Figure 1: Effects of size of a disk grating on the monkey's OFR response. We present here the gain of the oculo-motor response to a central grating (temporal frequency 10 Hz) as a function of its diameter for the macaque monkey (open circles) and the model (continuous line). Fits were performed as a function of the diameter d thanks to equations derived in the text. (Left) At low frequencies (0.12~cpd) and contrasts, the gain increases monotonically with the diameter. The curves are well fitted by the simpler equation with only an excitatory pooling. (Right) However in more general conditions (here 0.7~cpd), the initial gain decreases after a given diameter suggesting a suppressive effect. This corresponds to a surround inhibition which is well captured by the equation, the inhibition being more pronounced when contrast is higher and contrary to intuition, the diameter $\omega$ extracted from the fits remains constant across curves. This provided a general explanation of the ratio-Of-Guassian model in a probabilistic framework.

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