motion and position. In the present study we examine the influences of motion of a background of randomly oriented lines on the subjects' perceptual judgements of the velocity and position of a running spider, and on the way they hit the spider with a rod. The target was a computer simulation of a red spider running to the right across a surface of randomly oriented yellow lines. The surface appeared to coincide with a transparent screen that was about 7 cm in front of the computer monitor. This screen protected the monitor from being hit by the rod. To make the visible surface coincide with the protective screen we presented different images to the two eyes (at 60 Hz per eye) using LCD shutter spectacles. The resulting