Organization of the projection from the superficial to the deep layers of the hamster's superior colliculus as demonstrated by the anterograde transport of Phaseolus vulgaris leucoagglutinin

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Abstract
Anterograde tracing with Phaseolus vulgaris leucoagglutinin (PHA‐L) was employed to describe the projection from the superficial to the deep layers of the hamster's superior colliculus (SC). Deposits of PHA‐L in the stratum griseum superficiale (SGS) resulted in labelled terminal swellings in the stratum opticum and all of the deep laminae (the stratum griseum intermediate [SGI], stratum album intermedium [SAI], stratum griseum profundum [SGP], and stratum album profundum [SAP]). Labelled terminals were also visible in the periaqueductal gray (PAG). Reconstructions of individual axons showed that many collaterals in the deep laminae arose from axons that projected to targets outside the colliculus. The projection from the superficial to the deep laminae had a loose topographic organization, and the trajectories of interlaminar axons were generally deflected laterally from “projection” lines that were orthogonal to the SC surface. Physiological recording and receptive field mapping were used to determine actual projection lines, which connect neurons in the superficial and deep layers that have receptive fields with the same elevation. These projection lines closely matched the trajectory of the pathway from the superficial to the deep laminae.