The perception of 2D orientation is categorically biased.

Three experimental paradigms were used to investigate the perception of orientation relative to internal categorical standards of vertical and horizontal. In Experiment 1, magnitude estimation of orientation (in degrees) relative to vertical and horizontal replicated a previously reported spatial orientation bias also measured using verbal report: Orientations appear farther from horizontal than they are, whether numeric judgments are made relative to vertical or to horizontal. Analyses of verbal response patterns, however, suggested that verbal reports underestimate the true spatial bias. A non-verbal orientation bisection task (Experiment 2) confirmed that spatial errors are not due to numeric coding and are larger than the 6° error replicated using verbal methods. A spatial error of 8.6° was found in the bisection task, such that an orientation of about 36.4° from horizontal appears equidistant from vertical and horizontal. Finally, using a categorization ("ABX") paradigm in Experiment 3, it was found that there is less memory confusability for orientations near horizontal than for orientations near vertical. Thus, three different types of measures, two of them non-verbal, provide converging evidence that the coding of orientation relative to the internal standards of horizontal and vertical is asymmetrically biased and that horizontal appears to be the privileged axis.