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A
Control: Fly is detected in the top visual field
Dorsal
visually-evoked activity
P
A
P
A
Ventral
Dorsal
Posterior
membrane
Anterior
membrane
A
P
A
P
A
P
A
P
A
P
A
P
A
Ventral
Te c t a l membrane
fragment striped
carpet
B
Eye Rotation: Fly is detected in the bottom visual field
A = anterior
P = posterior
Dorsal
visually-evoked activity
Ventral
Ventral
D
N
Eye
T
Nasal
Temporal
A
Retinal Strip
V
Dorsal
A
P
A
P
A
P
A
P
A
P
A
P
A
Heat treated or
PI-PLC treated
posterior
membranes
FIGURE 6.10 Maladaptive topography implies chemospeci-
ficity. A. A normal frog sees a fly above on its ventral retina, which
projects to the dorsal tectum, leading to a snap in the appropriate
upwards direction. B. A frog with a rotated eye sees the same fly on
what used to be the dorsal retina, which projects as ever to the
ventral tectum, leading to a snap in the wrong downward direction.
(After Sperry, 1943)
that topographic nerve connections between the retina
and its main central target, the optic tectum, were the
result of anatomical rather than experiential features of
the nervous system, as Sperry's unlucky frogs never
did learn to snap in the correct direction. Sperry rea-
soned that the retinal fibers mapped onto the tectum
according to original anatomical coordinates of the eye.
The explanation he gave was the possible existence of
biochemical tags across the retina and tectum. He pos-
tulated the existence of two or more cytochemical gra-
dients “that spread across and through each other with
their axes roughly perpendicular” (Sperry, 1963). These
separate gradients successively superimposed on the
retinal and tectal fields and surroundings would stamp
each cell with its appropriate latitude and longitude
expressed in a kind of chemical code with matching
values between retinal and tectal maps.”
The chemoaffinity hypothesis inspired many biolo-
gists and biochemists to try to find the molecules that
were responsible for topographic targeting in the
retinotectal system. Such studies often took an in vitro
approach, and for over 20 years, not much progress
was made. Friedrich Bonhoffer and colleagues made a
Nasal
Temporal
B
Retinal Strip
FIGURE 6.11 The striped carpet assay. A. An equatorial strip of
retina spanning the nasal (N) temporal (T) extent is positioned on a
striped carpet of alternating anterior (A) and posterior (P) tectal
membranes. The nasal fibers from the retinal explant grow on both
A and P tectal membranes, but the temporal fibers grow only on the
A membranes. B. If the tectal membranes are denatured or treated
with PI-PLC, which releases PI-linked membrane proteins, the tem-
poral axons also grow on both types of membranes, suggesting that
the P membranes normally have a PI-linked repulsive guidance
molecule. (After Walter et al., 1987a; Walter et al., 1987b; Walter
et al., 1990)
breakthrough when they used membranes from ante-
rior and posterior parts of the tectum to make a striped
carpet. When retinal tissue from the temporal retina
was cultured on such striped carpets, they found that
these retinal axons grow preferentially on anterior
tectal membranes (Walter et al., 1987b) (Figure 6.11).
Surprisingly, when the posterior membranes were
heated, treated with formaldehyde, or exposed to an
enzyme (PI-PLC) that removes PI-linked membrane
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