The mean difference is 0.02, also demonstrating contrast invariance in SI.