Extensive attempts have been made to explain the neurobiological basis of the greater sensitivity of the visual system to vertically or horizontally oriented information than to information presented at oblique angles. However, investigators have largely ignored the overlap of the representation of a given angle with the representation of nearby angles. Recordings based on intrinsic optical signals were obtained in area 17 from 12 adult cats during the presentation of contours in various orientations. A method investigating both amplitude and statistical significance of changes was proposed to evaluate the orientation tuning properties for cell populations in the central area retinotopically corresponding to 0-15° of visual field. Cardinal orientations were found to activate significantly greater areas in the exposed cortical area than the areas activated by oblique orientations. Areas activated by cardinal or oblique contours and those separated from them by 10° were compared. A significantly lower degree of overlap was seen between areas activated by presentation of cardinal contours and areas activated by neighboring orientations compared to those for oblique orientations which overlapped more extensively with neighboring orientations. In addition, areas activated only by cardinal contours were significantly larger than areas activated only by oblique contours. These results demonstrated in cell population level that more cells prefer horizontal or vertical orientations, and these cells are tuned more sharply than oblique selective cells.