Figure

The stimuli shown in (1) are superimposed on, and alternated with, those shown in (2), creating the impression that a white square is moving right and left. Adapted from Ramachandran and Anstis (1986).

A different kind of evidence that apparent motion can be produced in more than one way was reported by Shiffrar and Freyd (1990). In part of their experiment, observers were presented in rapid succession with two photographs of a man with his hand held out and his palm facing forward. In one photograph, his hand was twisted as far left as possible, and in the other photograph it was twisted as far right as possible. There was a rotation of wrist of about 270° between the two photographs. Apparent motion could be seen either in the longer (270°) but biologically possible direction, or in the shorter (90°) but biologically impossible direction. When there was rapid alternation of the photographs, the shorter rotation was perceived. However, the longer and more plausible rotation was perceived when the rate of alternation was slower.

What do these findings mean? Shiffrar (1994) speculated that they can be understood in terms of the distinction between the dorsal (where is it?) and ventral (what is it?) streams of processing. It is only at the slower rate of alternation that it was possible to access knowledge from the ventral stream about hands and about what is physically possible.

There are various problems with the distinction between short-range and long-range motion (see Mather, 1994, for a review). For example, the spatial range of so-called short-range motion is almost certainly much greater when visual stimuli are presented a long way away from the fovea or central part of the retina, and this helps to undermine the distinction between short-range and long-range motion. As Wandell (1995, p. 365) concluded, "The long- and short-range motion classification is still widely used.But I suspect the classification will not last."

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