Laminar organization of receptive-field properties in tree shrew superior colliculus.

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Abstract
Based on a constellation of receptive-field properties, cells in the tree shrew superior colliculus were grouped into 5 receptive-field categories: stationary-responsive (S-R), movement-sensitive (M-S), elongated-field (E-F), diffuse-field (D-F) and poorly-responsive (P-R). The distribution of these 5 cell classes varied across 4 cytoarchitectonic zones: upper stratum griseum superficiale, lower stratum griseum superficiale, stratum opticum and the deep layers below stratum opticum. The upper stratum griseum superficiale (upper SGS) contained almost exclusively S-R cells, which had small, well-defined receptive fields and gave brisk, consistent responses to both stationary and moving visual stimuli. The lower stratum griseum superficiale (lower SGS) contained all 5 cell classes. The stratum opticum also contained all 5 classes; there were fewer S-R, M-S and E-F cells, and the most prevalent cell class was the D-F cell. The deep layers (below stratum opticum) contained few of the S-R, M-S and E-F cells which characterized the upper and lower SGS. The prevalent cell classes in the superficial gray layer (S-R, M-S and E-F) shared common characteristics, having relatively small receptive fields with well-defined boundaries and giving consistent responses to stimulus edges. The subdivision within the stratum griseum superficiale seems unique to the tree shrew and suggests a structural and functional specialization of the upper and lower SGS. The behavioral significance of this functional division within the SGS is unknown. In normal tree shrews the large, homogeneous population of S-R cells may participate in the relay of form vision information to the extrastriate cortex. The relative absence of striate cortex terminals in the upper SGS suggests that the cells in this region may be little affected by striate cortex ablation. The cells in the upper SGS, and in particular S-R cells may, therefore, continue to relay information following the removal of striate cortex, allowing destriate tree shrews to perform visual form discriminations.