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A
Normal binocular neuron in tectum
(star activates one site in tectum)
Contralateral eye rotated 180
Isthmal projection moves
(star activates one site in tectum)
°
(star activates two sites in tectum)
visual
field
N
T
T
eye
T
N
N
optic
tectum
nucleus
isthmus
B
FIGURE 9.18 Remapping in the frog visual pathway. A. In adult frogs (left), the optic tectum receives
two retinotopic projections: a direct projection from the contralateral eye and an indirect projection from the
ipsilateral eye (via the nucleus isthmus). These two projections are aligned such that a visual stimulus acti-
vates the same position in the tectum through either eye. In this example, the temporal eye activates the ante-
rior tectum. When the contralateral eye is rotated 180°, a visual stimulus activates two different positions in
the tectum, one via the eye and the other via the projection from nucleus isthmus (middle). This is because
the retinal ganglion cells in nasal eye project to the posterior tectum. Over time, the projection from nucleus
isthmus to the tectum adjusts its position so that a visual stimulus once again activates the same tectal posi-
tion through either eye (right). B. Photographs of axons from nucleus isthmus as they course through the
tectum of an adult control, and an eye-rotated animal (left). Note that control axons grow parallel to one
another from rostral to caudal. Axons from eye-rotated animals display less ordered growth. Drawings of
individual axons from nucleus isthmus in control and eye-rotated animals are shown at two postmetamor-
phic (PM) ages (right). Note that the axons of eye-rotated animals can make two terminal zones. (Adapted
from Udin and Keating, 1981; Guo and Udin, 2000)
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