A rigid pencil can appear to bend when wiggled. Pomerantz (1983) suggested that this phenomenon only occurs when there is a curved envelope of the motion blur pattern, but this hypothesis has not been rigorously tested. Thus, the present experiments were designed to provide further insights about the mechanisms that underlie this illusion.

Observers were shown computer displays of moving line-segments undergoing translation and rotation, and they were asked to indicate the apparent 'rubberiness' of the motion on a 10-point scale. The manipulation of speed, amplitude and relative phase of the rotational and translational motion components and introduction of additional motion enabled us to produce a wide variety of motion envelopes. We also manipulated the dynamic luminance contrast, the pattern of eye movements, and the structure and motion of the surrounding context.

The results indicate that the illusion is unaffected by manipulations of the blur envelope or the dynamic contrast. Perceived ‘rubberiness’ is greatest when the rotational and translatory oscillations have a relative phase of approximately 110° and a frequency of 2.5-3 cycles/second. The strength of the illusion can be increased or decreased by changing the pattern of eye movements so as to alter the movement trajectories on the retina. It can also be greatly attenuated for certain types of backgrounds, when the movement of the background is identical to that of the moving line. These results suggest that the illusion is based on spatiotemporal stimulus dynamics and perceptual grouping mechanisms rather than the envelope of the motion blur pattern.