Comparison of the distribution and somatodendritic morphology of tectotectal neurons in the cat and monkey

Abstract

The presence of a commissure connecting the two superior colliculi suggests they do not act independently, but the function of the tectotectal connection has never been firmly identified. To develop a better understanding of this commissural system, the present study determined the distribution and morphology of tectotectal neurons in the cat and macaque monkey, two animals with well-studied, but different orienting strategies. First, we compared the distribution of tectotectal cells retrogradely labeled following WGA-HRP injections into the contralateral superior colliculus. In monkeys, labeled tectotectal cells were found in all layers, but were concentrated in the intermediate gray layer (75%), particularly dorsally, and the adjacent optic layer (12%). Tectotectal cells were distributed throughout nearly the entire rostrocaudal extent of the colliculus. In cats, tectotectal cells were found in all the layers beneath the superficial gray, but the intermediate gray layer contained the greatest concentration (56%). Labeled cells were almost exclusively located in the rostral half of the cat superior colliculus, in contrast to the monkey distribution. In the context of the representation of visuomotor space in the colliculus, the distribution of monkey and cat tectotectal cells suggests a correspondence with oculomotor range. So these neurons may be involved in directing orienting movements performed within the oculomotor range. The somatodendritic morphology of tectotectal cells in these two species was revealed by homogeneous retrograde labeling from injections of biocytin or biotinylated dextran amine into the contralateral colliculus. The cell classes contributing to this pathway are fairly consistent across the two species. A variety of neuronal morphologies were observed, so there is no single tectotectal cell type. Instead, cell types similar to those found in each layer, excepting the largest neurons, were present among tectotectal cells. This suggests that a sample of each layer's output is sent to the contralateral colliculus.

Topographic Organization of Excitatory and Inhibitory Commissural Connections in the Superior Colliculi and Their Functional Roles in Saccade Generation

“…Previous anatomical studies have shown the laminar location and rostrocaudal distribution of commissural neurons, the morphological characteristics of their cell bodies and dendrites, and the properties of tectotectal axon terminals (Behan and Kime 1996;Grantyn and Grantyn 1982;Grantyn et al 2004;Olivier et al 1998). Specifically, the cell bodies of tectotectal CNs were observed primarily in the intermediate gray layer [stratum griseum intermedium (SGI)] and the deep gray layer [stratum griseum profundum (SGP)] (Edwards 1977;Fish et al 1982;May 2006;Olivier et al 1998;Yamasaki et al 1984).…”

“…Specifically, the cell bodies of tectotectal CNs were observed primarily in the intermediate gray layer [stratum griseum intermedium (SGI)] and the deep gray layer [stratum griseum profundum (SGP)] (Edwards 1977;Fish et al 1982;May 2006;Olivier et al 1998;Yamasaki et al 1984). Similarly, the main terminal field of tectotectal neurons arborized in the SGI and SGP (Behan and Kime 1996;Moschovakis and Karabelas 1985).…”

“…TRNs receive excitatory and inhibitory inputs from many areas, such as the visual association cortex, the frontal eye field, substantia nigra pars reticulata, and the cerebellum. Commissural inputs from the opposite SC are also important sources of input to TRNs (Maeda et al 1979;May 2006;Mize 1992;Moschovakis et al 1988;Olivier et al 1998;Takahashi et al 2005aTakahashi et al , 2007.…”

Oculomotor capture and Inhibition of Return: Evidence for an oculomotor suppression account of IOR

“…Together, build-up and fixation cells form a saccade map in which eye movements are based on the vector average of the activity in the build-up and fixation cells (e.g., Wurtz et al, 2000). An important feature of this saccade map is that there is substantial evidence for short distance excitation and long distance inhibition within the SC (e.g., Olivier et al, 1998;Munoz & Istvan, 1998). Within the SC saccade programming can basically be considered as a competition between activity at different locations of the saccade programming map (e.g., Trappenberg et al, 2001).…”

Tectotectal Neurons and Projections: A Proposal to Establish a Consistent Nomenclature

“…Table 1 also includes the terms that are a source of ambiguity and should therefore be avoided. For instance, the commissural neurons and the commissural projections of the SC are frequently referred to as ''tectotectal'' (e.g., Rhoades et al, 1981;Behan and Kime, 1996;Olivier et al, 1998Olivier et al, , 2000. Even if this common usage is understood by SC experts, it will certainly lead to confusion among a more general readership.…”