Civil Engineering Reference
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sensitivity to light. To understand this trade-off, we need to examine the structure of the retina in more
detail. As shown in Figure 23.4, the retina consists not only of rods and cones but also includes amacrine,
bipolar, horizontal, and ganglion cells.
Adetailed treatment of the functions of these cells is beyond the scope of this chapter. Basically, both rods
and cones connect to bipolar cells which, in turn, transfer impulses to a second set of neurons, called
ganglion cells. Horizontal cells synapse with both bipolar cells and receptor cells as well as with other
horizontal cells. And amacrine cells synapse with both bipolar cells and ganglion cells as well as with
other amacrine cells. The horizontal and amacrine cells, thus, help transfer information laterally
among different elements of the retina. In contrast, bipolar and ganglion cells play a role in determining
the sensitivity and the acuity of the photopic (cone-based) and scotopic (rod-based) visual systems.
To understand this phenomenon, remember that there are approximately 100 to 120 million rods and
4 to 6 million cones in the retina but only approximately 1 million ganglion cells. This implies that a
considerable amount of convergence and compression of information must occur as information is
passed from the receptor cells via the bipolar cells to the ganglion cells. Convergence, that is, the sum-
mation of input from several receptors to a ganglion cell, supports increased sensitivity because a
weak stimulus can activate several receptors to a limited extent and, once their input is combined and
sent off to the ganglion cell, it may exceed the threshold for neural activity, that is, for an action potential
to occur. As many as 1000 rods may pass information via their bipolar cells to a single ganglion cell, thus
exhibiting a high degree of convergence.
In contrast, cones show very little convergence. There is typically a 1:1 relationship and ratio between
cones in the fovea and the corresponding ganglion cells. For cones outside the fovea, the ratio is some-
what larger but never reaches that of rods. This explains the high acuity of cones, which have smaller
receptive fields than rods. A receptive field can be defined as “a circumscribed area on the retina that pro-
vides the input to a ganglion cell” (Soderquist, 2002).
Table 23.1 summarizes the main differences between rods and cones that have been discussed so far.
So far, we have focused on monocular vision, that is, the structures and perceptual processes associated
with the individual eye. To understand other important visual functions, such as depth perception, we
need to consider binocular vision and its affordances. For example, binocular vision grants us a larger
field of view, it reduces the risk of becoming disabled following damage to one eye, and it supports stereo-
scopic vision and thus depth perception.
Depth perception is important for a variety of tasks (e.g., flying an aircraft, driving a car), where a
person needs to be able to judge distance from and between objects in the environment. It is supported
by three main classes of depth cues: (a) oculomotor cues, (b) visual binocular cues, and (c) visual
monocular cues. Oculomotor cues include accomodation (discussed earlier), which provides depth
information by informing higher-level brain regions about the extent to which the ciliary muscles
had to change the lens shape in order to bring the object of interest in focus. This information
indirectly indicates the distance of an object from the observer. Convergence, that is, the amount to
which inward rotation of the two eyeballs is necessary to bring an image to rest on corresponding
areas of the retina of both eyes, is an example of a binocular depth cue. If an object is 6 m or more
away from the observer, the line of sight is parallel. If the object moves closer, the eyeballs begin to
turn inward progressively. Binocular disparity, that is, the disparity between the views obtained by
each eyeball, is another example of such cues and also provides information on distance. These
three mechanisms relate, for the most part, to depth perception for objects that are close to the obser-
ver (within a few meters).
Depth perception for more distant objects requires so-called “pictorial” cues, which are based on
past experience and thus represent a top-down influence on perception. They include, but are not
limited to:
.
Linear perspective, that is, convergence of parallel lines toward a more distant point (Figure 23.5)
.
Relative size, that is, if two objects that are known to be of the same size occupy different visual
angles, then the one occupying a smaller angle is perceived to be farther away
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