Perceptual Coherence : Hearing and Seeing

Perception of Motion 201
Figure 5.3. The two perceived
movements in the Ternus display.
The double-ended arrows represent
the back-and-forth motion as
the frames are alternated.
outcomes are shown in figure 5.3. The perception of group movement also
occurred if the two stimuli were presented to different eyes. The three-dot
shift, which is perceived at all but the shortest time intervals, makes each of
the dots move in exactly the same way.
7. The perceived motion can be influenced by prior movements within
the display. Antis and Ramachandran (1987) primed either horizontal or
vertical perceived motion by initially displaying one of the two movements.
The perceived motion follows the direction of the priming movement;
it is as if the dots do not want to change direction. It is interesting to
note that it is only the direction of the priming motion that influences the
apparent motion; speed or distance do not. The visual system expects objects
to move in rather consistent paths and not to change direction quickly
or erratically.
8. The perceived motion can be influenced by the perceptions from prior
trials, a phenomenon termed hysteresis. Imagine starting with a narrow
horizontal rectangle so that the perceived motion is vertical (along the short
dimension). If that rectangle is gradually changed over time so that it becomes
tall and vertical, observers will continue to perceive vertical motion
far beyond the height-to-length ratio that would normally bring about a
shift to horizontal motion. Conversely, starting with a tall vertical rectangle
yielding horizontal motion will bring about horizontal movement even for
narrow horizontal rectangles. Hysteresis can be understood as a type of
priming, in which the visual (and auditory) system expects to see one kind
of motion and will cling to that percept even if an alternative perception
would usually be preferred if that stimulus configuration were presented in
isolation.