Visual intensity and pattern discrimination deficits after lesions of the optic lobe in pigeons.

Abstract

Pigeons were trained to discriminate stimuli that differed in luminance or geometric pattern. Following discrimination training, bilateral stereotaxic lesions were made in the optic tectum and in the associated nuclei isthmi, located within the depth of optic lobe. In some cases the nuclei isthmi were destroyed with only minimal injury to the tectal strata. After recovery from surgery, the discrimination performance of the birds was again tested. Cases with massive destruction of the optic tectum and either direct or indirect damage to the nuclei isthmi showed extremely severe postoperative deficits in visual discrimination performance. Some cases were unable to reliably discriminate gross stimulus differences even after extensive postoperative retraining. Others eventually were able to perform the simpler discrimination tasks, but failed to recover their preoperative performance capabilities on the more difficult discrimination tasks. Cases with injury confined principally to the nuclei isthmi had less impairment of performance than the ones with injury to the tectum and the isthmic nuclei. In general, the results of this study suggested that lesions of the optic tectum in pigeons produced more severe and intractable deficits of visual discrimination performance than did lesions of structures more rostral in the tectofugal pathway (i.e. nucleus rotundus or ectostriatum). The data further suggest a separate role of the nuclei isthmi in the processing of visual information. The implications of the results for the understanding of the evolution of the vertebrate visual system are discussed.