The decreased visibility of obliquely oriented patterns as compared to horizontal or vertical ones is termed the oblique effect. The origin of the oblique effect in the chain of visual processing was examined by comparing the potency of oblique adapting gratings to the potency of horizontal ones. Oblique gratings (which were less visible but of equal physical contrast) were as powerful or more powerful than horizontal gratings as adapting stimuli. Obliquely oriented stimuli also produced a slightly stronger tilt aftereffect than stimuli near the cardinal axes. These results suggest that the diminished neural representation of oblique stimuli arises in the human cortex, rather than from impairments of sensitivity or resolution in the initial geniculo-cortical projection.