Interactions between components of the avian visual system.

Pigeons were trained to perform in a psychophysical procedure for determining luminance difference thresholds. After the psychometric data had stabilized, lesions were made in specific cell populations of the tectofugal visual pathway. In one group of pigeons, the lesions were confined to nucleus rotundus (Rt). In a second group, the lesions included not only Rt, but also the pars ventralis of the lateral geniculate nucleus (GLv). In the third group, the lesions included Rt, GLv and nucleus subpretectalis (SP). The postoperative performance of the birds in the Rt group showed elevations in threshold that represented substantial losses in sensory capacity. In contrast, the birds with lesions of Rt + GLv showed little or no change in their postoperative thresholds. Finally, the birds in the Rt + GLv + SP group were as impaired postoperatively as the birds with lesions of Rt alone. The results are interpreted in the context of the interaction between components of parallel routes within the tectofugal pathway and parallel pathways within the visual system.