Regulation and Integration of the Body
slightly diﬀerent view (Figure 15.19a). Te visual cortex fuses
the slightly diﬀerent images delivered by the two eyes, providing
accurate means of locating objects in space.
In contrast, many animals (pigeons, rabbits, and others) have
. Teir eyes are placed more laterally on the
head, so that the visual ﬁelds overlap very little, and crossover
of the optic nerve ﬁbers is almost complete. Consequently, each
visual cortex receives input principally from a single eye and a
totally diﬀerent visual ﬁeld.
Depth perception depends on the two eyes working together.
If only one eye is used, depth perception is lost, and the person
must learn to judge an object’s position based on learned cues
visual reﬂex centers controlling the extrinsic muscles of the eyes.
Another set comes from a small subset of ganglion cells in the
retina that contain the visual pigment
, dubbed the
circadian pigment. Tese ganglion cells respond directly to light
stimuli and their ﬁbers project to the
mediate pupillary light reﬂexes, and to the
of the hypothalamus, which functions as the “timer” to set
our daily biorhythms.
Notice that both eyes are set anteriorly and look in approxi-
mately the same direction. Teir visual ﬁelds, each about 170
degrees, overlap to a considerable extent, and each eye sees a
(a) The visual fields of the two eyes overlap considerably.
Note that fibers from the lateral portion of each retinal field
do not cross at the optic chiasma.
(b) Photograph of human brain, with the right side
dissected to reveal internal structures.
Visual pathway to the brain and visual ﬁelds, inferior view.