Retinal projections to the pretectum in the pigeon (Columba livia).

The retinal projection to the pretectum in the pigeon has previously been described in detail only by means of anterograde degeneration techniques (Repérant, '73). The present study reinvestigated these retinal projections by using the more sensitive anterograde autoradiographic technique. In general, our results confirm and extend those of Repérant ('73). We have found that three pretectal nuclei--the nucleus lentiformis mesencephali, tectal gray, and the area pretectalis--received heavy retinal input. A fourth pretectal nucleus, pretectalis diffusus, receives a slight retinal input. The nucleus lentiformis mesencephali can be divided into two closely apposed subnuclei that are cytoarchitecturally similar. We have termed them "lentiformis mesencephali, pars medialis" and "lentiformis mesencephali, pars lateralis." The tectal gray can be divided into a rostral, retinorecipient region and a narrow, caudal, nonretinorecipient region. The cytoarchitecture and retinal terminal field in area pretectalis have been described previously by us (Gamlin et al., '84). Pretectalis diffusus is located caudal to the retinorecipient dorsal thalamus and rostral to area pretectalis. Localized retinal injections of 3H proline delineated the number and extent of the retinal representations in the pretectum. Separate retinal representations were present in lentiformis mesencephali, pars medialis, lentiformis mesencephali, pars lateralis, the tectal gray, area pretectalis, and pretectalis diffusus. Only in the lentiformis mesencephali, pars medialis, lentiformis mesencephali, pars lateralis, and the tectal gray could the retinal representations be analyzed. Whereas the retinal representations in the lentiformis mesencephali, pars medialis and the tectal gray are comparable, the retinal representation in the lentiformis mesencephali, pars lateralis is different, being a mirror-image mediolaterally. In this study we introduce a conservative nomenclature for the retino-recipient pretectal nuclei that is consistent with earlier studies, in particular, those of Kuhlenbeck ('39), but has been modified in the light of our findings. We believe that this nomenclature, combined with the detailed cytoarchitectural descriptions provided, should facilitate future studies of the avian pretectum.