Perception of the orientation of objects is important in our interaction with the environment. So far, research has focused on the orientation of lines and gratings on a fronto-parallel plane with the main finding that vertical and horizontal orientations are perceived more accurately and precisely than oblique ones (oblique effect).

We tested the orientation perception of fronto-parallel ellipses with different length-to-width (aspect) ratios in various orientations. A circle was included in the test set. Six naive subjects adjusted a broken line (probe) to match the major axis orientation of an ellipse (stimulus) that was placed at the center of the probe.

The precision of the settings as quantified by the circular standard deviation (CSD), increased with increasing aspect ratio (more elongated ellipses). Reparametrizing aspect ratio as roundness (defined as the inverse of (aspect ratio minus one)), CSD increased linearly with roundness. This result could be captured by an ideal observer model where the vertices of the polygon making up the ellipse were perturbed with noise: a single noise level for each subject was sufficient to capture the results.

The accuracy results show large biases, especially for the low aspect ratios (close to a circle). For the circle, subjects displayed non-uniform distributions in their settings. Furthermore, there are large individual differences among the subjects. We can capture these differences by an empirical Bayesian model that takes the distribution of settings to the circle as a prior distribution. Thus the prior is obtained from the settings to a neutral stimulus. Going beyond the domain of perception of orientation, we believe empirical Bayesian modeling to be a new useful tool for vision research.