Abstract
In this paper, we review the connections and physiology of visual pathways to the cerebellum in birds and consider their role in flight. We emphasize that there are two visual pathways to the cerebellum. One is to the vestibulocerebellum (folia IXcd and X) that originates from two retinal-recipient nuclei that process optic flow: the nucleus of the basal optic root (nBOR) and the pretectal nucleus lentiformis mesencephali (LM). The second is to the oculomotor cerebellum (folia VI-VIII), which receives optic flow information, mainly from LM, but also local visual motion information from the optic tectum, and other visual information from the ventral lateral geniculate nucleus (Glv). The tectum, LM and Glv are all intimately connected with the pontine nuclei, which also project to the oculomotor cerebellum. We believe this rich integration of visual information in the cerebellum is important for analyzing motion parallax that occurs during flight. Finally, we extend upon a suggestion by Ibbotson (2017) that the hypertrophy that is observed in LM in hummingbirds might be due to an increase in the processing demands associated with the pathway to the oculomotor cerebellum as they fly through a cluttered environment while feeding.
Highlights
We extend upon a suggestion by Ibbotson (2017) that the hypertrophy that is observed in lentiformis mesencephali (LM) in hummingbirds might be due to an increase in the processing demands associated with the pathway to the oculomotor cerebellum as they fly through a cluttered environment while feeding
Optic flow is analyzed by specialized visual pathways in the avian brain, which originate from two retinal-recipient nuclei: the nucleus of the basal optic root of the Accessory Optic System, and the pretectal nucleus lentiformis mesencephali (LM)
The hypertrophy of LM in hummingbirds may not be related to hovering alone, but could be related to an increase in the processing demands associated with the pathway to the oculomotor cerebellum as they fly through a cluttered environment while feeding
Summary
In the generation of the optokinetic response for retinal image stabilization (Fite et al, 1979; Gioanni et al, 1983a,b), their involvement in the visual control of flight has yet to be demonstrated. Iwaniuk and Wylie (2007) noted that the LM was 2 to 5X larger in hummingbirds compared to other birds (Figures 2A,B). As hovering represents a specialized case of stabilization, Iwaniuk and Wylie (2007) postulated that the hypertrophy of the LM was to facilitate hovering. Pakan and Wylie (2006) noted that nBOR and LM give rise to two optic flow pathways to the cerebellum; to the posterior part, folia IXcd and X, which collectively are know as the vestibulocerebellum (VbC), and to the oculomotor cerebellum, which is comprised of folia VI-VIII (see below). We will expand upon the proposed role of the LM and visual cerebellar pathways in avian flight by exploring several questions: What other visual nuclei should show hypertrophy in concert with the LM? Which part of the LM is hypertrophied in hummingbirds? And is the expansion of the LM in hummingbirds driven by the visual demands of behaviors other than hovering, as preciously suggested?
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