BILATERALLY PROJECTING NEURONS IN THE 2 VISUAL PATHWAYS OF CHICKS

By means of a double-labeling technique, we have investigated the orga nization of the bilateral thalamo-Wulst and tecto-rotundal projections in 2-day old chicks. After injecting fluorogold (FG) into one side of the visual Wulst and rhodamine B isothiocyanate (RITC) into the other side of the visual Wulst, the labeled neurons in the nucleus genicula tus lateralis pars dorsalis (GLd) were examined. Although the distribu tion areas of ipsilaterally and contralaterally labeled neurons overla p partly, very few double-labeled neurons were found (only 0.01% doubl e-labeled neurons). This suggests that the ipsilateral and contralater al projections to the Wulst come from different neuronal populations o f the thalamus. The FG and RITC were also injected into the rotundal n uclei (Rt) on each side of the thalamus and the labeled neurons in the optic tectum (TeO) were examined. In the TeO, the distribution areas of the neurons labeled ipsilaterally and contralaterally to Rt overlap completely and we found that up to 45% of the tectal cells were doubl e-labeled by both FC and RITC. Therefore, many tectal neurons have axo n collaterals so that they project to the Rt on both sides of the thal amus and must send information simultaneously to both sides of the bra in. The differences in the structural organization of the two visual p athways are discussed with reference to the transmission of informatio n to higher centers on both sides of the brain. (C) 1998 Elsevier Scie nce B.V. All rights reserved.