Nucleus rotundus and nucleus dorsolateralis posterior (DLP) are the thalamic components of two parallel pathways within the tectofugal division of the pigeon visual system. An earlier study (Hodos, Weiss & Bessette, 1986) had shown that lesions in direct telencephalic recipients of projections from rotundus and DLP produced postoperative elevations in size-difference thresholds only if the lesion included both structures. What was not revealed by their study was whether the integrity of both thalamic components is necessary for pigeons to discriminate small differences in the size of stimuli or whether the birds could still make the discrimination with only one of the two nuclei intact. This question was particularly important because no prior behavioral evidence existed to indicate that DLP plays a role in visual information processing. Therefore, 14 pigeons were tested preoperatively using a variant of the method of constant stimuli to determine the smallest difference between the size of two annuli that the subjects could discern. The comparison stimuli, which were presented in a successive discrimination procedure, ranged from 3.5–15 mm in diameter. After surgery, in which lesions were placed bilaterally in rotundus, DLP, or both structures, the subjects' size-difference thresholds were again determined. Combined lesions of rotundus and DLP resulted in impaired psychophysical performance. The postoperative behavior was characterized by initial elevations in threshold followed by a gradual improvement in performance. Some birds returned to their preoperative level. By comparison, subjects with lesions in rotundus or DLP alone showed an immediate return to their preoperative sensitivity level. These results indicate that both nuclei can process information about the size of visual stimuli. Moreover, the processing that occurs within either nucleus is sufficient for the pigeon to discriminate size differences. The present experiment provides the first behavioral evidence that DLP participates in visual information processing.

The responses of single cells to luminance, color and computer-generated spots, bars, kinematograms, and motion-in-depth stimuli were studied in the nucleus rotundus of pigeons. Systematic electrode penetrations revealed that there are several functionally distinct subdivisions within rotundus where six classes of visual-selective cells cluster. Cells in the dorsal-posterior zone of the nucleus respond selectively to motion in depth (i.e. an expanding or contracting figure in the visual field). Most cells recorded from the dorsal-anterior region responded selectively to the color of the stimulus. The firing rate of the cells in the anterior-central zone, however, is dramatically modulated by changing the level of illumination over the whole visual field. Cells in the ventral subdivision strongly respond to moving occlusion edges and very small moving objects, with either excitatory or inhibitory responses. These results indicate that visual information processing of color, ambient illumination, and motion in depth are segregated into different subdivisions at the level of nucleus rotundus in the avian brain.