DETERMINATION OF NEURAL CONNECTIONS
185
differences of localization inside a given set of neurons. In the following
paragraphs, some experimental results will be mentioned which are
very difficult to explain in terms of such a system. They raise the question of whether the assumed selective connections are the only factor
in the establishment of specific nervous functions. An attempt is made,
therefore, to find alternative interpretations that might overcome the
difficulties mentioned. For clarity, the experiments will be divided into
two groups. In the next section are collected those results from which an
embryonic determination of functional specificity in neurons is apparent.
This will be called "inherent specificity." Section III deals with the
acquired specificities, i.e., when the neuron gains its specificity by making
contact with the periphery. Then in Section IV the possibility of establishing a neural organization on a basis other than functional specificity
is considered.
II. Inherent Specificities
A. Visual System
As previously mentioned, from a series of eye rotation experiments
Sperry (1944, 1945) concluded that each neuron in the retina and in
the optic center differs qualitatively from the others according to its
position within the retinal or tectal field, respectively. The qualitative
differences ensure that the optic fibers are predisposed to establish highly
selective connections in appropriate focal areas of the optic tectum,
which results in the characteristic retinotopic projection. A few experiments are selected in this section to study the development of the
retinotectal specificity, the tectal projection of optic fibers under various
experimental conditions, and the histology of the visual system.
1. Embryonic
Determination
of the
Functional
Polarity of the Eye
Primordium
Stone (1960) succeeded in demonstrating the functional polarity of
the retina in an early embryonic stage when no neural connections had
yet been established. According to his experiments, rotated visual fields
developed following rotated transplantation of an eye cup at a late
embryonic stage (Harrison stage 36) in Ambystoma.
Earlier transplantation of the eye primordium gave rise to normal vision.
This problem was subjected to a more detailed study on
Triturus
vulgaris and T. cristatus
(Székely, 1954). Although normal vision was
found after the rotation of the eye field in the medullary plate stage,
reversed visual fields developed when the eye rotation was done at the
next earliest stage accessible for eye surgery, i.e., immediately after the
185
differences of localization inside a given set of neurons. In the following
paragraphs, some experimental results will be mentioned which are
very difficult to explain in terms of such a system. They raise the question of whether the assumed selective connections are the only factor
in the establishment of specific nervous functions. An attempt is made,
therefore, to find alternative interpretations that might overcome the
difficulties mentioned. For clarity, the experiments will be divided into
two groups. In the next section are collected those results from which an
embryonic determination of functional specificity in neurons is apparent.
This will be called "inherent specificity." Section III deals with the
acquired specificities, i.e., when the neuron gains its specificity by making
contact with the periphery. Then in Section IV the possibility of establishing a neural organization on a basis other than functional specificity
is considered.
II. Inherent Specificities
A. Visual System
As previously mentioned, from a series of eye rotation experiments
Sperry (1944, 1945) concluded that each neuron in the retina and in
the optic center differs qualitatively from the others according to its
position within the retinal or tectal field, respectively. The qualitative
differences ensure that the optic fibers are predisposed to establish highly
selective connections in appropriate focal areas of the optic tectum,
which results in the characteristic retinotopic projection. A few experiments are selected in this section to study the development of the
retinotectal specificity, the tectal projection of optic fibers under various
experimental conditions, and the histology of the visual system.
1. Embryonic
Determination
of the
Functional
Polarity of the Eye
Primordium
Stone (1960) succeeded in demonstrating the functional polarity of
the retina in an early embryonic stage when no neural connections had
yet been established. According to his experiments, rotated visual fields
developed following rotated transplantation of an eye cup at a late
embryonic stage (Harrison stage 36) in Ambystoma.
Earlier transplantation of the eye primordium gave rise to normal vision.
This problem was subjected to a more detailed study on
Triturus
vulgaris and T. cristatus
(Székely, 1954). Although normal vision was
found after the rotation of the eye field in the medullary plate stage,
reversed visual fields developed when the eye rotation was done at the
next earliest stage accessible for eye surgery, i.e., immediately after the