Based on a constellation of receptive-field properties, cells in the tree shrew superior colliculus were grouped into five 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 five cell classes varied across four 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 five cell classes: however, S-R, M-S, and E-F cells were more prevalent than the other cell classes in this region. M-S cells had relatively small, well-defined receptive fields and gave consistent response to moving, but not to stationary, stimuli. E-F cells had elongated activating regions with suppressive flanking zones and responded optimally to appropriately oriented, elongated stimuli. The stratum opticum also contained all five classes; however, there were fewer S-R, M-S, and E-F cells, and the most prevalent cell class was the D-F cell. D-F cells had large activating regions with vague borders and responded better to small dark stimuli than to stimuli lighter than the background. 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. Instead, D-F and P-R cells predominated. P-R cells gave extremely poor, if any, responses to visual stimuli. 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. In contrast, the D-F and P-R cells characteristic of the deep layers all had large, poorly defined visual receptive fields and gave weak, inconsistent responses to visual stimuli. This difference in prevalent cell classes complements the previously demonstrated subdivision of the tree shrew superior colliculus into a superficial division, which has projections via the thalamus to the extrastriate cortex and is involved in sensory discriminations, and a deep division, which has descending connections and is involved with visuomotor behaviors.
