DETERMINATION OF NEURAL CONNECTIONS
193
specificity does not determine, therefore, a strict cell-to-cell selective
connection pattern between retinal and tectal neurons, as was inferred
from the behavioral experiments. There is undoubtedly some kind of
specificity, which ensures that the fibers can roughly find one or other
part of the tectum, but it remains to be investigated whether it is a
peculiar neural mechanism or something else that elaborates the final
minute projection pattern found in normal animals.
3. Histological Investigations
of the Visual
System
Not very much histological work has been done on the visual system
of amphibia, and for the purpose of the present review only two studies
will be mentioned. One of them concerns the retinotectal projection, and
the other deals with the structure of the optic center.
Attardi and Sperry (1963) have reported selective reconnections of
optic fibers following nerve regeneration from partially ablated retinas
in goldfish. In this species the optic nerve, before entering the midbrain,
divides into two main bundles. The lateral tract contains fibers from the
dorsal half of the eye and the medial tract consists of fibers of ventral
origin. As they traverse the medial and lateral circumference of the
optic lobe, respectively, the fibers gradually turn radially into the
tectum where they run in a superficial parallel layer and terminate in a
retinotopic distribution with the nasal fibers located in the caudal, the
temporal fibers in the rostral, the dorsal fibers in the lateral, and the
ventral fibers in the medial part of the optic lobe. With the dorsal half
of the retina destroyed and the optic nerve cut, the regenerating optic
fibers selectively entered the medial tract to connect with the medial
tectum. Conversely, when the dorsal retina remained intact, the regenerating fibers grew into the lateral tract and into the lateral tectum. After
removing the temporal retina, the surviving nasal fibers entered both
tracts and, after bypassing the rostral part of the tectum, filled the
parallel layer in the caudal part; in the reverse case, the temporal fibers
grew into the rostral part. The authors regarded these results as direct
microscopical evidence for the orderly selective termination of optic
fibers in the brain. An unexpected finding was also obtained: namely,
that the regenerating fibers became segregated immediately beyond the
nerve scar and selectively entered the proper tract. The authors suggested
a presumably chemotactic selectivity that determined the route to be
followed by the fibers in the optic nerve. This assumption seemed to
gain support from the experiment in which the medial and lateral tracts
were cross-united, each to the opposite central stump, and in the tectum
the fibers crossed back toward their original pathway (Arora and Sperry,
1962).
193
specificity does not determine, therefore, a strict cell-to-cell selective
connection pattern between retinal and tectal neurons, as was inferred
from the behavioral experiments. There is undoubtedly some kind of
specificity, which ensures that the fibers can roughly find one or other
part of the tectum, but it remains to be investigated whether it is a
peculiar neural mechanism or something else that elaborates the final
minute projection pattern found in normal animals.
3. Histological Investigations
of the Visual
System
Not very much histological work has been done on the visual system
of amphibia, and for the purpose of the present review only two studies
will be mentioned. One of them concerns the retinotectal projection, and
the other deals with the structure of the optic center.
Attardi and Sperry (1963) have reported selective reconnections of
optic fibers following nerve regeneration from partially ablated retinas
in goldfish. In this species the optic nerve, before entering the midbrain,
divides into two main bundles. The lateral tract contains fibers from the
dorsal half of the eye and the medial tract consists of fibers of ventral
origin. As they traverse the medial and lateral circumference of the
optic lobe, respectively, the fibers gradually turn radially into the
tectum where they run in a superficial parallel layer and terminate in a
retinotopic distribution with the nasal fibers located in the caudal, the
temporal fibers in the rostral, the dorsal fibers in the lateral, and the
ventral fibers in the medial part of the optic lobe. With the dorsal half
of the retina destroyed and the optic nerve cut, the regenerating optic
fibers selectively entered the medial tract to connect with the medial
tectum. Conversely, when the dorsal retina remained intact, the regenerating fibers grew into the lateral tract and into the lateral tectum. After
removing the temporal retina, the surviving nasal fibers entered both
tracts and, after bypassing the rostral part of the tectum, filled the
parallel layer in the caudal part; in the reverse case, the temporal fibers
grew into the rostral part. The authors regarded these results as direct
microscopical evidence for the orderly selective termination of optic
fibers in the brain. An unexpected finding was also obtained: namely,
that the regenerating fibers became segregated immediately beyond the
nerve scar and selectively entered the proper tract. The authors suggested
a presumably chemotactic selectivity that determined the route to be
followed by the fibers in the optic nerve. This assumption seemed to
gain support from the experiment in which the medial and lateral tracts
were cross-united, each to the opposite central stump, and in the tectum
the fibers crossed back toward their original pathway (Arora and Sperry,
1962).