Inability of 2D human retina to specify direction in 3D space produces optical illusions, say researchers

January 5, 2009--Researchers at Duke University (Durham, NC) have issued a new study that helps explain why a person observing an object through an aperture sees movement differently than when looking at the same object directly. Dale Purves MD, Director of the Center for Cognitive Neuroscience and colleagues report that looking through various-shaped apertures can dramatically change the direction that objects appear to move. For example, a tilted rod moving horizontally from left to right appears to move downward at an angle when viewed through a circular hole, as seen in this video clip. The researchers hypothesize that humans perceive such peculiar effects because, evolutionarily speaking, the projection of light onto a two-dimensional retina cannot specify the direction of objects moving in three-dimensional space.

In their study, Purves and colleagues Kyongje Sung, William T. Wojtach, who co-authored an article describing the work for Proceedings of the National Academy of Sciences, asked volunteers to describe how they perceived the motion of moving lines viewed through apertures. Previously, the researchers had generated a computer database cataloguing the actual directional movement of virtual rod-shaped objects in 3-D space projected onto two dimensions. The results showed that the volunteers' perceptions of motion of the objects, viewed through various apertures, were nearly identical to the predictions of the database. The directions of moving objects that we perceive must therefore be based on past experience, rather than analysis of the image on the retina, the authors say.

For more information see an abstract of the paper, "An empirical explanation of aperture effects," and a PDF with supporting information including details of the simulation model, etc.