The aim of this investigation was to understand the functions of long horizontal connections projecting from area 17 to area 18 in cats. The animals were anesthetized and prepared for recording single-cell responses to sine-wave gratings in area 18. Neuronal activity was analyzed under three conditions: prior to, during, and after inactivation of a circumscribed region of area 17. The latter was depressed with micro-injections of GABA. Cells in both areas were in close retinotopic correspondence. Cells were classified as simple and complex types. Globally, simple cells were less affected than complex units, and those which were affected shifted their optimal spatial frequency to higher values. Complex neurons were more often influenced by the interruption of area 17 input. Namely, the peaks of the tuning curves were displaced on the x-axis to a new optimal spatial frequency. This effect was obtained by a dual change: a decline in the discharge strength to the optimal spatial frequency and an enhancement to nonoptimal spatial frequency. Contrast sensitivity function disclosed similar shifts of optimal spatial frequencies. Likewise bandwith, spatial resolution, high cutoff, and low cutoff were modified to a greater extent in complex cells. It appears that there is no relationship between areas 17/18 orientation difference and the modifications observed in tuning curves to spatial frequencies. The results suggest that neurons of area 18 may carry multiple-frequency channels and that area 17 facilitates the emergence of one particular spatial frequency.