Many mammalian visual cortex neurons show orientation selective responses to “illusory contours” formed by abutting gratings. Our recent results demonstrate two distinct modes of neuronal response to these stimuli (Song and Baker, SFN 2003). Here we demonstrate how spatial filtering models of receptive fields, like those previously proposed for responses to other kinds of second-order stimuli, can predict both these modes of response. The model sums two parallel pathways. One path is a simple linear filter (F0), whose parameters determine the selectivity to first-order stimuli such as sinewave gratings. The other path is a filter-rectify-filter (F1-R-F2) cascade, in which F1 is tuned to much higher spatial frequencies than F2. Responses of this model were simulated and compared to single neuron responses in cat area 18. Illusory contour stimuli consisted of abutting (inducing) gratings whose relative spatial phase periodically reverses. If the inducing grating’s spatial frequency is relatively low, then th...