The perception of 2D orientation is categorically biased.

From the time of Wundt (1862) and Jastrow (1892), it has been argued that the perception of 2D angles is distorted. Fisher (1968) showed that acute angles presented about a horizontal axis were overestimated while acute angles presented about a vertical axis were underestimated. Such a pattern suggests that lines tend to appear steeper than they are. Although Fisher argued the numeric errors did not fully support this simple interpretation, his findings strongly suggested that oblique orientations appeared steeper than they were in both cases. Several authors have argued on theoretical and empirical grounds that the perception of 2D orientation is indeed biased. For example Dick and Hochstein (1989) asked people to categorize oriented lines into bins separated by 5° and found that orientations near horizontal were seen as farther from horizontal than they were, with a maximum error of about 6° for lines 30° from horizontal. Howe and Purves (2005) argued that the misperception of angles was due to the statistical predominance of shallow angles in natural scenes. However, because their argument was expressed in terms of angles rather than orientations, it does not directly predict the asymmetry observed by Fisher (1968) for angles about horizontal vs. vertical axes, even though it is consistent with such an asymmetry. With the exception of the work of Dick and Hochstein, the data supporting the idea that 2D orientation is distorted has come predominantly from judgments of angles rather than of orientation. Although verbal estimates are often regarded as unreliable psychophysical measures of perceptual experience, verbal estimates of surface orientation have been shown to be surprisingly consistent with a variety of non-verbal methods (Durgin, Li & Hajnal, 2010; Li & Durgin, 2009, 2010). In the present paper we will employ verbal methods, as well as two different non-verbal methods to extend the findings of Dick and Hochstein (1989) in three ways. The results of all three methods support the same conclusions. First, in Experiment 1, we show that even when participants are not constrained to coarse categorical judgments, but are asked to make verbal estimates in degrees, a bias quite similar to that shown by Dick and Hochstein (1989) is present. This bias is argued not to be a numeric bias because it is spatially the same whether verbal estimates are made relative to horizontal or to vertical. This experiment provides an important replication of the patterns of bias reported by Dick and Hochstein. In a second experiment we rule out verbal/numeric bias altogether by conducting an orientation bisection task to discover what 2D orientation appears to be equidistant from horizontal and vertical. These data show that, if anything, numeric reports have underestimated the vertical bias in 2D orientation perception. Moreover, the effect cannot be attributed to the vertical-horizontal illusion. Finally, we use an “ABX” categorization task to demonstrate that the perceptual categories of vertical and horizontal are not symmetrically represented. The novel application of this task contrasts with prior tasks that have found little evidence of difference in information transmission between vertical and horizontal for simple oriented stimuli (i.e., Dick & Hochstein).