Laminar organization of superior colliculus in the rabbit: a study of receptive-field properties of single units.

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Abstract
Single units (284) were studied in the superior colliculus of 20 paralyzed Dutch-belted rabbits. The upper 2/3 of the stratum griseum superficial contained units that responded briskly to small visual stimuli. The receptive fields (mean diameter, 2.86.degree.) were the smallest encountered in the superior colliculus. The majority of units responded well to stationary stimuli. The lower 1/3 of the stratum griseum superficiale contained visual units that had larger receptive fields (mean diameter, 8.63.degree.) and more often preferred moving stimuli than did units in the upper 2/3 of the stratum griseum superficiale. These fields were more elongated in the anterior-posterior dimension than those of units in the rest of the colliculus. Layers deep to the stratum griseum superficiale had units that responded to visual, somatosensory and auditory stimuli. Visual units had larger receptive fields and were movement sensitive and direction selective more frequently than those in the stratum griseum superficiale. More units showed habituation to visual stimuli, and more unresponsive units were encountered. Specifically, units in the stratum griseum intermediate had visual receptive fields of a mean diameter of 19.86.degree.. The somatosensory fields were large, sometimes including 1/4 of the body surface. The few visual receptive fields in the stratum griseum profundum included much of the contralateral visual space. New evidence for a functional subdivision of the stratum griseum superficiale into 2 substrata in the rabbit is provided. Evidence in other mammalian species, such as the tree shrew, gray squirrel, cat and squirrel monkey, suggests that such a subdivision may be a common mammalian pattern.