Illusory contour orientation discrimination in the cat.

We present the first evidence that a non-human species (the cat) is able to discriminate the orientation of illusory contours. Following Vogels and Orban45, we used two types of illusory contours. In one type, the illusory contour was defined by a number of contour-inducing semicircles, of which the endpoints were separated by a gap. In the other pattern, the inducing semicircles were shifted in phase along their diameter and their endpoints were aligned along the contour. Just noticeable differences in orientation were measured (at the 73.5% correct level), using a Wetherill and Levitt49 staircase procedure. Values in the order of 11 degrees were obtained when using the first type of illusory contour. Just noticeable differences with the second type were in the order of 17 degrees. Reducing the salience of the illusory contour, whether by scrambling the contour, or by decreasing the number or the contrast of inducing semicircles, systematically increased discrimination thresholds.