Cover and Figure 1. As a bird such as a zebra finch flies through its environment, the surfaces and objects appear to move across the retina, a visual signal known as optic flow. Numerous studies have demonstrated that two retinal recipient midbrain areas, the nucleus lentiformis mesencephali and the nucleus of the basal optic root (and their homologues in other vertebrates) contain neurons that respond to optic flow. Both of these nuclei send projections to pre-motor areas in the brainstem and cerebellum. Numerous anatomical and physiological studies are revealing how this optic flow information is integrated with sensory signals and how cerebellar output may be driving different flight behaviours. Illustrations by Sylvia Heredia
Abstract
Avian flight is guided by optic flow—the movement across the retina of images of surfaces and edges in the environment due to self-motion. In all vertebrates, there is a short pathway for optic flow information to reach pre-motor areas: retinal-recipient regions in the midbrain encode optic flow, which is then sent to the cerebellum. One well-known role for optic flow pathways to the cerebellum is the control of stabilizing eye movements (the optokinetic response). However, the role of this pathway in controlling locomotion is less well understood. Electrophysiological and tract tracing studies are revealing the functional connectivity of a more elaborate circuit through the avian cerebellum, which integrates optic flow with other sensory signals. Here we review the research supporting this framework and identify the cerebellar output centres, the lateral (CbL) and medial (CbM) cerebellar nuclei, as two key nodes with potentially distinct roles in flight control. The CbM receives bilateral optic flow information and projects to sites in the brainstem that suggest a primary role for flight control over time, such as during forward flight. The CbL receives monocular optic flow and other types of visual information. This site provides feedback to sensory areas throughout the brain and has a strong projection the nucleus ruber, which is known to have a dominant role in forelimb muscle control. This arrangement suggests primary roles for the CbL in the control of wing morphing and for rapid maneuvers.