The intercollicular pathway in the golden hamster: an anatomical study.

The intercollicular pathway of the hamster was studied by means of a combination of horseradish peroxidase (HRP), autoradiographic, and double-labeling (nuclear yellow-HRP) techniques. Small deposits of HRP marked significant numbers of cells in the contralateral colliculus only when the injection site included the laminae ventral to the stratum opticum. Anterior deposits labeled many more neurons than injections into the caudal part of the tectum. Of the cells labeled by our injections, 18.2% were located in the superficial collicular laminae (stratum griseum superficiale and stratum opticum), and the remainder (81.8%) were in the deep layers. A wide variety of morphological cell types contributed axons to the intercollicular projection, and in a given animal the loci of the labeled neurons were generally symmetrical with the injection site. Small deposits of [3H]-leucine resulted in contralateral labeling only when the injection included the deep collicular laminae. The transported label was most dense in the stratum griseum intermediale and stratum griseum profundum, and its location was generally homotopic with the injection site. Experiments in which collicular HRP deposits were combined with large cervical spinal or pontine reticular injections of Nuclear Yellow indicated that intertectal neurons did not, in most cases, contribute axon branches to the spinal or pontine reticular projections of the colliculus. Receptive field data obtained at the time of the HRP and/or [3H]-leucine deposits demonstrated that the collicular representations of the ipsilateral and at least 45 degrees of the contralateral hemifields were encompassed by intercollicular connections. This was also true for the somatosensory representation of the entire head and a portion of the neck.