Bilateral and ipsilateral ascending tectopulvinar pathways in mammals: A study in the squirrel (spermophilus beecheyi)

Abstract

The mammalian pulvinar complex is a collection of dorsal thalamic nuclei related to several visual and integrative processes. Previous studies have shown that the superficial layers of the superior colliculus project to multiple divisions of the pulvinar complex. Although most of these works agree about the existence of an ipsilateral tectopulvinar projection arising from the stratum griseum superficialis, some others report a bilateral projection originating from this same tectal layer. We investigated the organization of the tectopulvinar projections in the Californian ground squirrel using cholera toxin B (CTb). We confirmed previous studies showing that the caudal pulvinar of the squirrel receives a massive bilateral projection originating from a specific cell population located in the superficial collicular layers (SGS3, also called the “lower SGS” or “SGSL”). We found that this projection shares striking structural similarities with the tectorotundal pathway of birds and reptiles. Morphology of the collicular cells originating this projection closely corresponds to that of the bottlebrush tectal cells described previously for chickens and squirrels. In addition, we found that the rostral pulvinar receives an exclusively ipsilateral projection from a spatially separate population of collicular cells located at the base of the stratum opticum, deeper than the cells projecting to the caudal pulvinar. These results strongly support, at a structural level, the homology of the pathway originating in the SGS3 collicular cells upon the caudal pulvinar with the tectorotundal pathway of nonmammalian amniotes and contribute to clarifying the general organization of the tectopulvinar pathways in mammals.

Attentional Capture? Synchronized Feedback Signals from the Isthmi Boost Retinal Signals to Higher Visual Areas

“…Major et al (2000) have proposed that in the squirrel, parabigeminal axons are directed to the bottlebrush endings of the TGCs, which send a non-topographic, bilateral projection to the caudal pulvinar in the thalamus, just like Rtprojecting TGCs in birds (Major et al, 2000, Fredes et al, 2011. The parabigeminal axons might exert similar control of the collicular-caudal pulvinar pathway as shown here for birds.…”

Response Normalization in the Superficial Layers of the Superior Colliculus as a Possible Mechanism for Saccadic Averaging

“…1 B, C). Our previous work demonstrated that these signals result from synaptic activity (Vokoun et al, 2010) and our more recent work shows that the spread of activation from intermediate layers into the superficial layers is mediated by a novel excitatory pathway from the stratum griseum intermediale (SGI; intermediate layers) to the stratum griseum superficiale (SGS; superficial layers; Ghitani et al, 2012).…”

“…A second possibility is that the superficial layers of the SC play a role in action selection when faced with options. Indeed, information from the motor layers of the SC encoding the location of the two potential targets could activate a single region within the sensory layers of the SC through a newly identified, ascending excitatory pathway (Ghitani et al, 2012). This activation would result in potentiating the activity of visual pathways through the projections of superficial layer neurons to the lateral geniculate nucleus and the pulvinar (Diamond et al, 1991;Chomsung et al, 2008;Fredes et al, 2012).…”

“…Indeed, information from the motor layers of the SC encoding the location of the two potential targets could activate a single region within the sensory layers of the SC through a newly identified, ascending excitatory pathway (Ghitani et al, 2012). This activation would result in potentiating the activity of visual pathways through the projections of superficial layer neurons to the lateral geniculate nucleus and the pulvinar (Diamond et al, 1991;Chomsung et al, 2008;Fredes et al, 2012). Activation of these visual pathways might lead to the subject "seeing" a third target which in turn, would lead to the programming of a saccade to the intermediate location.…”

Circuits for active vision : parallel tectothalamocortical visual pathways in the mouse.

“…Neurons that innervate the dLGN are always located in the upper onehalf of the stratum griseum superficiale (SGS) and the majority of these cells are narrow field vertical cells (NFV), whereas those neurons that project to the pulvinar are located in the lower half of the SGS and stratum opticum (SO), and the majority of these cells are wide field vertical cells (WFV) (Mooney et al, 1988;Fredes et al, 2012;Gale and Murphy, 2014). It has also been noted that the somata of tectopulvinar cells are larger than those of tectogeniculate neurons (Albano et al, 1979;Kawamura et al, 1980;Caldwell and Mize, 1981;Graham and Berman, 1981;Harrell et al, 1982).…”