Connectional organization of the superior colliculus

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The mammalian superior colliculus is a prominent subcortical sensorimotor integrator that plays an important and specific role in allowing the animal to orient to visual, auditory and somatosensory stimuli 1–8. Over the past decade, anatomical and physiological studies have revealed some important organizational correlates of this functional specificity. In the early 1970s, combined anatomical and behavioral studies in the laboratory of I.T. Diamond 1,5, as well as electrophysiological analyses by B. Gordon 9 and B. Stein 7 and colleagues, demonstrated fundamental differences between the superficial collicular layers (I–III) and the deep layers (IV–VII). This ‘superficial-deep’ distinction was in turn refined as a result of studies that revealed connectional and functional differences between individual layers comprised in these two major regions. Recent data indicate that the layers are themselves divisible according to criteria based on anatomical connections and physiology. In light of these recent findings we suggest that the connectional subdivisions of individual superficial layers differ in type from those of individual deep layers. We further suggest that the different types of subdivision reflect the different quantities and qualities of connections of the superficial versus the deep layers.