Biology Reference
In-Depth Information
The optic nerves from both eyes come together at the midline of the body in an area called
the 'optic chiasm'. The chiasm is important in combining signals from each eye that pertain to
the same area in the visual fields of the two eyes. In animals without binocular vision, in
which there is no overlap between visual field, the optic nerves simply cross at the chiasm
and project contralaterally
d
that is, to the opposite side of the brain. In mice, the eyes of
which show hardly any overlap in visual field, over 97 percent of axons do this.
54,55
In
animals with binocular vision, however, axons representing areas of overlap segregate out
and project ipsilaterally
d
that is, to the same side of the brain. Even in animals with strong
binocular vision, the very periphery of the visual field is perceived by only one eye (as
anyone with two working eyes can verify by holding a hand beside their face as far back
towards the ear as it is visible, and then closing the nearest eye). Because of the inverting
action of a simple lens such as the one in the eye, the edge of the human visual field projects
to the nasal retina while the central parts of the field are perceived by the temporal retinas of
both eyes. It is, therefore, the temporal axons that can sort out at the chiasm to project ipsi-
laterally. The placing of the eyes on the face and the shape of the face in different species
means that binocular vision may also vary along the dorsoventral axis.
The decision between contralateral or ipsilateral projection at the optic chiasm boils down,
once again, to a decision about whether or not to cross the midline of the CNS. The molecular
mechanisms that operate there can be regarded as analogous to those operating in the
midline of the spinal cord or of insect CNS, in that they seem to use an inhibitory signal at
the midline to which only some growth cones are sensitive, but the molecules involved are
different. The midline of the mouse optic chiasm expresses Ephrin-B2 at the time that axons
growing through it have to make their contralateral/ipsilateral choice. Axons coming from
the dorsal retina around day 15 of mouse development represent parts of the eye that will
not produce binocular information. They express little or no EphB1, the receptor for Eph-
rin-B2, and are therefore able to cross the midline unimpeded. Those coming from the ventral
temporal zone express high amounts of EphB1, however (
Figure 12.5
), and are repelled by
the midline and forced to follow an ipsilateral pathway
56
(
Figure 12.7
). The importance of
FIGURE 12.7
Mechanism for deciding whether to cross the midline at the optic chiasm.
