We need to locate objects not only in space but also in time. Without the temporal dimension spatial location alone would be of little use to us. Of course, change in spatial location over time is motion. Retinal image motion can be a consequence of displacement either of an object relative to a stationary environment or of an observer relative to a stationary object. In addition both object and observer could move together. All these conditions result in changes in the pattern of stimulation at the eyes, but despite these we retain an appropriate representation both of our position in space and that of the object. Occasionally, errors do occur in our perception of motion, and these can be very instructive in understanding the nature of motion perception.
One example that most people have experienced is the false allocation of motion that can occur when seated in a stationary train as a neighbouring train pulls slowly away from the station. We initially perceive the carriage in which we are seated to be moving, and this false allocation is only later replaced by the veridical percept when the other train is moving faster. Why does this happen? In order to understand it we need to examine the frames of reference that are present. First consider the situation when the train in which we are seated moves out of a station. There is a relative displacement of the platform with respect to the windows of the carriage. Some aspects that were initially visible disappear from view and others that were previously occluded become visible. The rate of these changes increases as the velocity of the train increases. Any stable environment will produce such an optic flow (to use one of Gibson's terms) with respect to a moving vehicle, and so it is this characteristic that usually defines vehicular motion. In the case where the neighbouring train starts to move the optic flow produced is ambiguous-it could be a consequence of either train moving, but it is more common for the motion of the whole visual field to specify self motion rather than motion of the environment. The illusion of motion only occurs if the neighbouring train is close enough to occupy most of the view visible through a large window; if it is several tracks away, so that other parts of the station are visible, then there is no illusion. One of the reasons that we cease to experience motion of our own carriage after some seconds is that the illusory motion is not