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568 22. Visual Perception
As shown in Figure 22.14, the packing density of rods drops to zero at the
center of the fovea. Away from the fovea, the rod density first increases and then
decreases. One result of this is that there is no foveal vision when illumination
is very low. The lack of rods in the fovea can be demonstrated by observing a
night sky on a moonless night, well away from any city lights. Some stars will
be so dim that they will be visible if you look at at point in the sky slightly to the
side of the star, but they will disappear if you look directly at them. This occurs
because when you look directly at these features, the image of the features falls
only on the cones in the retina, which are not sufficiently light sensitive to detect
the feature. Looking slightly to the side causes the image to fall on the more
light sensitive cones. Scotopic vision is also limited in acuity, in part because
of the lower density of rods over much of the retina and in part because greater
pooling of signals from the rods occurs in the retina in order to increase the light
sensitivity of the visual information passed back to the brain.
22.2.5 Motion
When reading about visual perception and looking at static figures on a printed
page, it is easy to forget that motion is pervasive in our visual experience. The
patterns of light that fall on the retina are constantly changing due to eye and body
motion and the movement of objects in the world. This section covers our ability
to detect visual motion. Section 22.3.4 describes how visual motion can be used
to determine geometric information about the environment. Section 22.4.3 deals
with the use of motion to guide our movement through the environment.
The detectability of motion in a particular pattern of light falling on the retina
is a complex function of speed, direction, pattern size, and contrast. The issue is
further complicated because simultaneous contrast effects occur for motion per-
ception in a manner similar to that observed in brightness perception. In the
extreme case of a single small pattern moving against a contrasting, homoge-
nous background, perceivable motion requires a rate of motion corresponding to
0.2
◦
–0.3
◦
/second of visual angle. Motion of the same pattern moving against a
textured pattern is detectable at about a tenth this speed.
With this sensitivity to retinal motion, combined with the frequency and ve-
locity of saccadic eye movements, it is surprising that the world usually appears
stable and stationary when we view it. The vision system accomplishes this in
three ways. Contrast sensitivity is reduced during saccades, reducing the visual
effects generated by these rapid changes in eye position. Between saccades, a
variety of sophisticated and complex mechanisms adjust eye position to compen-
sate for head and body motion and the motion of objects of interest in the world.
Finally, the visual system exploits information about the position of the eyes to