Anomaly of the perception of the length of sloped lines

This paper describes psychophysical experiments in which three observers estimated the length of lines that differ in orientation and lie at various distances from each other. Distortions of the estimated length of up to 15%-20% are obtained, depending on how the lines are oriented. The average strength of the illusion was less by a factor of 1.3 in the case of contiguous lines than when the lines were located in space at angular distances of 2.4 degrees and 5.8 degrees. The distortions were insignificantly less in the case of short lines. The maximum illusion is obtained for an orientation of 60 degrees and 120 degrees of contiguous lines with angular length 2.1 degrees. In most of the other cases, the maximum distortions are detected for an orientation of 90 degrees. Possible mechanisms for the appearance of the illusion are discussed-in particular, those that explain the illusion by the statistics of two-dimensional projections of three-dimensional scenes, the theory of perspective, or the association of the illusion with an elliptical shape of the field of view. (C) 2020 Optical Society of America