An autoradiographic study of the projections from the lateral geniculate body of the rat.

The projections from the lateral geniculate body of the rat were followed using the technique of autoradiography after injections of [3H] proline into the dorsal and/or ventral nuclei of this diencephalic structure. Autoradiographs were prepared from either frozen or paraffin coronal sections through the rat brain. The dorsal nucleus of the lateral geniculate projected via the optic radiation to area 17 of the cerebral cortex. There was also a slight extension of label into the zones of transition between areas 17, 18 and 18a. The distribution of silver grains in the various layers of the cerebral cortex was analyzed quantitatively and showed a major peak of labeling in layer IV with minor peaks in outer layer I and the upper half and lowest part of layer VI. The significance of these peaks is discussed in respect to the distribution of geniculocortical terminals in other mammalian species. The ventral nucleus of the lateral geniculate body had 5 major projections to brain stem structures both ipsilateral and contralateral to the injected nucleus. There were two dorsomedial projections: (1) a projection to the superior colliculus which terminated mainly in the medial third of the stratum opticum, and (2) a large projection via the superior thalamic radiation which terminated in the ipsilateral pretectal area; a continuation of this projection passed through the posterior commissure to attain the contralateral pretectal area. The three ventromedial projections involved: (1) a geniculopontine tract which coursed through the basis pedunculi and the lateral lemniscus to terminate in the dorsomedial and dorsolateral parts of the pons after giving terminals to the lateral terminal nucleus of the accessory optic tract, (2) a projection via Meynert's commissure to the suprachiasmatic nuclei of both sides of the brain stem as well as to the contralateral ventral lateral geniculate nucleus and lateral terminal nucleus of the accessory optic tract, and (3) a medial projection to the ipsilateral zona incerta. The results obtained in these experiments are contrasted with other data on the rat's central visual connections to illustrate the importance of these connections in many subcortical visual functions.