The importance of velocity gradients in the perception of three-dimensional rigidity.

Sequential presentation of a number of random-dot patterns which when super-imposed yield an expanding flow field leads to the perception of a coherent motion towards the observer. The motion vectors in this type of flow field all radiate from the origin. This percept of a global coherent expanding flow results only when the local speeds (magnitude of the local motion vectors) are zero at the centre and increase linearly towards the periphery. If all the dots radiate outwards but have the same speed, a clear percept of three-dimensional nonrigidity arises.