A morphological study of the nucleus subpretectalis of the pigeon

Abstract

In pigeons, the tectofugal system is functionally as well as structurally lateralized. So for example the right nucleus rotundus is less modulated by right forebrain influences than the left nucleus rotundus by the left ones. This functional lateralization pattern may depend on a dynamic balance between left and right tectal processing. Apart from inhibitory interactions at tectal level, suppressive influences might directly affect rotundal neurons by GABAergic input from a cluster of nuclei, the bed nuclei of the tecto-thalamic tract. A major afferent of these nuclei is the side branch of the tectorotundal projection which is of bilateral origin and which is involved in the regulation of ipsilateral as well as bilateral visual processing. Hence, an important role of the bed nuclei could be the interhemispheric communication and in turn the mediation of functional asymmetries. In a first step to unravel asymmetric influences of these nuclei, the present study investigated if the largest of the bed nuclei, the nucleus subpretectalis displays morphological asymmetries in the pigeon. We found that the nucleus subpretectalis in fact exhibits asymmetric cell sizes with larger cell bodies on the left side. This asymmetrical pattern was not present in dark-incubated animals indicating that cell size asymmetries within nucleus subpretectalis are induced by asymmetric photic stimulation during embryonic development.

Shaping a lateralized brain: Asymmetrical light experience modulates access to visual interhemispheric information in pigeons

“…Structural asymmetries in the visual pathways of pigeons and chickens are shaped by the ontogenetic light conditions39132021222319. This suggests that the proportion of crossing tectorotundal fibers could be regulated by asymmetrical photic input.…”

“…But a direct relation between the degree of bilateral input and efficiency of accessing interhemispheric visual information would imply the presence of an asymmetrical tectorotundal projection in dark-incubated birds, too. Although not tested directly yet, this is questionable since asymmetries of tectal and rotundal soma sizes, which would indicate asymmetries of axodendritic complexity, are missing in light-deprived pigeons222319. Therefore, it is unlikely that lateralized access to transfer information is only caused by asymmetries of ascending projections and hence, simply by the degree of input from the ipsilateral eye.…”

“…The resulting differences in retinal activity induce asymmetrical differentiation processes in left and right hemispheric visual circuits, which ultimately establish the adult functional lateralization pattern17318. As a result, depriving the embryos from light during development prevents the formation of visuomotor as well as anatomical asymmetries in chickens and pigeons1319202122. In the altricial pigeon, manipulations of the visual experience after hatching still modify the typical pattern20232425.…”

Functional and structural comparison of visual lateralization in birds – similar but still different

“…But in each species, different visual systems are affected ( Figure 1 ). In pigeons, the tectofugal pathway (corresponding to the extrageniculate pathway in mammals) is lateralized, with soma size asymmetries of mesencephalic and diencephalic neurons indicating left-right differences in the complexity of their neuronal connections (Güntürkün, 1997; Manns and Güntürkün, 1999a, 2003; Freund et al, 2008). Moreover, projections of the right optic tectum to the contralateral nucleus rotundus are stronger than the projections of the left tectum to the right rotundus.…”

“…In pigeons, this causes an asymmetry in the projections of the tectofugal pathway (Güntürkün et al, 1998) while in chickens projections of the thalamofugal pathway are affected (Rogers and Bolden, 1991; Rogers and Deng, 1999; Koshiba et al, 2003). Dark incubation of eggs prevents establishment of several asymmetries (Rogers and Sink, 1988; Skiba et al, 2002; Manns and Güntürkün, 2003; Freund et al, 2008) and impairs interhemispheric cooperation (Manns and Römling, 2012). Furthermore, in the altricial pigeon, monocular light deprivation during a short plastic period after hatch can strengthen or even alter the direction of visual asymmetries (Manns and Güntürkün, 1999a, b).…”