Functional subdivisions of the ascending visual pathways in the pigeon.

This study represents an attempt to examine an alternative view of the functional architecture of the ascending visual pathways in pigeons. According to this conception the pars dorsalis (GLd) of the thalamofugal system represents the lateral monocular field of view and is frontally blind to a large extent. The tectofugal system, on the other hand, processes frontal visual input within the framework of asymmetrical tectorotundal connections. As a result, the left, but not the right, rotundus should be able to integrate to an important degree the input from both eyes via the tecta of both hemispheres. Two lesion studies were conducted to test these assumptions. In the first psychophysical experiment, the visual acuity was determined in head-fixed pigeons. After thresholds were determined, stereotaxic lesions were placed in the GLd and/or the rotundus. Multiple regressions between structure specific lesion extents and postoperative threshold alterations demonstrated that only GLd lesions contributed to acuity reductions. In the second experiment the acuity threshold of pigeons under binocular and monocular conditions was determined in a conventional skinner box before GLd and/or rotundus lesions. Multiple regression analyses showed that rotundus--but not GLd lesions--contributed to performance losses. The left rotundus lesions were significantly related to threshold elevations under both monocular conditions, while the right rotundus only contributed together with the left rotundus to binocular performance. The double dissociation revealed in these experiments indicates that the ascending pathways in pigeons are functionally segregated and differentially process frontal and lateral as well as left- and right-sided inputs.