Ronald L. Meyer’s research while affiliated with California Institute of Technology and other places

In normal goldfish, lesions of various size were made in nasal or temporal retina immediately prior to retinal labeling with tritiated proline. The resulting gaps in retinal innervation of tectum indicated that the projection is retinotopographically ordered to a precision of about 50 micrometer. Similarly, acute tectal incisions transecting the optic pathways were combined with immediate retinal labeling. The resulting tectal denervation confirmed that most fibers follow highly ordered paths through the stratum opticum of tectum; but a few fibers were found to follow unusual paths to their appropriate tectal positions. In other fish, the optic nerve was crushed. At various times afterwards, retinotopography and pathway order were similarly analyzed by making retinal lesions or tectal incisions just prior to labeling. For up to 40 days after crush, the projection lacked any refined retinotopic order. Only a gross topography could be demonstrated. Over several months, retinotopography gradually improved eventually approaching that of normals. Correlated with this was an initial stereotypic growth through the pathways of the stratum opticum followed by a long period of highly anomalous growth through the innervation layer. Evidently, many regenerated fibers grew in through inappropriate routes to the wrong region of tectum but subsequently arrived at their appropriate locus by circuitous routes within the innervation layer.

An abnormal, ipsilateral projection was formed by deflecting optic fibers that normally innervate the posterior part of one tectum into the anterior end of the opposite tectum. When anterior recipient tectum was simultaneously denervate, the deflected fibers formed a retinotopic map in this region that was reversed with respect to the anterior-posterior tectal axis.

A small group of selected optic fibers were surgically deflected from one tectum into the other, thus creating a novel additional projection originating from a small area of ipsilateral retina. The normal fibers to this "recipient" tectum were also severed so that both the deflected and the normal fibers regrew into this tectum at about the same time. The reinnervation pattern was analyzed by autoradiography and electrophysiologic mapping. Both techniques showed that the deflected fibers and the "normal" fibers failed to intermix. The deflected fibers typically formed several well-defined patches of innervation in roughly the appropriate region of denervated recipient tectum. The normal fibers filled in the remaining uninnervated tectal areas and were completely or nearly completely exculded from the patches occupied by the deflected fibers. This segregation was often quite sharp having an apparent average overlap less than 25-50 micron. The electrophysiology indicated that the projections of both deflected and normal fibers were retinotopically organized but that the mapping by the normal fibers was compressed. This compression, an apparent consequence of being squeezed onto a smaller than normal region of tectum, was similar to that previously observed following ablations of part of the tectum. The negligible surgical damage in the present experiment, however, excludes the kind of cytochemical reorganization previously suggested to produce compression. The findings also provide evidence for a competitive type of interaction between optic fibers.

In goldfish, one eye was enucleated, and after two or more weeks a select fraction of optic fibers from the remaining eye was deflected into the ipsilateral optic tectum. At varying intervals, the optic reinnervation of the ipsilateral tectum was measured by autoradiography and electrophysiologic mapping. Both methods indicated the deflected optic fibers not only innervated the appropriate region of tectum but also spread beyond this, occupying a total area that was several times greater than normal. Correlated with this spreading were low grain density in the autoradiograms and reduction in the number of amplitudes of units recorded electrophysiologically. The electrophysiology also revealed this projection to be almost devoid of topographic organization, showing only a crude but appropriate polarity.

Tritiated thymidine was injected intraperitoneally into juvenile goldfish that were 3 to 6 cm in body length. After 6 h to 32 months these fish were then killed for autoradiography of retina and tectum. The autoradiographs indicated that despite the maturity of these goldfish both retina and tectum were still growing by the accretion of new cells at their margins. As was shown for Xenopus, new retinal cells were added around the entire retinal circumference, while new tectal cells were added principally around the posterior tectal margin and none to the anteriormost edge. However, goldfish of this size have a well-defined and functional retinotopic projection of retina onto tectum. This means that newly created ganglion cells of temporal retina, destined to send axons to anterior tectum, will find only old tectal cells at their target locus with which to connect. Presumably preexisting retinal terminals in the tectum must be shifted posteriorly to allow the new ganglion cells to find tectal synaptic sites.

An eye-in-water electrophysiological recording method was used to map accurately the retinotectal projection of normal goldfish, and the results were compared with those obtained using the more customary technique of recording with the eye in air. In air, the receptive fields were found not only enlarged, as expected from the extreme myopia, but were also shifted toward the periphery by as much as 25°, resulting in a sizable deficit in the map. In view of this mapping error, the eye-in-water technique was used to reinvestigate the projection plasticity previously found with the eye-in-air recording method. After ablation of the posterior half-tectum or medial quartertectum, with and without optic nerve crush, recordings were obtained in 40 fish, 1 to 18 months postsurgically. The results substantially confirmed that compression, whereby the visual field, including regions surgically deprived of their normal target zones, comes to project onto the tectal remnant in compressed retinotopic fashion, but the compression found was not homogeneous and as complete as previously thought. Rather, the degree of compression was greatest near the lesion and progressively decreased toward the opposite tectal pole where it was frequently absent. This compression difference between these tectal regions was typically a factor of two and in early postsurgical stages as great as three to four. In addition, regions of the peripheral field corresponding to the tectal lesion were found to be missing from the recorded projection even after long survival times. Some projection anomalies, not previously seen, were also observed. These findings have implications for theories on the growth of selective nerve connections.

Mapping of retinotectal projections in the tree frog Hyla regilla was carried out by both behavioral and electrophysiological recording techniques following tectal ablations, with and without optic nerve regeneration. Scotomata produced by unilateral and bilateral half tectum ablations and by unilateral rectangular midtectal excisions were found to remain essentially unaltered in all cases through recovery periods up to 334 days. Similarly, electrophysiological mapping of the rostral half tectum separated by Gelfilm implants from the caudal tectum for up to 191 days yielded a normal rostral visual field. The stability of the retinotectal projection in adult Hylidae observed in these experiments contrasts with the plastic readjustments obtained in young goldfish in which the retinotectal system is still probably growing and presumably capable of field regulation. The results are taken to support the original explanatory model for developmental patterning of retinotectal connections in terms of selective cytochemical affinities between retinal and tectal neurons.