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GEORGE SZÉKELY
the eye polarity in the sense that, after partial mutilation of the eye
primordium, the already determined part reestablishes the functional
polarity by influencing the differentiation of the nondetermined part,
without involving any regeneration.
To establish a highly selective connection pattern by the specified optic
fibers in the optic lobe, it is logical to assume, as Sperry did, that the
central neurons upon which they terminate should be specified as well.
However, very little is known about the optic tectum in this context. In
Ambystoma,
Crelin (1952) found reduced but normal vision following
rotation of the tectum. Unfortunately, the experiments were done in an
early embryonic stage in which functional determination of the tectum
opticum was not to be expected, and Crelin's experiments yield little
information about this important question.
2. Electrophysiological
Studies on the Retinotectal
Projection
Electrophysiological investigations have given a great deal of information about possible nerve connections in the optic lobe. A systematic
representation of the retina on the optic lobe was first shown by Buser
(1955). This investigation was then extended by Gaze (1958), who
succeeded in demonstrating that fibers of a regenerating optic nerve,
even after previous eye rotation, grew back to their original place in
the optic lobe (Gaze, 1959), which supported Sperry's earlier suggestion.
Having found four types of optic fiber terminals of different function
(contrast, convexity, movement, and dimming detection) in four separate layers of the optic lobe, Lettvin et al. (1959) showed not only that
the fibers grew back in a retinotopic order following optic nerve transection, but also that the appropriate terminals found their proper layers.
Thus, Sperry's hypothesis seemed to be fully corroborated. However, a
few other experiments in which electrophysiological methods were used
for analysis deserve more detailed comment.
a. Projection of Optic Fibers from Regenerating
Retina. This experiment was carried out by Graf stein and Burgen (1964) who, in adult
newts, cut the eye along its equator and removed the corneal part, which
was then replaced with a 180° rotation. Three months later, after confirming the recovery of vision, they mapped the evoked potentials on the
surface of the tectum opticum upon focal illumination of the retina.
Their findings showed three different types of projection: (i) the projection was normal, as if nothing had happened to the eye; (ii) the projection was completely rotated; or (iii) within a rotated projection pattern
they found a double localization of the central area—one of these was
in a normal position, the other corresponded to the rotated projection.
It is extremely difficult to interpret these results since many unknown
GEORGE SZÉKELY
the eye polarity in the sense that, after partial mutilation of the eye
primordium, the already determined part reestablishes the functional
polarity by influencing the differentiation of the nondetermined part,
without involving any regeneration.
To establish a highly selective connection pattern by the specified optic
fibers in the optic lobe, it is logical to assume, as Sperry did, that the
central neurons upon which they terminate should be specified as well.
However, very little is known about the optic tectum in this context. In
Ambystoma,
Crelin (1952) found reduced but normal vision following
rotation of the tectum. Unfortunately, the experiments were done in an
early embryonic stage in which functional determination of the tectum
opticum was not to be expected, and Crelin's experiments yield little
information about this important question.
2. Electrophysiological
Studies on the Retinotectal
Projection
Electrophysiological investigations have given a great deal of information about possible nerve connections in the optic lobe. A systematic
representation of the retina on the optic lobe was first shown by Buser
(1955). This investigation was then extended by Gaze (1958), who
succeeded in demonstrating that fibers of a regenerating optic nerve,
even after previous eye rotation, grew back to their original place in
the optic lobe (Gaze, 1959), which supported Sperry's earlier suggestion.
Having found four types of optic fiber terminals of different function
(contrast, convexity, movement, and dimming detection) in four separate layers of the optic lobe, Lettvin et al. (1959) showed not only that
the fibers grew back in a retinotopic order following optic nerve transection, but also that the appropriate terminals found their proper layers.
Thus, Sperry's hypothesis seemed to be fully corroborated. However, a
few other experiments in which electrophysiological methods were used
for analysis deserve more detailed comment.
a. Projection of Optic Fibers from Regenerating
Retina. This experiment was carried out by Graf stein and Burgen (1964) who, in adult
newts, cut the eye along its equator and removed the corneal part, which
was then replaced with a 180° rotation. Three months later, after confirming the recovery of vision, they mapped the evoked potentials on the
surface of the tectum opticum upon focal illumination of the retina.
Their findings showed three different types of projection: (i) the projection was normal, as if nothing had happened to the eye; (ii) the projection was completely rotated; or (iii) within a rotated projection pattern
they found a double localization of the central area—one of these was
in a normal position, the other corresponded to the rotated projection.
It is extremely difficult to interpret these results since many unknown