A comparison of colour, shape, and flash induced illusory line motion

When a bar suddenly appears between two boxes, the bar will appear to shoot away from the box that matches it in colour or in shape-a phenomenon referred to as attribute priming of illusory line motion (ILM; colourILM and shapeILM, respectively). If the two boxes are identical, ILM will still occur away from a box if it changes luminance shortly before the presentation of the bar (flashILM). This flash condition has been suggested to produce the illusory motion due to the formation of an attentional gradient surrounding the flashed location. However, colourILM and shapeILM cannot be explained by an attentional gradient as there is no way for attention to select the matching box prior to the presentation of the bar. These findings challenge the attentional gradient explanation for ILM, but only if it is assumed that ILM arises for the same underlying reason. Two experiments are presented that address the question of whether or not flashILM is the same as colourILM or shapeILM. The results suggest that while colourILM and shapeILM reflect a common illusion, flashILM arises for a different reason. Therefore, the attentional gradient explanation for flashILM is not refuted by the occurrence of colourILM or shapeILM, which may reflect transformational apparent motion (TAM).