Tectal mosaic: organization of the descending tectal projections in comparison to the ascending tectofugal pathway in the pigeon.

The optic tectum of vertebrates is an essential relay station for visuomotor behavior and is characterized by a set of connections that comprises topographically ordered input from the eyes and an output that reaches premotor hindbrain regions. In the avian tectofugal system, different ascending cell classes have recently been identified based on their dendritic and axonal projection patterns, although comparable information about the descending cells is missing. By means of retrograde tracing, the present study describes the detailed morphology of tectal output neurons that constitute the descending tectobulbar and tectopontine pathways in pigeons. Descending cells were more numerous in the dorsal tectum and differed in terms of 1) their relative amount of ipsi- vs. contralateral projections, 2) the location of the efferent cell bodies within different tectal layers, and 3) their differential access to visual input via dendritic ramifications within the outer retinorecipient laminae. Thus, the descending tectal system is constituted by different cell classes presumably processing diverse aspects of the visual environment in a visual field-dependent manner. We demonstrate, based on a careful morphological analysis and on double-labeling experiments, that the descending pathways are largely separated from the ascending projections even when they arise from the same layers. These data support the concept that the tectum is arranged as a mosaic of multiple cell types with diverse input functions at the same location of the tectal map. Such an arrangement would enable the tectal projections onto diverse areas to be both retinotopically organized and functionally specific.