The mean proportion correct for each pair of orientations are plotted in
Figure 4. The paired orientations are represented by their mean orientation, so that “5” refers to any ABX trial in which one of the initial orientations was horizontal and the other 10° from horizontal. Similarly, “85” refers to an ABX trial where one of the initial orientations was vertical and the other 10° from vertical. If the categories of vertical and horizontal were equivalent, then ABX performance for “5” and “85” should be equivalent. Based on the exaggeration of deviations from horizontal however, we should expect that ABX performance would be better for “5” than for “85.” Consistent with this expectation, mean performance for pairs that included a horizontal orientation (87.3%) was reliably better than that for pairs including vertical (79.9%),
t(9) = 3.33,
p = 0.0087.
The oblique effect (Appelle,
1972; Essock,
1980) is generally described as the precedence of cardinal orientations over oblique orientations. Essock distinguished between class 1 (roughly “sensory”) and class 2 (roughly “encoding”) oblique effects, but both are characterized as better representing cardinal than oblique orientations. This pattern is well known, and it is present in our ABX data as well. More importantly, however, our data indicate that there is an asymmetry between the categories of vertical and horizontal both in explicit measures of perceived orientation (
Experiments 1 and
2) and in categorical discrimination (
Experiment 3). This categorical difference between the two cardinal axes may help account for the asymmetrical distortions found by Dick and Hochstein and replicated and extended in
Experiments 1 and
2. That is, all three experiments can be understood as reflecting greater categorical sensitivity to deviations from horizontal.
This difference between vertical and horizontal contrasts with the traditional class 1 and class 2 oblique effects (Essock,
1980). Essock, Krebs, and Prather (
1997) have described the results of Dick and Hochstein (
1989) as typical of class 2 oblique effects, but we emphasize that Dick and Hochstein found no difference between horizontal and vertical orientations in information transmission (though greater transmission for both cardinal orientations than for oblique lines). Similarly, some studies of environmental image statistics intended to explain oblique effects have emphasized the relative paucity of oblique lines, without reporting differences between vertical and horizontal (e.g., Coppola, Purves, McCoy, & Purves,
1998; but see Hansen & Essock,
2004). Essock et al. specifically identify “memory confusability” (p. 524) as indicative of a class 2 effect. However, the current ABX task, which measures confusability in memory, demonstrates a striking anisotropy between vertical and horizontal, which is consistent with the non-verbal spatial biases we measured in
Experiment 2. Thus, this experiment provides additional support, in a non-verbal task, for the conclusion that the visual coding of vertical and horizontal orientations is not symmetrical.