thumb|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.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. It is named after Johann Christian 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. Although Zöllner was focused on a different illusion, the misalignment of the diagonal lines revealed a distinct visual phenomenon.
The magnitude of the illusion depends on the properties of the obscuring pattern and the nature of its borders. Many detailed studies of the illusion, including "amputating" various components point to its principal cause: acute angles in the figure are seen by viewers as expanded
Theoretical explanations
Multiple explanations have been proposed for the Poggendorff illusion, most attributing it to a combination of perceptual and cognitive factors rather than a single cause.
Angle misperception
One of the most established accounts attributes the illusion to distortions in how the brain processes angles and spatial orientation. Ross H. Day and Ross G. Dickinson (1976) argued that the illusion results from a combination of perceptual biases, including the horizontal–vertical effect, the longitudinal–transverse effect, and misjudgment of acute and obtuse angles. Acute angles tend to be perceived as wider than they are, shifting the apparent continuation of the oblique line. The illusion persists even when the orientation of the lines remains constant, suggesting the distortion occurs in the space between them rather than in the lines themselves. When visual cues like luminance are used to make the obliques appear in front of the rectangle, the illusion is reduced, supporting the depth misperception explanation. Recent neuroimaging research by Shen et al. (2016) shows that real and illusory contours engage different brain regions. Real lines activate ventral visual areas involved in detailed processing, while inferred lines engage dorsal regions linked to spatial construction. The Poggendorff illusion may result from a discrepancy between these systems, where the ventral stream encodes visible segments accurately but the dorsal stream reconstructs the hidden continuation incorrectly, causing misalignment.
Pressey and Sweeney (1972) suggested an assimilation theory, where the brain averages competing line projections near the intersection, giving more weight to shorter segments. This shifts the diagonal's apparent direction and explains both classical and reversed versions of the illusion.
Although grounded in established perceptual theories, these models have received less empirical attention in Poggendorff research. Also, illusion strength has been linked to language and reasoning skills, but not to basic visual alignment skills, suggesting cognitive development is more influential than visual perception alone. This suggests that the cerebellum may play a role in how the brain processes spatial relationships, and that visual illusions like the Poggendorff could potentially serve as diagnostic tools in neurological assessment.