# Poggendorff illusion

The Poggendorff illusion

The Poggendorff illusion is a geometrical-optical illusion that involves the misperception of the position of one segment of a transverse line that has been interrupted by the contour of an intervening structure (here a rectangle). It is named after Poggendorff, the editor of the journal, who discovered it in the figures Johann Karl Friedrich Zöllner submitted when first reporting on what is now known as the Zöllner illusion, in 1860.[1]

In the picture to the right, a straight black and red line is obscured by a grey rectangle. The blue line, rather than the red line, appears to be a continuation of the black one, which is clearly shown not to be the case on the second picture. Instead there is an apparent position shift of the lower portion of the line.[2] The magnitude of the illusion depends on the properties of the obscuring pattern and the nature of its borders.[3]

Many detailed studies of the illusion, including "amputating" various components[4][5] point to its principal cause: acute angles in the figure are seen by viewers as expanded[6][7] though the illusion diminishes or disappears when the transverse line is horizontal or vertical. Other factors are involved.[8][9]

## References

1. ^ Zöllner F (1860). "Ueber eine neue Art von Pseudoskopie und ihre Beziehungen zu den von Plateau und Oppel beschrieben Bewegungsphaenomenen". Annalen der Physik 186 (7): 500–25. Bibcode:1860AnP...186..500Z. doi:10.1002/andp.18601860712.
2. ^ Greist-Bousquet S., Schiffman H.R. (1981). "The Poggendorff illusion: an illusion of linear extent?". Perception 10 (2): 155–64. doi:10.1068/p100155. PMID 7279544.
3. ^ Westheimer G, Wehrhahn C. (1997). "Real and virtual borders in the Poggendorff illusion". Perception 26 (12): 1495–501. doi:10.1068/p261495. PMID 9616477.
4. ^ Day, R.H. (1973). "The oblique line illusion: The poggendorff effect without parallels". The Quarterly Journal of Experimental Psychology 25 (4): 535–541. doi:10.1080/14640747308400375. PMID 4767535.
5. ^ Spehar B, Gillam B J, 1998, "Modal and amodal completion in the Poggendorff illusion" Perception 27 ECVP Abstract Supplement
6. ^ Weintraub, D.J., Krantz, D.H., & Olson, T. (1980). The Poggendorff illusion: consider all the angles. J. exp. Psychol. Hum Percept Perform., 6 718-725
7. ^ Morgan M.J. (1999). "The Poggendorff illusion: a bias in the estimation of the orientation of virtual lines by second-stage filters". Vision Research 39 (14): 2361–2380. doi:10.1016/S0042-6989(98)00243-0. PMID 10367057.
8. ^ Green R.T., Hoyle E.M. (1963). "The Poggendorff Illusion as a Constancy Phenomenon". Nature 200 (4906): 611–612. Bibcode:1963Natur.200..611G. doi:10.1038/200611a0. PMID 14082259.
9. ^ Gregory, R.L. (1968). "Perceptual Illusions and Brain Models". Proceedings of the Royal Society B 171 (1024): 279–296. Bibcode:1968RSPSB.171..279G. doi:10.1098/rspb.1968.0071. JSTOR 75828.