Abstract
Targets flashed within a frame that moves back and forth are perceptually offset by as much as the frame’s displacement (Özkan et al, 2021; Cavanagh et al, 2022), a far larger offset than that seen on stationary or continuously moving targets (induced motion, Duncker, 1929; Wallach et al, 1978). To understand the difference between continuous and flashed targets we varied the frequency from a single flash per transit of the frame up to continuously present (30 Hz). We find a monotonic decrease in the illusory offset as the flash frequency increases. However, the setting for a stationary target, where there is little or no effect, does not lie on this function, suggesting that the stationary target may group with the steady background instead of the frame. To test this, we presented continuously visible targets that were steady or jiggled vertically while the frame moved horizontally. When the continuous target had any movement, the illusory horizontal movement was visible (induced motion). It did not matter if the frame itself also jiggled vertically, synchronously or asynchronously with the probe. In our original frame effect where the target flashes once per transit, the apparent separation between the two flashes is maximum and there is no sense of motion between the two flashes. When there are multiple flashes or continuous movement, there is illusory movement but the apparent distance travelled is reduced compared to the separation seen when there is just the one flash at each end of the frame’s travel. When the target is stationary, no illusory movement is seen at the frame speeds used here, most likely because it groups with the stationary background.