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sensory coding properties at the most peripheral level
of the nervous system. For example, mouse retinal
ganglion cells (RGC) initially respond to both an
increase and decrease in luminescence, called an ON-
OFF response . During postnatal development, the RGC
dendritic arbor is refined, and most neurons end up
producing either ON or OFF responses to visual stim-
ulation, but not both. When mice are reared in the
dark, the RGC dendrites are not refined, and the cells
continue to produce ON-OFF responses (Tian and
Copenhagen, 2003). Therefore, environmental rearing
studies may influence maturation at many levels, and
the cumulative effect is assessed in the cortex.
Kittens see vertical orientiation
Vertical Preference
Strobe light:
Kittens see stationary objects
When peripheral sensory axons reach the central
nervous system, they usually innervate the target in an
orderly manner, forming topographic maps that are
quite accurate due to molecular gradients that direct
axons to an approximate location within the target
(Chapter 6). However, synaptic activity also influences
the development of topography. This is most evident
when two maps must become aligned with one another
in the same structure. For example, binocular neurons in
the frog optic tectum respond to the same visual position
in space when activated through each eye. We have
already studied the direct contralateral projection from
retina to tectum in Chapter 6. The ipsilateral eye activates
the tectum via an indirect projection. Tectal neurons
project to a structure called nucleus isthmus , and isthmal
neurons project to the contralateral tectal lobe (Figure
9.18A, left panel). Therefore, there are two perfectly
aligned maps of visual space in the tectum, one from the
contralateral eye and one from the ipsilateral eye.
In order to test whether visual activity plays a role
in this precise alignment, frogs were reared in the dark.
Direct retinal projections from the contralateral eye
continue to form a precise map, but the indirect pro-
jection via the nucleus isthmus is poorly organized
(Keating and Feldman, 1975). That is, the formation of
one map depends largely on molecular cues, while the
other map depends on activity. The effect is so power-
ful that one can actually cause isthmal axons to move
to a new location in the tectum when the contralateral
map is disrupted (Figure 9.18A, center panel). If the
contralateral eye is rotated by 180°, a single point in
space will activate different positions in the tectum via
each eye. When this manipulation is performed while
the animals are still tadpoles, then the isthmal projec-
tion to the tectum will shift so that the ipsilateral
retinal map comes into register with the direct
Motion Selective
FIGURE 9.17 The visual environment influences orientation and
motion processing. A. Kittens were reared with goggles that per-
mitted visual experience with only vertically oriented stimuli. The
response to oriented stimuli was then obtained from single visual
cortex neurons. The cell shown responds best to vertical stimuli. In
goggle-reared cats, 87% of neurons were selective for vertical
stimuli, compared to only 50% in controls. B. Kittens were reared in
stroboscopic light that permitted visual experience with only sta-
tionary objects. The response to moving stimuli was then obtained
from single visual cortex neurons. The cell shown responds best to
stimuli moving down and to the right. In strobe-reared cats, only
10% of neurons were motion selective, compared to 66% in controls.
(Adapted from Stryker et al., 1978; Pasternak et al., 1985)
recorded in the cortex, the majority of them responded
selectively to the orientation that was present in the
rearing environment (Figure 9.17A). To determine
whether moving visual stimuli also influences the
development of motion selectivity, kittens were raised
in stroboscopic light. As nightclub enthusiasts know,
this stimulus permits one to see a full range of shapes
and colors, but it eliminates smooth motion. When
cortical neurons were recorded after a period of
strobe-rearing, the majority could no longer respond
selectively to the direction of movement (Figure 9.17B).
As with ocular dominance columns, sensory ex-
perience appears to alter normal development, rather
than shape the adult pattern. For example, orientation
columns form normally in the kitten visual cortex in
the absence of experience (Crair et al., 1998).
These results, and many others like them, show that
environmental stimuli influence a broad range of func-
tional properties. Although this discussion has focused
on the cortex, sensory experience may influence
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