Shape Constancy

Shape constancy refers to our ability to see an object's shape as remaining the same (thus "constant") despite changes in the angle from which we view it. When we look straight on at a rectangular door, for example, the door projects into our eyes a shape that is approximately rectangular (allowing for minor distortions, the curvature of our retina, and so forth). As the door swings open, however, the shape of the door's image projected onto our retina changes into a trapezoid. Fortunately, we are not fooled into thinking the door is actually changing shape. Similarly, a coin viewed head on will appear circular. If the coin is rotated, the shape of the image projected to our eyes becomes elliptical; but again we see the coin as circular. In short, our visual system solves an "inverse" problem by working backwards from both the retinally projected shape and information about viewing angle to create a conscious perception of the object's actual shape and orientation in 3-D space. In this regard, it is similar to size constancy (in which a coin moving toward us is not seen as growing larger) or lightness constancy (in which a coin brought into a spotlight is not seen as brighter). Instead, we see the permanent, unchanging properties of objects irrespective of their momentary appearance. Shape constancy is an essential component to our perceiving objects in the visual world accurately.

The Shepard Illusion demonstrated below may be the most striking demonstration of shape constancy ever produced. In the figure below, the orange/tan shape at the bottom is the same size and shape as the yellow tops of the upper shapes. Since it is hard to believe that this is true, the demo we developed allows you to move the bottom figure on top of another figure to see for yourself. This doesn't work in the figure below, but does work in the demo you can run by clicking on the link "The Shepard Illusion demo." You can then move the bottom shape with the mouse. To rotate it into postion, click on one of its corners and move the mouse.

Click here to start the demo