Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1212 Chapter 21: Development of Multicellular Organisms
(A)
(B)
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Figure 21–85 Ocular dominance columns in the visual cortex of a monkey’s brain, and their
sensitivity to visual experience. (A) Normally, stripes of cortical cells driven by the right eye
alternate with stripes, of equal width, driven by the left eye. The stripes, set up before birth, are
revealed here by injecting a radioactive tracer molecule into one eye, allowing time for this tracer to
be transported to the visual cortex, and detecting radioactivity there by autoradiography, in sections
cut parallel to the cortical surface. (B) If one eye is kept covered after birth, during the sensitive
period of development, and thus deprived of visual experience, its stripes shrink and those of the
active eye expand. In this way, the deprived eye may lose the power of vision almost entirely. (From
D.H. Hubel, T.N. Wiesel and S. LeVay, Philos. Trans. R. Soc. Lond. B Biol. Sci. 278:377–409, 1977.
With permission from The Royal Society.)
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however, occurring spontaneously and independently in each retina before birth,
leads to a remarkable pattern of ocular dominance columns in the visual cortex:
stripes of cells driven by inputs from the right eye alternating with stripes driven
by inputs from the left eye (Figure 21–85).
The basis for these phenomena became clear from ingenious experiments
interfering artificially with visual experience and altering the coordination of
electrical signaling in the two eyes. These studies, and many others subsequently,
have highlighted a simple but profoundly important principle that seems to
govern synapse reinforcement and elimination throughout the nervous system.
When two (or more) neurons synapsing on the same target cell fire at the same
time, they reinforce their connections to that cell; when they fire at different times,
they compete, so that all but one of them tend to be eliminated. This firing rule is
expressed in the catchphrase “neurons that fire together wire together.”
The firing rule provides a simple interpretation of the developmental phenomenon
we have just described in the mammalian visual system. A pair of axons
bringing information from neighboring sites in the left eye will frequently fire
together, and therefore wire together, as will a pair of axons from neighboring sites
in the right eye; but a right-eye axon and a left-eye axon will rarely fire together,
and will instead compete. Indeed, if activity from both eyes is silenced using toxins
that block axonal electrical activity or synaptic signaling, as described above,
the inputs fail to segregate correctly.
The segregation of inputs from the two eyes is only the first of a series of activity-dependent
adjustments of visual connections, whose maintenance is extraordinarily
sensitive to experience early in life. If, during a certain sensitive period
(ending at about 5 years of age in humans), one eye is kept covered for a time so
as to deprive it of visual stimulation, while the other eye is allowed normal stimulation,
the deprived eye loses its synaptic connections to the cortex and becomes
almost entirely, and irreversibly, blind. In accordance with what the firing rule
would predict, a competition has occurred in which synapses in the visual cortex
made by inactive axons are eliminated while synapses made by active axons are
consolidated. In this way, cortical territory is allocated to axons that carry information
and is not wasted on those that are silent.
Activity-dependent synaptic changes are not confined to early life. They
also occur in the adult brain, where many synapses show both functional and
morphological alterations with use. This synaptic plasticity is thought to have a