Visual responses to contrast-defined contours with equally spatial-scaled carrier in cat area 18

Contrast-defined contours are one type of second-order contours, across which there are no differences in luminance. Although they can be always perceived, their responses have been only investigated when the spatial frequency of carrier, the background texture whose contrast is modulated to form contours, is much higher than that of contrast-defined contours, due to the interference of responses to luminance contours in other cases. In the present study, we examined visual responses in cat area 18 to the contrast-defined contours with carrier at same spatial frequency equal to neuron's preferred value for luminance contours, by establishing a control stimulus including all the luminance components but lack of the contrast contour information. Using single unit recording and intrinsic optical imaging, we demonstrated that contrast gratings with equally spatial-scaled carrier induced responses in a proportion of cat area 18 neurons with the preferred orientation similar to that for luminance contours, and the responses generated orientation maps similar to those for luminance contours. Our finding suggests that early visual cortex can process second-order contours regardless of the spatial frequency of carriers, in a way similar to the processing of luminance contours. This uniform manner of early visual processing might underlie the visual detection of both luminance contours and non-luminance second-order contours.