Structural organization of parallel information processing within the tectofugal visual system of the pigeon

Visual information processing within the ascending tectofugal pathway to th e forebrain undergoes essential rearrangements between the mesencephalic te ctum opticum and the diencephalic nucleus rotundus of birds. The outer tect al layers constitute a two-dimensional map of the visual surrounding, where as nucleus rotundus is characterized by functional domains in which differe nt visual features such as movement, color, or luminance are processed in p arallel. Morphologic correlates of this reorganization were investigated by means of focal injections of the neuronal tracer choleratoxin subunit B in to different regions of the nuclei rotundus and triangularis of the pigeon. Dependent on the thalamic injection site, variations in the retrograde lab eling pattern of ascending tectal efferents were observed. All rotundal pro jecting neurons were located within the deep tectal layer 13. Five differen t cell populations were distinguished that could be differentiated accordin g to their dendritic ramifications within different retinorecipient laminae and their axons projecting to different subcomponents of the nucleus rotun dus. Because retinorecipient tectal layers differ in their input from disti nct classes of retinal ganglion cells, each tectorotundal cell type probabl y processes different aspects of the visual surrounding. Therefore, the dif ferential input/output connections of the five tectorotundal cell. groups m ight constitute the structural basis for spatially segregated parallel info rmation processing of different stimulus aspects within the tectofugal visu al system. Because-two of five rotundal projecting cell groups additionally exhibited quantitative shifts along the dorsoventral extension of the tect um, data also indicate visual field-dependent alterations in information pr ocessing for particular visual features. (C) 2001 Wiley-Liss, Inc.