Visual illusions affecting perception of the roentgen image.

The common denominator of any roentgenographic study is the ability of the radiologist to interpret the image on the film. We are aware that the makeup of that image is a composite of the shadows of all the structures through which the roentgen beam passes. Less well appreciated, however, is a variety of illusory phenomena which results from overlapping shadows, differences in background illumination, and subjective contour formation. The result is a variety of false images which exists in the "mind's eye" only and may be misinterpreted as significant pathologic abnormalities. The three most common causes of illusory phenomena are Mach bands, background density effect, and subjective contours. These are all related and are derived from the process of lateral inhibition. It is important for radiologists to recognize these phenomena in order to avoid their pitfalls. This paper will describe these phenomena and will review previous work on perception.