Perceptual Coherence : Hearing and Seeing

196 Perceptual Coherence
Up to the late 1800s, motion was thought to be perceived by tracking
one’s own eye movements. But even at that time, it was known that motion
was perceived when the eye did not move. Exner (1875) provided convincing
evidence that motion was a distinct perceptual quality, and not a derivative
of static views spread across space and time. Exner produced two
slightly displaced sparks, separated by a short time interval. The critical
outcome occurred when the sparks were so close spatially that both sparks
were always seen together (they could not be separated by any type of eye
movement). Even in this case, there was a clear perception of a single spark
moving back and forth between locations at intermediate time intervals. To
demonstrate that these apparent movements were not due to small, nonperceptible
eye movements, Wertheimer (1912) constructed an apparatus that
created two out-of-phase motions, for example a red light moving leftright-left
and simultaneously a green light moving right-left-right. Observers
saw both motions, and Wertheimer argued that the eye could not
move in both directions at once.
One-Dot Apparent Movement
The Gestalt psychologists used apparent motion to denounce what they
claimed was the orthodox sensation + experience theory of perceiving. Actually,
the Gestalt psychologists had constructed a straw man, as no theorist
adhered to the viewpoint that perception emerged only from the compounding
of sensations. The positive outcome was that extensive research
was undertaken to map out the optimal space-time relationships characterizing
apparent motion.
In the simplest case of apparent motion, a single target at position x 1 is
flashed at time t 1 , and a second target is flashed at position x 2 at time t 2 . The
basic case occurs when the targets are identical in the two frames and are
either brighter or darker than the background. Given the appropriate interval
(t 2 − t 1 ), the target seems to move from x 1 to x 2 , regardless of whether
the target is brighter or darker than the background. The outcomes are more
complex if the two targets differ, and those will be discussed later.
Now consider the cases in which the targets are continuously alternated.
Korte (1915) summarized experiments that incorporated three factors:
(1) the discriminability of two lights; (2) the distance between the lights;
and (3) the time interval between the onset of each light. Korte originally
used the time interval between the offset of the first light and the onset of
the second, but the results are simpler to interpret if the timing between the
onsets is used. (This differs from hearing, in which the offset-to-onset interval
determines some types of organization, as discussed in chapter 9;
Bregman, Ahad, Crum, & O’Reilly, 2000.) The important law here is that
the onset-to-onset interval is directly proportional to the spatial distance. In