Maybe they are all circles: clues and cues.

When several cues provide information about the same property of a visual scene, a weighted average of the singe-cue estimates can provide a more reliable estimate than that of any individual cue. Some cues rely on assumptions about the scene, such as that shapes are isotropic. Assuming that an elliptical image arises from viewing a circle at an angle allows one to extract the circle's angle from the aspect ratio in the image. This study investigates whether the weight given to image shape as a slant cue depends on the prevailing circumstances. Neither rotating an object to provide direct evidence that it is circular, nor surrounding an object with circles rather than ellipses increased the weight assigned to image shape relative to that assigned to binocular information. Thus the weight given to slant cues does not seem to rely on an elaborate analysis of the scene.