Receptive-field structure of optic flow responsive Purkinje cells in the vestibulocerebellum of pigeons

From the eye to the wing: neural circuits for transforming optic flow into motor output in avian flight

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.

Selective processing of all rotational and translational optic flow directions in the zebrafish pretectum and tectum

Background

The processing of optic flow in the pretectum/accessory optic system allows animals to stabilize retinal images by executing compensatory optokinetic and optomotor behavior. The success of this behavior depends on the integration of information from both eyes to unequivocally identify all possible translational or rotational directions of motion. However, it is still unknown whether the precise direction of ego-motion is already identified in the zebrafish pretectum or later in downstream premotor areas.

Results

Here, we show that the zebrafish pretectum and tectum each contain four populations of motion-sensitive direction-selective (DS) neurons, with each population encoding a different preferred direction upon monocular stimulation. In contrast, binocular stimulation revealed the existence of pretectal and tectal neurons that are specifically tuned to only one of the many possible combinations of monocular motion, suggesting that further downstream sensory processing might not be needed to instruct appropriate optokinetic and optomotor behavior.

Conclusion

Our results suggest that local, task-specific pretectal circuits process DS retinal inputs and carry out the binocular sensory computations necessary for optokinetic and optomotor behavior.

Electronic supplementary material

The online version of this article (10.1186/s12915-019-0648-2) contains supplementary material, which is available to authorized users.

Inferior olivary projection to the zebrin II stripes in lobule IXcd of the pigeon flocculus: A retrograde tracing study

Zebrin II (ZII; a.k.a. aldolase C) is expressed heterogeneously in Purkinje cells (PCs) such that there are sagittal stripes of high expression (ZII+) interdigitated with stripes of little or no expression (ZII-). The pigeon flocculus receives visual-optokinetic information and is important for generating compensatory eye movements. It consists of 4 sagittal zones based on PC complex spike activity (CSA) in response to rotational optokinetic stimuli. There are two zones where CSA responds best to rotation about the vertical axis (VA), interdigitated with two zones where CSA responds best to rotation about an horizontal axis (HA). These optokinetic zones relate to the ZII stripes in folium IXcd of the flocculus, such that an optokinetic zone spans a ZII+/- pair: the HA zones span the P5+/- and P7+/- ZII stripe pairs, whereas the VA zones correspond to ZII stripe pairs P4+/- and P6+/-. In the present study, we used fluorescent retrograde tracing to determine the olivary inputs to the ZII+ and ZII- stripes within the functional pairs. We found that separate but adjacent areas of the medial column of the inferior olive (mcIO) project to the ZII+ and ZII- stripes within each of the functional pairs. Thus, although a ZII+/- stripe pair represents a functional unit in the pigeon flocculus insofar as the CSA of all PCs in the stripe pair encodes similar sensory information, the olivary inputs to the ZII+ and ZII- stripes arise from different, although adjacent, regions of the mcIO.

The biophysics of bird flight: functional relationships integrate aerodynamics, morphology, kinematics, muscles, and sensors

Bird flight is a remarkable adaptation that has allowed the approximately 10 000 extant species to colonize all terrestrial habitats on earth including high elevations, polar regions, distant islands, arid deserts, and many others. Birds exhibit numerous physiological and biomechanical adaptations for flight. Although bird flight is often studied at the level of aerodynamics, morphology, wingbeat kinematics, muscle activity, or sensory guidance independently, in reality these systems are naturally integrated. There has been an abundance of new studies in these mechanistic aspects of avian biology but comparatively less recent work on the physiological ecology of avian flight. Here we review research at the interface of the systems used in flight control and discuss several common themes. Modulation of aerodynamic forces to respond to different challenges is driven by three primary mechanisms: wing velocity about the shoulder, shape within the wing, and angle of attack. For birds that flap, the distinction between velocity and shape modulation synthesizes diverse studies in morphology, wing motion, and motor control. Recently developed tools for studying bird flight are influencing multiple areas of investigation, and in particular the role of sensory systems in flight control. How sensory information is transformed into motor commands in the avian brain remains, however, a largely unexplored frontier.

Processing of visual signals related to self-motion in the cerebellum of pigeons

In this paper I describe the key features of optic flow processing in pigeons. Optic flow is the visual motion that occurs across the entire retina as a result of self-motion and is processed by subcortical visual pathways that project to the cerebellum. These pathways originate in two retinal-recipient nuclei, the nucleus of the basal optic root (nBOR) and the nucleus lentiformis mesencephali, which project to the vestibulocerebellum (VbC) (folia IXcd and X), directly as mossy fibers, and indirectly as climbing fibers from the inferior olive. Optic flow information is integrated with vestibular input in the VbC. There is a clear separation of function in the VbC: Purkinje cells in the flocculus process optic flow resulting from self-rotation, whereas Purkinje cells in the uvula/nodulus process optic flow resulting from self-translation. Furthermore, Purkinje cells with particular optic flow preferences are organized topographically into parasagittal "zones." These zones are correlated with expression of the isoenzyme aldolase C, also known as zebrin II (ZII). ZII expression is heterogeneous such that there are parasagittal stripes of Purkinje cells that have high expression (ZII+) alternating with stripes of Purkinje cells with low expression (ZII-). A functional zone spans a ZII± stripe pair. That is, each zone that contains Purkinje cells responsive to a particular pattern of optic flow is subdivided into a strip containing ZII+ Purkinje cells and a strip containing ZII- Purkinje cells. Additionally, there is optic flow input to folia VI-VIII of the cerebellum from lentiformis mesencephali. These folia also receive visual input from the tectofugal system via pontine nuclei. As the tectofugal system is involved in the analysis of local motion, there is integration of optic flow and local motion information in VI-VIII. This part of the cerebellum may be important for moving through a cluttered environment.

Zebrin-immunopositive and -immunonegative stripe pairs represent functional units in the pigeon vestibulocerebellum

The cerebellum is a site of complex sensorimotor integration and contains up to 80% of the neurons in the brain, yet comparatively little is known about the organization of sensorimotor systems within the cerebellum. It is known that afferent projections and Purkinje cell (PC) response properties are organized into sagittal "zones" in the cerebellum. Moreover, the isoenzyme aldolase C [also known as zebrin II (ZII)] is heterogeneously expressed in cerebellar PCs such that there are sagittal stripes of PCs with high expression (ZII+) interdigitated with stripes of little or no expression (ZII-). In the present study, we show how the ZII stripes in folium IXcd of the vestibulocerebellum in pigeons are related to response properties of PCs. IXcd consists of seven pairs of ZII+/- stripes denoted P1+/- (medial) to P7+/- (lateral). Electrophysiological studies have shown that vestibulocerebellar PCs respond to particular patterns of optic flow resulting from self-motion in three-dimensional space. In our study, we recorded optic flow preferences from PCs in IXcd, marked recording locations with injections of fluorescent tracer, and subsequently immunoreacted coronal sections for ZII. We found that the PCs within a ZII+/- stripe pair all responded best to the same pattern of optic flow. That is, a ZII+/- stripe pair forms a functional unit in the cerebellum. This is the first demonstration that the function of PCs is associated with ZII stripes across the mediolateral extent of an entire folium.

Differential projections from the vestibular nuclei to the flocculus and uvula-nodulus in pigeons (Columba livia)

The pigeon vestibulocerebellum is divided into two regions based on the responses of Purkinje cells to optic flow stimuli: the uvula-nodulus responds best to self-translation, and the flocculus responds best to self-rotation. We used retrograde tracing to determine whether the flocculus and uvula-nodulus receive differential mossy fiber input from the vestibular and cerebellar nuclei. From retrograde injections into the both the flocculus and uvula-nodulus, numerous cells were found in the superior vestibular nucleus (VeS), the cerebellovestibular process (pcv), the descending vestibular nucleus (VeD), and the medial vestibular nucleus (VeM). Less labeling was found in the prepositus hypoglossi, the cerebellar nuclei, the dorsolateral vestibular nucleus, and the lateral vestibular nucleus, pars ventralis. In the VeS, the differential input to the flocculus and uvula-nodulus was distinct: cells were localized to the medial and lateral regions, respectively. The same pattern was observed in the VeD, although there was considerable overlap. In the VeM, the majority of cells labeled from the flocculus were in rostral margins on the ipsilateral side, whereas labeling from uvula-nodulus injections was distributed bilaterally throughout the VeM. Finally, from injections in the flocculus but not the uvula-nodulus, moderate labeling was observed in a paramedian area, adjacent to the medial longitudinal fasciculus. In summary, there were clear differences with respect to the projections from the vestibular nuclei to functionally distinct parts of the vestibulocerebellum. Generally speaking, the mossy fibers to the flocculus and uvula-nodulus arise from regions of the vestibular nuclei that receive input from the semicircular canals and otolith organs, respectively.

Projections of the nucleus of the basal optic root in pigeons (Columba livia): A comparison of the morphology and distribution of neurons with different efferent projections

The avian nucleus of the basal optic root (nBOR) is a visual structure involved in the optokinetic response. nBOR consists of several morphologically distinct cell types, and in the present study, we sought to determine if these different cell types had differential projections. Using retrograde tracers, we examined the morphology and distribution of nBOR neurons projecting to the vestibulocerebellum (VbC), inferior olive (IO), dorsal thalamus, the pretectal nucleus lentiformis mesencephali (LM), the contralateral nBOR, the oculomotor complex (OMC) and a group of structures along the midline of the mesencephalon. The retrogradely labeled neurons fell into two broad categories: large neurons, most of which were multipolar rather than fusiform and small neurons, which were either fusiform or multipolar. From injections into the IO, LM, contralateral nBOR, and structures along the midline-mesencephalon small nBOR neurons were labeled. Although there were no differences with respect to the size of the labeled neurons from these injections, there were some differences with the respect to the distribution of labeled neurons and the proportion of multipolar vs. fusiform neurons. From injections into the VbC, the large multipolar cells were labeled throughout nBOR. The only other cases in which these large neurons were labeled were contralateral OMC injections. To investigate if single neurons project to multiple targets we used paired injections of red and green fluorescent retrograde tracers into different targets. Double-labeled neurons were never observed indicating that nBOR neurons do not project to multiple targets. We conclude that individual nBOR neurons have unique projections, which may have differential roles in processing optic flow and controlling the optokinetic response.