The overestimation of gaze for horizontal, vertical, and diagonal fixation points

A widely known result from gaze-perception research is the overestimation effect where gaze endpoints are seen farther to the side than they actually are. While horizontal gaze directions have been studied repeatedly, there is scarce research on other directions after early reports that vertical gaze is perceived accurately. It is argued that if participants base their judgment on the movements of the iris-pupil-complex in relation to eye size, vertical gaze should be overestimated similarly as horizontal gaze. This is what was found in the reported experiment. However, horizontal gaze was actually overestimated more than diagonal and vertical gaze. The small difference in overestimation between the axes may be explained by the horizontal-vertical illusion, entailing that horizontal extensions are seen as shorter than vertical extensions.