Near-field acuity changes after visual system lesions in pigeons. II. Telencephalon

Pigeons were trained to perform in a psychophysical task that measured their minimum-separable visual acuity. After their performance stabilized, lesions were made in telencephalic components of the visual system. In one group, lesions were made in the ectostriatum, which is the telencephalic target of the tectofugal visual pathway. These cases showed severe to moderate losses of acuity. The magnitude of the loss was correlated with the extent of ectostriatal damage. In another group, lesions were made in the visual Wulst, a portion of which receives the ascending fibers of the thalamofugal visual pathway. Within this group, only lesions that were large and included all components of the visual Wulst were effective in decreasing visual acuity to a moderate degree. A partial correlation analysis indicated that the components of the visual Wulst that were responsible for the acuity changes were the accessory hyperstriatum and the hyperstriatum ventrale. However, lesions that were generally confined to these regions alone were ineffective. Also ineffective were lesions of the granular components of the visual Wulst, which receive the ascending thalamofugal fibers. The results raise questions about the presumed roles of the tectofugal pathway as a background-vision mechanism and the thalamofugal pathway as a fine-detail vision mechanism.