Angle-matching illusions and perceived line orientation

Spatial illusions which occur in angle-matching tasks were examined in six experiments, using two different kinds of display. In experiments 1 and 2, illusory errors generally were in the direction predicted by Lennie's hypothesis which states that angle arms are attracted perceptually towards the oblique axes of space, although the display used in these experiments differed from Lennie's. However, experiment 3 showed that these errors might equally be explained by the addition of interactive effects between angle arms (tilt illusions). Parametric investigation of Lennie's figures (experiments 4 and 5) showed that the largest angular illusion occurred with the largest angle used (45°) and an intermediate line length (2 deg 7 min). These angular illusions were not explicable by the additon of tilt illusions (experiment 6), suggesting that different judgmental processes may underline orientation and angle estimations.