Behavior: Basic Psychology of the User - Foundations for Designing User-Centered Systems - page 98

results here have different interpretations, as to whether they are the result of
vision or the result of cognition. A comprehensive theory of the user would
incorporate this interaction between vision and cognition. Being aware of the
higher level perceptual issues can help you design better interfaces.
4.5.1 Movement and Spatial Perception
Movement can be detected either by moving ourselves (even though the image
falls on the same part of the retina) or by staying still whilst the image moves
across the retina. The processing of the stimuli related to body movements and
visual stimuli are combined because we can track a still object with our eyes while
moving our bodies and yet be sure that it is still.
Above a certain speed of movement the eye can spontaneously track a moving
object. Ask someone to slowly wave a small light (about as bright as a lit match)
around in a darkened room and follow it with your eyes. The movement of the
light will be seen even though no image is moving across the retina (because your
eyes will keep it constantly on the retina). The periphery of the retina is the area
most sensitive to movement, but it is very difficult to identify an object at the
extreme periphery of the field of view. The detection of a moving object in the
periphery of the eye is what usually initiates the movement of the eye in pursuit of
that object so that it can be brought into focus on the fovea. Movement may also be
perceived as afterimages when both the eye and the retinal image are stationary
(this effect is due to adaptation of the motion detectors in the eye). Thus, the eye
and the brain combine their results to get this apparent motion.
4.5.2 Depth Cues
Spatial perception is determined from the muscular activity of the two eyes and
discrepancies between the two images that are formed. When we want to display a
3D image on a screen, we have to represent the 3D image using just two
dimensions, although better 3D displays continue to appear. We can simulate
depth perception using the discrepancy between the two images, as well as the
perceptual depth cues listed in Table 4.3 .
In the real world, motion parallax may be one of the most important cues that
enable us to perceive distance and depth. It occurs when we move our heads from
side to side and we see objects displaced at different rates. Objects that are further
away appear to move more slowly than objects that are closer. In screen design,
the trick is to move the viewpoint of the ''camera'' so that the image on the screen
moves according to the principles of motion parallax. This is not used in most non-
game interfaces, although virtual reality systems provide it, and video games
provide it through motion of objects.