Visual perception of orientation is categorical near vertical and continuous near horizontal

Four experiments were conducted to examine whether visual-orientation information is perceived categorically. In experiments 1 and 3, adult participants sorted oriented line stimuli into broad oblique and narrow vertical or horizontal categories. Experiments 2 and 4 showed that categorical discrimination of orientation occurred only near the vertical-oblique boundary. The data indicate that there is categorical perception near vertical and more continuous perception near horizontal. The results are relevant to the debate over whether categorical perception is derived from perceptual structure, verbal coding, or within-task learning. In addition, the asymmetrical perception of orientation around vertical and horizontal is consistent with the possibility that there may be differences in the functional significance of orientation near the two main axes.