| When two pictures contain an object at different spatial locations, and these pictures are shown in rapid succession, we are likely to see the object move. This is apparent motion, familiar to us from movies. I use the phenomenon of apparent motion to understand how vision combines space and time in the perception of dynamic scenes: (a) in grouping between successive elements, and (b) in grouping between successive aggregates of elements.; A. According to the traditional view, if we increase the spatial distance between locations, we cannot maintain the same vividness of motion without increasing the temporal interval between the views. This observation is known as Korte's Third Law; we will call it the space-time coupling theory. Later studies, using better methodology, found the opposite result. If we increase the spatial distance between locations, we cannot maintain the same propensity to see motion without decreasing the temporal interval between the views. We will call this the space-time tradeoff theory. I show that both theories are correct, but at different speeds of motion.; B. When motion is seen, each visual element may either have a separate identity and move separately, or it may merge with other elements to form a larger-scale object, so that the element's motion will be the same as the object's. A tacit assumption in the literature on motion perception has been that grouping between concurrent entities (spatial grouping) and grouping between successive entities (temporal grouping) are separable and sequential. I show that the two processes are tightly integrated, and it is the competition between two kinds of grouping which determines whether we see the elements move as separate entities or as parts of an object. |
