Decision factors affecting line orientation judgments in the method of single stimuli

Just noticeable differences in orientation are smaller at principal standard orientations than at oblique standard orientations when they are measured with the method of single stimuli. We determined whether this oblique effect is due to an anisotropy in decision factors. A first series of experiments showed that the subjects compare the stimulus with an internal criterion, and that this decision rule is used at all standard orientations. A second series of experiments determined the influence on the oblique effect of nonsensorial variables related to criterion setting. The results strongly suggest that the effect is not due to a criterial noise anisotropy and that criterion-setting processes are similar at principal and oblique standard orientations. The latter conclusion was also supported by an analysis of the sequential stimulus and response dependencies in this task. Hence, it appears that the oblique effect in line orientation discrimination, when it is measured with the method of single stimuli, is due not to decision factors but to a sensorially based anisotropy.