Immunohistochemical and tracing techniques were used in combination to reveal the source of a neuropeptide Y-like immunoreactive (NPY-LI) plexus in the nucleus of the basal optic root (nBOR) of the pigeon accessory optic system. Injections of rhodamine-labeled latex microspheres into nBOR produced retrograde labeling of a population of neurons interposed between the principal optic nucleus of the dorsolateral thalamus (equivalent to the mammalian dorsal lateral geniculate nucleus) and the ventral lateral geniculate nucleus. The retrogradely labeled neurons were distributed mainly in the immediate vicinity of the lateral, dorsal, and ventral aspects of the nucleus rotundus. Immunohistochemical methods revealed many NPY-containing somata within the same intergeniculate thalamic area. Double-labeling immunohistochemical and retrograde tracing experiments evidenced that many NPY-LI neurons in the intergeniculate area contained rhodamine microspheres that had been previously injected into the ipsilateral nBOR. The projection of that general thalamic area to the nBOR was then confirmed by means of anterograde transport of Phaseolus vulgaris leucoagglutinin. In these experiments, the intergeniculate region was demonstrated to project to all divisions of the nBOR and to every other retino-recipient structure, including the suprachiasmatic nucleus. Finally, electrolytic lesions of the intergeniculate area produced a dramatic reduction in the number of NPY-LI axons and terminals within the ipsilateral nBOR and also within other retino-recipient structures. These data indicate the existence of a thalamic NPY-LI projection to the pigeon nBOR of the accessory optic system. This chemically specific projection originates from the intergeniculate area, which was shown in this study to project to all other retino-recipient structures. Thus, NPY may have a role in the functional organization of the accessory optic system and also of the avian visual system as a whole.

Various residual visual capacities have been reported for the phenomenally blind field of hemispherectomized patients, providing evidence for the relative roles of cortical and subcortical pathways in vision. We attempted to characterize these functions by examining the ability of five patients to detect, localize, and discriminate high-contrast flashed, flickering and moving targets. Dependent measures were verbal, manual, and oculomotor responses. As a control for light scatter, intensity thresholds for monocular detection of targets in the hemianopic field were compared with thresholds obtained when using an additional half eyepatch to occlude the blind hemiretina of the tested eye. One unilaterally destriate patient was tested on the same tasks. In photopic conditions, none of the hemispherectomized patients could respond to visual cues in their impaired fields, whereas the destriate patient could detect, discriminate, and point to targets, and appreciate the apparent motion of stimuli across his midline. Under reduced lighting, the threshold luminance required by hemispherectomized patients to detect stimuli presented monocularly was similar to that required for their detection when all visual information was occluded in the blind field, and only available to the visual system indirectly via light scatter. In contrast, the destriate patient's monocular threshold in his blind field was substantially lower than that for stimuli directly occluded in the blind field. As we found no range of stimuli which the hemispherectomized patients could detect or discriminate that was not also associated with discriminable scattered light, we conclude that the subcortical pathways which survive hemispherectomy cannot mediate voluntary behavioural responses to visual information in the hemianopic field.

The directional selectivity of units within the nucleus of the basal optic root (nBOR) of the accessory optic system (AOS) was studied before and after lesions of the visual telencephalon (visual Wulst) in urethane-anesthetized pigeons. In intact pigeons, most nBOR units preferred upward motion with a temporal component or downward motion with a nasal component. The ipsilateral and bilateral telencephalic lesions generated a dramatic reduction in the number of cells with optimal responses to upward motion. The overall distribution of preferred directions was still bimodal following ipsilateral or bilateral Wulst lesions, with most units showing best responses to a straight temporal or to downward-nasal directions. The contralateral Wulst lesions produced, instead, a marked reduction in downward preferences. The nBOR units which were studied in these cases showed mainly upward-temporal and upward-nasal responses. These data suggest an involvement of the visual Wulst in the determination of the dictional selectivity of nBOR neurons in the pigeon. Specifically, the responses of nBOR units to upward motion appeared to depend on the integrity of the telencephalic descending systems which impinge, in both direct and indirect ways, upon that AOS nucleus. Taken together with data for the mammalian AOS, the present results indicate that nonretinal afferents to AOS nuclei have an important role in the functional organization of that subcortical visual pathway.

The B fragment of cholera toxin (CTb) is a highly sensitive anterograde tracer for the labelling of retinal axons. It can reveal dense retinofugal projections to well-known retinorecipient nuclei along with sparse but distinct input to target areas that are not commonly recognized. Following a unilateral injection of CTb into the vitreous chamber of seven adult cats, we localized the toxin immunohistochemically in order to identify direct retinal projections in these animals. Consistent with previous findings, the strongest projections were observed in the superficial layers of the superior colliculus, the dorsal and ventral lateral geniculate nuclei, the pretectal nuclei, the accessory optic nuclei, and the suprachiasmatic nucleus of the hypothalamus. However, we also found labelled terminals in several other brain areas, including the zona incerta, the medial geniculate nucleus, the lateral posterior-pulvinar complex, the lateral habenular nucleus, and the anterior and lateral hypothalamic regions. The morphological characteristics of the retinal axon terminals in most of the identified novel target sites are described.