The response (R) of V1 neurons to elongated stimuli at the preferred
orientation varies sigmoidally as a function of contrast (C). This
contrast response function (CRF) accelerates at low C and saturates at
high C. It is characterized by the maximum firing rate (Rmax), the
steepness and the contrast at which R equals half Rmax (C50).
Experiments reveal a large diversity in these parameters. What are the
respective contributions of the feedforward input, the recurrent
intra-cortical input and the intrinsic properties of the neurons in
shaping the CRF? To answer this question we have developed a rate
model of a hypercolumn in V1, in which we assumed power-law
input-output transfer-function (TF) for the neurons. Assuming that the
exponent of the TF is larger than 1, the width of the orientation
tuning curve (OT) of the output is narrower than the OT width of the
input, and it is approximately contrast-invariance (CI). The response
varies sigmoidally with C, the acceleration at low C reflects the TF
non-linearity, whereas the saturation is strongly affected by the
intra-cortical inhibition and by the saturating properties of the
input. We have found similar behavior while testing the model with
conductance-based neurons, in which power-law TFs emerge naturally as
a result of the presence of noise in the inputs. A great deal of the
diversity observed in the shape of the CRF can be accounted for in our
model by assuming a large diversity in the input and cellular
properties. Based on these findings we predict a correlation between
the steepness and C50 of the CRF of neurons in V1.