DETERMINATION OF NEURAL CONNECTIONS
189
factors might contribute to the variety of projections. During surgery the
retina had been removed and had regenerated from the pigment epithelium. There is no way of finding out which pigment cell gives origin
to any particular retinal neuron. It is impossible to decide whether a
kind of "field" was at work or whether a peculiar nervous mechanism
gave rise to this curious result. It is, however, evident that the assumed
specificity of the retinal elements is not very exclusive ; this is especially
obvious from the double representation of the central retina.
b. Projection of Regenerating
Optic Fibers. Gaze and Jacobson (1963)
mapped the retinotectal projection on Rana temporaria at various intervals from 23 to 247 days after section of the optic nerve. They classified
the results obtained from sixty-seven successful recordings into four
groups according to the degree of normality. The earliest responses
recorded after nerve section comprised pattern 1, in which they found
disorganized anomalous regeneration. Responses from the optic lobe
could be evoked only from small regions of the visual field localized in
the nasal and/or temporal half. Although each half-field projected to
its appropriate region of the lobe, a more detailed retinotopic projection
was not apparent. In the normal frog, for each position of the electrode
on the tectum there was a region in the visual field, 5° to 15° in diameter, from which a response could be evoked. This was extended to 10°
to 50° in the operated frogs. In pattern 2 the abnormal projection showed
a partial organization in only the anteroposterior (nasotemporal) axis
of the retina. This partial organization was found at the rostral end of
the lobe. In pattern 3 there was a normal retinotectal projection. In
pattern 4 there was a partial or complete recovery of the normal projection, together with an anomalous but retinotopically organized projection to the wrong side of the brain. There was not a strict correlation
between the time elapsed after nerve section and the pattern of projection, but the time sequence of the result was roughly in the order given.
Although the results were obtained from recordings from a series of
frogs at different times after section of the optic nerve, rather than from
the same frog at different stages of regeneration, the authors suggested
that the incomplete types of regeneration (patterns 1 and 2) were stages
in the normal process of recovery.
Again, the interpretation of this extremely interesting experiment is
difficult and manifold. A trial-and-error mechanism with functional
correction is not feasible because of the complete inability of the animals
to relearn their erroneous optical reflexes (Sperry, 1951). One may
surmise that the regenerating fibers randomly arrive at the tectum and,
after testing the specificity of several tectal neurons by sending out
widely arborizing branches, finally settle down at the appropriate loci.
It seems to be unlikely, however, that regenerating fibers should initially
189
factors might contribute to the variety of projections. During surgery the
retina had been removed and had regenerated from the pigment epithelium. There is no way of finding out which pigment cell gives origin
to any particular retinal neuron. It is impossible to decide whether a
kind of "field" was at work or whether a peculiar nervous mechanism
gave rise to this curious result. It is, however, evident that the assumed
specificity of the retinal elements is not very exclusive ; this is especially
obvious from the double representation of the central retina.
b. Projection of Regenerating
Optic Fibers. Gaze and Jacobson (1963)
mapped the retinotectal projection on Rana temporaria at various intervals from 23 to 247 days after section of the optic nerve. They classified
the results obtained from sixty-seven successful recordings into four
groups according to the degree of normality. The earliest responses
recorded after nerve section comprised pattern 1, in which they found
disorganized anomalous regeneration. Responses from the optic lobe
could be evoked only from small regions of the visual field localized in
the nasal and/or temporal half. Although each half-field projected to
its appropriate region of the lobe, a more detailed retinotopic projection
was not apparent. In the normal frog, for each position of the electrode
on the tectum there was a region in the visual field, 5° to 15° in diameter, from which a response could be evoked. This was extended to 10°
to 50° in the operated frogs. In pattern 2 the abnormal projection showed
a partial organization in only the anteroposterior (nasotemporal) axis
of the retina. This partial organization was found at the rostral end of
the lobe. In pattern 3 there was a normal retinotectal projection. In
pattern 4 there was a partial or complete recovery of the normal projection, together with an anomalous but retinotopically organized projection to the wrong side of the brain. There was not a strict correlation
between the time elapsed after nerve section and the pattern of projection, but the time sequence of the result was roughly in the order given.
Although the results were obtained from recordings from a series of
frogs at different times after section of the optic nerve, rather than from
the same frog at different stages of regeneration, the authors suggested
that the incomplete types of regeneration (patterns 1 and 2) were stages
in the normal process of recovery.
Again, the interpretation of this extremely interesting experiment is
difficult and manifold. A trial-and-error mechanism with functional
correction is not feasible because of the complete inability of the animals
to relearn their erroneous optical reflexes (Sperry, 1951). One may
surmise that the regenerating fibers randomly arrive at the tectum and,
after testing the specificity of several tectal neurons by sending out
widely arborizing branches, finally settle down at the appropriate loci.
It seems to be unlikely, however, that regenerating fibers should initially