Thalamic connections of the ground squirrel superior colliculus and their topographic relations

The connections of the superior colliculus (SC) of the ground squirrel Spermophilus tridecemlineatus were studied with the horseradish peroxidase (HRP) method. Multiple pressure injections of HRP served to define the total pattern of SC projections while iontophoretic injections allowed differentiation of connections of the deep and superficial layers and determination of topographic relations of SC with its associated nuclei. The deep laminae were mainly connected with auditory, somatosensory and reticular regions of the brain, including the inferior colliculus, zona incerta, substantia nigra, mesencephalic central grey, pontine nuclei, spinal trigeminal nucleus, nucleus of the posterior commissure, thalamic reticular nucleus, raphe nuclei, lateral vestibular nucleus, the lateral superficial reticular formation of the medulla, the mesencephalic reticular formation, nucleus gracilis and the cervical spinal cord. The superficial laminae were connected with visual system structures. They were reciprocally connected with the dorsal and ventral lateral geniculate nuclei, the pretectum, nucleus lateralis posterior (LP), the parabigeminal nucleus and the contralateral SC. Connections between the SC and the dorsal lateral geniculate were topologic. LP was found to consist of three divisions: rostrolateral, rostromedial and caudal. SC was interconnected with the rostrolateral and caudal divisions. The connections between the SC and the rostrolateral division were topologic; those with the caudal division were not. The connections of the deep collicular layers in ground squirrels were similar to those which have been reported for cats and monkeys. The connections of the superficial laminae were more extensive than has been reported in other species. These elaborate interconnections indicate extensive interaction between primary retinal projection nuclei in the processing of visual information.