Orientation discrimination was measured with a yes-no signal detection procedure at two reference orientations, 90° and 45°. For the first of two experiments, the stimuli were disk-shaped gratings, 2.5° in diameter. Their spatial frequencies ranged from 1.8 cpd to 17.6 cpd. For the second experiment, the stimuli were either gratings or bars consisting of the center cycle of the corresponding grating. As expected, the results showed a highly significant reference orientation factor, with smaller just noticeable differences at 90° than at 45° (theoblique effect). However, orientation discrimination was independent of spatial frequency and bar width. Implications of the findings are discussed in relation to a holographic model that relates the oblique effect to meridional differences in the number of orientation-tuned cortical cells.