The MacKay Phenomenon: Is it All Hand-Waving?

Abstract

When a table-tennis bat with a light-bulb in its centre is waved about by another person in a stroboscopically lit room, the light appears to float free from the bat, although it is physically fixed. A related phenomenon was reported originally by MacKay [1958 Nature (London) 180 507 – 508)]. This illusion disappears in an active condition, where observers move the bat with their own hands, which suggests a proprioceptive basis for the illusion. In our implementation with a Power Macintosh computer, the illusion occurred in both conditions. Thus ‘hand-waving’ is not inherent to the illusion but may exert its effect indirectly by promoting predictive eye-tracking movements.

To test the hypothesis that the perceived position of the bulb is extrapolated between a visual icon, tied to retinal coordinates, and its actual physical location thus causing the displace-ment illusion, we performed two experiments. From experiment 1 we found that for simple one-dimensional movement, the displacement illusion occurs only when there is a change in direction such that the bulb appears to ‘overshoot’ the bat. In experiment 2, using random two-dimensional movement, we manipulated the length of the dark (non-visible) period. The overshoot increased from around 30 min arc at 65 ms to around 60 min arc at 300 ms. For longer dark periods (up to 2000 ms), the illusion occurred providing the change in direction was within the 300 ms window after stimulus offset. These results show that during discontinuous illumination, perceived location is determined by extrapolation.