Intracortical processing of visual contour information in cats.

Fine slices made through the gray matter perpendicular to the surface of cortical area 17-18 in the cat produced no change in behavioral measures of grating acuity, a modest reduction in vernier acuity and a proportionately larger effect on contour orientation acuity. The introduction of vertical gaps in the vernier acuity targets produced increases in the vernier offset threshold proportional to the gap size both before and after cortical damage, except at the largest vertical gap values employed (1-1.5 degrees) where no change in thresholds was observed after surgery. Neither the preoperative vernier thresholds nor the postoperative changes in vernier thresholds were predicted by the measured contour orientation thresholds and thus an orientation discrimination model of vernier acuity was not supported. The results of this study support the view that intracortical processing in area 17-18 is important in both vernier and orientation acuity, but the neural mechanisms mediating the two capacities are different.