Induction of compression in the re‐established visual projections on to a rotated tectal reimplant that retains its original topographic polarity within the halved optic tectum of adult goldfish.

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Abstract
The topographic pattern of re-established retinotectal projections following various surgical manipulations of the optic tectum was studied in adult goldfish [Carassius auratus] with neurophysiological mapping methods. Immediately following excision of the caudal half of the tectum, a piece of the tectal tissue was dissected from the remaining rostral half-tectum, and then reimplanted to the same half-tectum after either 180 or 90.degree. anticlockwise rotation around the dorsoventral axis in the 1st experimental group. A majority (21 of 23) of these operated fish, in which the reimplanted tectal tissue degenerated, showed no sign of a field compression: only the nasal half of the visual field (with a localized partial scotoma corresponding to the area of the degenerated reimplant) projected on to the remaining intact area of the rostral half-tectum. In 7 fish, the re-established visual projections on to the 180 or 90.degree. rotated reimplants showed a corresponding localized 180 or 90.degree. rotation with reference to the other projections on to the surrounding intact area of the same half-tectum. Only 1 of these 7 fish showed a compression in the re-established projections from the entire visual field on to the operated half-tectum with the 90.degree. rotated reimplant. When a field compression was induced 1st in the intact rostral half-tectum following excision of the caudal half, and then a piece of the 90.degree. rotated tectal tissue was reimplanted later within the rostral half-tectum, the previously induced field compression persisted, regardless of whether the reimplanted tissue degenerated or survived. In the latter case, the compression in the re-established visual projections on to the surviving reimplant occurred according to the original topographic polarity of the 90.degree. rotated tectal tissue. A field compression could also be induced within a rotated tectal reimplant, which retained its original polarity, as follows. A piece of the tectal tissue was dissected from the central area of the whole tectum, and then reimplanted after either 180 or 90.degree. rotation. When the reimplanted tectal tissue became reinnervated later, the caudal half of the operated tectum (including the posterior half of the reimplant) was excised. The re-established visual projections on to the remaining part of the halved reimplant within the rostral half-tectum showed later a field compression in accordance with the original topographic polarity of the 180 or 90.degree. rotated tectal tissue. This is direct evidence for the compatibility between the retention of original topographic polarity by a reimplanted tectal tissue and the capability of the same tectal tissue to readjust to a disparity in size. Histological examination of the operated half-tectum with a reimplant, stained by a modified rapid Golgi method, revealed that the reimplanted tectal tissues retained highly organized cytoarchitectonic structures. Various types of tectal neurones maintained intricate interconnexions within the reimplanted tissue. These neuronal elements of the reimplanted tectal tissue may be responsible for the retention of original topographic polarity of the tectal tissue, and for its dynamic capability in readjusting to a disparity in size.