Temporal frequency and velocity-like tuning in the pigeon accessory optic system

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.

Topography of optic flow processing in olivo-cerebellar pathways in zebra finches (Taeniopygia guttata)

In birds, the nucleus of the basal optic root (nBOR) and the nucleus lentiformis mesencephali (LM) are brainstem nuclei involved in the analysis of optic flow. A major projection site of both nBOR and LM is the medial column of the inferior olive (IO), which provides climbing fibers to the vestibulocerebellum. This pathway has been well documented in pigeons, but not other birds. Recent works have highlighted that zebra finches show specializations with respect to optic flow processing, which may be reflected in the organization of optic flow pathways to the IO. In this study, we characterized the organization of these pathways in zebra finches. We found that the medial column consists of at least eight subnuclei (i-viii) visible in Nissl-stained tissue. Using anterograde traces we found that the projections from LM and nBOR to the IO were bilateral, but heavier to the ipsilateral side, and showed a complementary pattern: LM projected to subnucleus i, whereas nBOR projected to subnuclei ii and v. Using retrograde tracers, we found that these subnuclei (i, ii and v) projected to the vestibulocerebellum (folia IXcd and X), whereas the other subnuclei projected to IXab and the lateral margin of VII and VIII. The nBOR also projected ipsilaterally to the caudo-medial dorsal lamella of the IO, which the retrograde experiments showed as projecting to the medial margin of VII and VIII. We compare these results with previous studies in other avian species.

Development of the horizontal optocollic reflex in juvenile barn owls (Tyto furcata pratincola)

Adult barn owls and primates possess an almost symmetric monocular rotational horizontal optocollic reflex. In primates, the reflex is initially asymmetric and becomes symmetric with time after birth. The condition in barn owls has not been studied so far. Here, we present data on the development of this reflex in this bird. We tested juvenile barn owls from the time before they open their eyes after hatching to the time they reach adult feather length. Wide-field visual patterns served as stimuli. They were presented at different rotational speeds in binocular and monocular settings. The binocular horizontal optocollic responses of juvenile barn owls were symmetric and adult-like on the first day that the birds responded to the stimulus. The monocular responses showed different rates of development in respect to stimulus velocity and stimulus direction. For velocities up to 20 deg/s, the monocular reflex was also adult-like on the first day that the birds responded to the stimulus. An initially higher asymmetry for 30 deg/s compared to adults disappeared within about two weeks. The development at even higher velocities remained unclear.

Response properties of optic flow neurons in the accessory optic system of hummingbirds versus zebra finches and pigeons

Optokinetic responses function to maintain retinal image stabilization by minimizing optic flow that occurs during self-motion. The hovering ability of hummingbirds is an extreme example of this behavior. Optokinetic responses are mediated by direction-selective neurons with large receptive fields in the accessory optic system (AOS) and pretectum. Recent studies in hummingbirds showed that, compared with other bird species, 1) the pretectal nucleus lentiformis mesencephali (LM) is hypertrophied, 2) LM has a unique distribution of direction preferences, and 3) LM neurons are more tightly tuned to stimulus velocity. In this study, we sought to determine if there are concomitant changes in the nucleus of the basal optic root (nBOR) of the AOS. We recorded the visual response properties of nBOR neurons to large-field-drifting random dot patterns and sine-wave gratings in Anna's hummingbirds and zebra finches and compared these with archival data from pigeons. We found no differences with respect to the distribution of direction preferences: Neurons responsive to upward, downward, and nasal-to-temporal motion were equally represented in all three species, and neurons responsive to temporal-to-nasal motion were rare or absent (<5%). Compared with zebra finches and pigeons, however, hummingbird nBOR neurons were more tightly tuned to stimulus velocity of random dot stimuli. Moreover, in response to drifting gratings, hummingbird nBOR neurons are more tightly tuned in the spatiotemporal domain. These results, in combination with specialization in LM, support a hypothesis that hummingbirds have evolved to be "optic flow specialists" to cope with the optomotor demands of sustained hovering flight.NEW & NOTEWORTHY Hummingbirds have specialized response properties to optic flow in the pretectal nucleus lentiformis mesencephali (LM). The LM works with the nucleus of the basal optic root (nBOR) of the accessory optic system (AOS) to process global visual motion, but whether the neural response specializations observed in the LM extend to the nBOR is unknown. Hummingbird nBOR neurons are more tightly tuned to visual stimulus velocity, and in the spatiotemporal domain, compared with two nonhovering species.

Sensory systems in birds: What we have learned from studying sensory specialists

"Diversity" is an apt descriptor of the research career of Jack Pettigrew as it ranged from the study of trees, to clinical conditions, to sensory neuroscience. Within sensory neuroscience, he was fascinated by the evolution of sensory systems across species. Here, we review some of his work on avian sensory specialists and research that he inspired in others. We begin with an overview of the importance of the Wulst in stereopsis and the need for further study of the Wulst in relation to binocularity across avian species. Next, we summarize recent anatomical, behavioral, and physiological studies on optic flow specializations in hummingbirds. Beyond vision, we discuss the first evidence of a tactile "fovea" in birds and how this led to detailed studies of tactile specializations in waterfowl and sensorimotor systems in parrots. We then describe preliminary studies by Pettigrew of two endemic Australian species, the plains-wanderer (Pedionomus torquatus) and letter-winged kite (Elanus scriptus), that suggest the evolution of some unique auditory and visual specializations in relation to their unique behavior and ecology. Finally, we conclude by emphasizing the importance of a comparative and integrative approach to understanding avian sensory systems and provide an example of one system that has yet to be properly examined: tactile facial bristles in birds. Through reviewing this research and offering future avenues for discovery, we hope that others also embrace the comparative approach to understanding sensory system evolution in birds and other vertebrates.

Visual Control of Walking Speed in Drosophila

An animal's self-motion generates optic flow across its retina, and it can use this visual signal to regulate its orientation and speed through the world. While orientation control has been studied extensively in Drosophila and other insects, much less is known about the visual cues and circuits that regulate translational speed. Here, we show that flies regulate walking speed with an algorithm that is tuned to the speed of visual motion, causing them to slow when visual objects are nearby. This regulation does not depend strongly on the spatial structure or the direction of visual stimuli, making it algorithmically distinct from the classic computation that controls orientation. Despite the different algorithms, the visual circuits that regulate walking speed overlap with those that regulate orientation. Taken together, our findings suggest that walking speed is controlled by a hierarchical computation that combines multiple motion detectors with distinct tunings. VIDEO ABSTRACT.

Visual-Cerebellar Pathways and Their Roles in the Control of Avian Flight

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.

The retinal projection to the nucleus lentiformis mesencephali in zebra finch (Taeniopygia guttata) and Anna's hummingbird (Calypte anna)

In birds, the nucleus of the basal optic root (nBOR) and the nucleus lentiformis mesencephali (LM) are retinal recipient nuclei involved in the analysis of optic flow and the generation of the optokinetic response. In both pigeons and chickens, retinal inputs to the nBOR arise from displaced ganglion cells (DGCs), which are found at the margin of the inner nuclear and inner plexiform layers. The LM receives afferents from retinal ganglion cells, but whether DGCs also project to LM is a matter of debate. Previous work in chickens had concluded that DGCs do not project to LM, but a recent study in pigeons found that both retinal ganglion cells and DGCs project to LM. These findings leave open the question of whether there are species differences with respect to the DGC projection to LM. In the present study, we made small injections of retrograde tracer into the LM in a zebra finch and an Anna's hummingbird. In both cases, retrogradely labeled retinal ganglion cells and DGCs were observed. These results suggest that a retinal input to the LM arising from DGCs is characteristic of most, if not all, birds.

Neurons Responsive to Global Visual Motion Have Unique Tuning Properties in Hummingbirds

Neurons in animal visual systems that respond to global optic flow exhibit selectivity for motion direction and/or velocity. The avian lentiformis mesencephali (LM), known in mammals as the nucleus of the optic tract (NOT), is a key nucleus for global motion processing [1-4]. In all animals tested, it has been found that the majority of LM and NOT neurons are tuned to temporo-nasal (back-to-front) motion [4-11]. Moreover, the monocular gain of the optokinetic response is higher in this direction, compared to naso-temporal (front-to-back) motion [12, 13]. Hummingbirds are sensitive to small visual perturbations while hovering, and they drift to compensate for optic flow in all directions [14]. Interestingly, the LM, but not other visual nuclei, is hypertrophied in hummingbirds relative to other birds [15], which suggests enhanced perception of global visual motion. Using extracellular recording techniques, we found that there is a uniform distribution of preferred directions in the LM in Anna's hummingbirds, whereas zebra finch and pigeon LM populations, as in other tetrapods, show a strong bias toward temporo-nasal motion. Furthermore, LM and NOT neurons are generally classified as tuned to "fast" or "slow" motion [10, 16, 17], and we predicted that most neurons would be tuned to slow visual motion as an adaptation for slow hovering. However, we found the opposite result: most hummingbird LM neurons are tuned to fast pattern velocities, compared to zebra finches and pigeons. Collectively, these results suggest a role in rapid responses during hovering, as well as in velocity control and collision avoidance during forward flight of hummingbirds.

Optic flow stabilizes flight in ruby-throated hummingbirds

Flying birds rely on visual cues for retinal image stabilization by negating rotation-induced optic flow, the motion of the visual panorama across the retina, through corrective eye and head movements. In combination with vestibular and proprioceptive feedback, birds may also use visual cues to stabilize their body during flight. Here, we test whether artificially induced wide-field motion generated through projected visual patterns elicits maneuvers in body orientation and flight position, in addition to stabilizing vision. To test this hypothesis, we present hummingbirds flying freely within a 1.2 m cylindrical visual arena with a virtual surround rotated at different speeds about its vertical axis. The birds responded robustly to these visual perturbations by rotating their heads and bodies with the moving visual surround, and by adjusting their flight trajectories; following the surround. Thus, similar to insects, hummingbirds appear to use optic flow cues to control flight maneuvers in addition to stabilize their visual inputs.

Retinal projection to the pretectal nucleus lentiformis mesencephali in pigeons (Columba livia)

In birds, the nucleus of the basal optic root (nBOR) and the nucleus lentiformis mesencephali (LM) are retinal-recipient nuclei involved in the analysis of optic flow and the generation of the optokinetic response. The nBOR receives retinal input from displaced ganglion cells (DGCs), which are found at the margin of the inner nuclear and inner plexiform layers, rather than the ganglion cell layer. The LM receives afferents from retinal ganglion cells, but whether DGCs also project to LM remains unclear. To resolve this issue, we made small injections of retrograde tracer into LM and examined horizontal sections through the retina. For comparison, we also had cases with injections in nBOR, the optic tectum, and the anterior dorsolateral thalamus (the equivalent to the mammalian lateral geniculate nucleus). From all LM injections both retinal ganglion cells and DGCs were labeled. The percentage of DGCs, as a proportion of all labeled cells, varied from 2-28%, and these were not different in morphology or size compared to those labeled from nBOR, in which the proportion of DGCs was much higher (84-93%). DGCs were also labeled after injections into the anterior dorsolateral thalamus. The proportion was small (2-3%), and these DGCs were smaller in size than those projecting to the nBOR and LM. No DGCs were labeled from an injection in the optic tectum. Based on an analysis of size, we suggest that different populations of retinal ganglion cells are involved in the projections to LM, nBOR, the optic tectum, and the anterior dorsolateral thalamus.

Spectral characteristics of phase sensitivity and discharge rate of neurons in the ascending tectofugal visual system

Drifting gratings can modulate the activity of visual neurons at the temporal frequency of the stimulus. In order to characterize the temporal frequency modulation in the cat’s ascending tectofugal visual system, we recorded the activity of single neurons in the superior colliculus, the suprageniculate nucleus, and the anterior ectosylvian cortex during visual stimulation with drifting sine-wave gratings. In response to such stimuli, neurons in each structure showed an increase in firing rate and/or oscillatory modulated firing at the temporal frequency of the stimulus (phase sensitivity). To obtain a more complete characterization of the neural responses in spatiotemporal frequency domain, we analyzed the mean firing rate and the strength of the oscillatory modulations measured by the standardized Fourier component of the response at the temporal frequency of the stimulus. We show that the spatiotemporal stimulus parameters that elicit maximal oscillations often differ from those that elicit a maximal discharge rate. Furthermore, the temporal modulation and discharge-rate spectral receptive fields often do not overlap, suggesting that the detection range for visual stimuli provided jointly by modulated and unmodulated response components is larger than the range provided by a one response component.

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.

Motion parallax processing in pigeon (Columba livia) pretectal neurons

In the visual system of invertebrates and vertebrates there are specialised groups of motion-sensitive neurons, with large receptive fields, which are optimally tuned to respond to optic flow produced by the animals' movement through the 3-D world. From their response characteristics, shared frame of reference with the vestibular or inertial system, and anatomical connections, these neurons have been implicated in the stabilisation of retinal images, the control of posture and balance, and the animal's motion trajectories through the world. Using standard electrophysiological techniques and computer-generated stimuli, we show that some of these flow-field neurons in the pretectal nucleus lentiformis mesencephali in pigeons appear to be processing motion parallax. Two large overlapping planes of random dots moving independently were used to simulate motion parallax, in which one with larger dots was moved fast and the other with smaller dots was moved slowly in the opposite direction. Their neural responses to these two superimposed planes were facilitated above those produced by a single plane of moving dots and those produced by two layers moving in the same direction. Furthermore, some of these neurons preferred backward motion in the visual field and others preferred forward motion, suggesting that they may separately code visual objects 'nearer' and 'farther' than the stabilised ('on') plane during forward translational motion. A simple system is proposed whereby the relative activity in 'near', 'far' and 'on' populations could code depth through motion parallax in a metameric manner similar to that employed to code color vision and stereopsis.

Organization of visual mossy fiber projections and zebrin expression in the pigeon vestibulocerebellum

Extensive research has revealed a fundamental organization of the cerebellum consisting of functional parasagittal zones. This compartmentalization has been well documented with respect to physiology, biochemical markers, and climbing fiber afferents. Less is known about the organization of mossy fiber afferents in general, and more specifically in relation to molecular markers such as zebrin. Zebrin is expressed by Purkinje cells that are distributed as a parasagittal array of immunopositive and immunonegative stripes. We examined the concordance of zebrin expression with visual mossy fiber afferents in the vestibulocerebellum (folium IXcd) of pigeons. Visual afferents project directly to folium IXcd as mossy fibers and indirectly as climbing fibers via the inferior olive. These projections arise from two retinal recipient nuclei: the lentiformis mesencephali (LM) and the nucleus of the basal optic root (nBOR). Although it has been shown that these two nuclei project to folium IXcd, the detailed organization of these projections has not been reported. We injected anterograde tracers into LM and nBOR to investigate the organization of mossy fiber terminals and subsequently related this organization to the zebrin antigenic map. We found a parasagittal organization of mossy fiber terminals in folium IXcd and observed a consistent relationship between mossy fiber organization and zebrin stripes: parasagittal clusters of mossy fiber terminals were concentrated in zebrin-immunopositive regions. We also describe the topography of projections from LM and nBOR to the inferior olive and relate these results to previous studies on the organization of climbing fibers and zebrin expression.

Novel, continuous visual motion induces c-fos expression in the avian optokinetic nuclei and optic tectum

We studied the stimulus characteristics necessary for the expression of c-fos protein in optokinetic system neurons using immunocytochemistry. Using whole-field visual motion as a stimulus, we found substantial c-fos expression in the optic tectum (TeO), the nucleus of the basal optic root (nBOR) and the pretectal nucleus lentiformis mesencephali (LM); in all cases immunostaining was seen only on the side contralateral to the eye viewing whole-field unidirectional motion; the side of the brain contralateral to the eye wearing a diffuser showed no staining. In the nBOR and the LM, different regions showed a remarkable specificity of c-fos expression depending on the direction of visual motion stimulation. Neurons were stained primarily in regions known from previous electrophysiological recordings to be maximally responsive to that direction of motion; little staining was seen after motion orthogonal to the preferred motion direction. Novel, continuous visual motion stimuli, lasting more than 30 min, was required for maximal c-fos expression, suggesting that brief periods of unidirectional optic flow, as would be experienced during normal life, do not stimulate the expression of c-fos. The largest number of neurons was labeled when birds raised from hatching with one eye covered by a diffuser were exposed to full-field visual motion immediately after the diffuser was switched from one eye to the other, so that only the previously naive eye was visually stimulated. We conclude that the expression of c-fos in the optokinetic nuclei is linked to near peak firing rates on the one hand, and the novelty and duration of the visual signals, on the other, supporting the assumption that this expression is mainly related to stimulus contexts leading to neuronal plastic changes.

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.

Optokinetic stimulation affects temporal estimation in healthy humans

The representation of time and space are closely linked in the cognitive system. Optokinetic stimulation modulates spatial attention in healthy subjects and patients with spatial neglect. In order to evaluate whether optokinetic stimulation could influence time perception, a group of healthy subjects performed "time-comparison" tasks of sub- and supra-second intervals before and after leftward or rightward optokinetic stimulation. Subjective time perception was biased by the direction of optokinetic stimulation. Rightward optokinetic stimulation induced an overestimation of time perception compared with baseline and leftward optokinetic stimulation. These results indicate a directional bias in time perception induced by manipulation of spatial attention and could argue for a mental linear representation of time intervals.

Spectral receptive field properties of neurons in the feline superior colliculus

The spatio-temporal frequency response profiles of 73 neurons located in the superficial, retino-recipient layers of the feline superior colliculus (SC) were investigated. The majority of the SC cells responded optimally to very low spatial frequencies with a mean of 0.1 cycles/degree (c/deg). The spatial resolution was also low with a mean of 0.31 c/deg. The spatial frequency tuning functions were either low-pass or band-pass with a mean spatial frequency bandwidth of 1.84 octaves. The cells responded optimally to a range of temporal frequencies between 0.74 cycles/s (c/s) and 26.41 c/s with a mean of 6.84 c/s. The majority (68%) of the SC cells showed band-pass temporal frequency tuning with a mean temporal frequency bandwidth of 2.4 octaves, while smaller proportions of the SC units displayed high-pass (19%), low-pass (8%) or broad-band (5%) temporal tuning. Most of the SC units exhibited simple spectral tuning with a single maximum in the spatio-temporal frequency domain, while some neurons were tuned for spatial or temporal frequencies or speed tuned. Further, we found cells excited by gratings moving at high temporal and low spatial frequencies and cells whose activity was suppressed by high velocity movement. The spatio-temporal filter properties of the SC neurons show close similarities to those of their retinal Y and W inputs as well as those of their inputs from the cortical visual motion detector areas, suggesting their common role in motion analysis and related behavioral actions.

A single mechanism can explain the speed tuning properties of MT and V1 complex neurons

A recent study by Priebe et al., (2006) has shown that a small proportion (27%) of primate directionally selective, complex V1 neurons are tuned for the speed of image motion. In this study, I show that the weighted intersection mechanism (WIM) model, which was previously proposed to explain speed tuning in middle temporal neurons, can also explain the tuning found in complex V1 neurons. With the addition of a contrast gain mechanism, this model is able to replicate the effects of contrast on V1 speed tuning, a phenomenon that was recently discovered by Priebe et al., (2006). The WIM model simulations also indicate that V1 neuron spatiotemporal frequency response maps may be asymmetrical in shape and hence poorly characterized by the symmetrical two-dimensional Gaussian fitting function used by Priebe et al., (2006) to classify their cells. Therefore, the actual proportion of speed tuning among directional complex V1 cells may be higher than the 27% estimate suggested by these authors.

The neural mechanisms of long distance animal navigation

Animal navigation is a complex process involving the integration of many sources of specialized sensory information for navigation in near and far space. Our understanding of the neurobiological underpinnings of near-space navigation is well-developed, whereas the neural mechanisms of long-distance navigation are just beginning to be unraveled. One crucial question for future research is whether the near space concepts of place cells, head direction cells, and maps in the entorhinal cortex scale up to animals navigating over very long distances and whether they are related to the map and compass concepts of long-distance navigation.

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

The avian nucleus lentiformis mesencephali (LM) is a visual structure involved in the optokinetic response. The LM consists of several morphologically distinct cell types. In the present study we sought to determine if different cell types had differential projections. Using retrograde tracers, we examined the morphology and distribution of LM neurons projecting to the vestibulocerebellum (VbC), inferior olive (IO), dorsal thalamus, nucleus of the basal optic root (nBOR), and midline mesencephalon. From injections into the latter two structures, small LM cells were labeled. More were localized to the lateral LM as opposed to medial LM. From injections into the dorsal thalamus, small neurons were found throughout LM. From injections into the VbC, large multipolar cells were found throughout LM. From injections into IO, a strip of medium-sized fusiform neurons along the border of the medial and lateral subnuclei was labeled. To investigate if neurons project to multiple targets we used fluorescent retrograde tracers. After injections into IO and VbC, double-labeled neurons were not observed in LM. Likewise, after injections into nBOR and IO, double-labeled neurons were not observed. Finally, we processed sections through LM for glutamic acid decarboxylase (GAD). Small neurons, mostly in the lateral LM, were labeled, suggesting that projections from LM to nBOR and midline mesencephalon are GABAergic. We conclude that two efferents of LM, VbC and IO, receive input from morphologically distinct neurons: large multipolar and medium-sized fusiform neurons, respectively. The dorsal thalamus, nBOR, and midline mesencephalon receive input from small neurons, some of which are likely GABAergic.

Influence of the behavioural context on the optocollic reflex (OCR) in pigeons (Columba livia)

We investigated the effects of several behavioural conditions on the properties of the horizontal optocollic reflex (OCR) in pigeons. The head reflex was triggered by rotating the visual surroundings at different velocities (stimuli steps of 30-300 deg. s-1) and the characteristics of the slow and fast phases of the OCR were analysed during,(i) the `resting condition', in which animals were hung in a harness, (ii) the`standing condition', in which animals were freely standing, (iii) the`walking condition', in which animals were walking on a treadmill at different velocities, and (iv) the `flying condition', in which animals were hung in a harness and subjected to a frontal air-stream, provoking a flying posture.

In the `resting' condition, irregularities were observed in the amplitude of nystagmic beats, in the beating field and in the slow phase velocity (SPV)of the OCR. These irregularities diminished progressively when the behavioural condition changed from `standing' to `walking', and disappeared in the`flying' condition. Correlatively, the working range of the OCR (evaluated by its gain at the plateau of SPV) was progressively extended toward higher stimulation velocities.

The velocity of the fast phases of the OCR (measured for all the conditions except the `walking condition') also increased in correlation with the SPV. The walking speed did not influence the OCR in the treadmill velocity range of 0.20-0.40 m s-1. The presence of a frontal airstream in the`standing condition' did not change the OCR properties. This fact (and other observations discussed in the paper) suggests that the adaptation of the OCR to the behavioural context is mediated by internal signals generated by each behavioural condition.

The accessory optic system: basic organization with an update on connectivity, neurochemistry, and function

The accessory optic system (AOS) is formed by a series of terminal nuclei receiving direct visual information from the retina via one or more accessory optic tracts. In addition to the retinal input, derived from ganglion cells that characteristically have large receptive fields, are direction-selective, and have a preference for slow moving stimuli, there are now well-characterized afferent connections with a key pretectal nucleus (nucleus of the optic tract) and the ventral lateral geniculate nucleus. The efferent connections of the AOS are robust, targeting brainstem and other structures in support of visual-oculomotor events such as optokinetic nystagmus and visual-vestibular interaction. This chapter reviews the newer experimental findings while including older data concerning the structural and functional organization of the AOS. We then consider the ontogeny and phylogeny of the AOS and include a discussion of similarities and differences in the anatomical organization of the AOS in nonmammalian and mammalian species. This is followed by sections dealing with retinal and cerebral cortical afferents to the AOS nuclei, interneuronal connections of AOS neurons, and the efferents of the AOS nuclei. We conclude with a section on Functional Considerations dealing with the issues of the response properties of AOS neurons, lesion and metabolic studies, and the AOS and spatial cognition.

Quantitative Reassessment of Speed Tuning in the Accessory Optic System and Pretectum of Pigeons

The correlation model of motion detection has been used to describe visual motion processing in the pretectum and accessory optic system (AOS). One feature of correlation detectors is that they are tuned to a particular temporal frequency (TF) independent of the spatial frequency (SF) but not to a particular stimulus speed (speed = TF/SF). Previous work has suggested that a subset of neurons in the AOS and pretectum of pigeons show apparent speed tuning. However, this study used relatively liberal between-groups statistics to assess speed tuning. From studies of the motion-sensitive neurons in primate cortex, a rigorous within-groups test of speed tuning has been offered. A meta-analysis of the spatiotemporal tuning of units in the AOS and pretectum of pigeons using this within-groups analysis of speed tuning has been performed. We conclude that speed tuning in the AOS and pretectum is rarer than previously estimated, and there is remarkable diversity in the impact of SF on tuning for speed. In total, 18.6% of the units showed significant speed tuning whereas 39.8% showed significant SF/TF independence. However, many cells (41.5%) fell along a continuum between speed tuning and SF/TF independence. This diversity has also been noted in primate cortex and may reflect a general property of motion-sensitive systems.

Role of Motor and Visual Experience During Development of Bipedal Locomotion in Chicks

The purpose of this research was to investigate the role of motor and visual experience during the development of locomotion in chicks. We have previously demonstrated that when locomotor activity is restricted immediately posthatching, chicks walk with shorter stride lengths and attenuated head bobbing movements. Head bobbing is an optokinetic response in birds, driven by the movement of the visual world across the retina (i.e., optic flow). During locomotion, optic flow is generated by forward translation, and we have shown that the magnitude of head bobbing movements and stride lengths are moderately correlated in walking chicks. In the present study, we investigated this relationship more closely by examining whether imposed changes in stride length could affect head excursions during head bobbing. We manipulated stride length by hobbling chicks immediately after hatching and subsequently quantified kinematic parameters, including step timing and head excursions, during walking. Imposition of shorter stride lengths induced chicks to take more frequent steps, spend less time in contact with the ground, and shortened head excursions during head bobbing. Nevertheless, the developmental changes in head excursions were not fully accounted for by altered stride lengths, so in a separate experiment, we investigated whether the development of head bobbing relies on the normal experience of optic flow. We raised chicks under stroboscopic illumination to eliminate chicks' experience of optic flow but found that this did not significantly alter head bobbing. These results are discussed along with related findings in other species and the possible neural and biomechanical constraints underlying development of walking and head bobbing in birds.

Telencephalic projections to the nucleus of the basal optic root and pretectal nucleus lentiformis mesencephali in pigeons

In birds, the nucleus of the basal optic root (nBOR) of the accessory optic system (AOS) and the pretectal nucleus lentiformis mesencephali (LM) are involved in the analysis of optic flow and the generation of the optokinetic response. In several species, it has been shown that the AOS and pretectum receive input from visual areas of the telencephalon. Previous studies in pigeons using anterograde tracers have shown that both nBOR and LM receive input from the visual Wulst, the putative homolog of mammalian primary visual cortex. In the present study, we used retrograde and anterograde tracing techniques to further characterize these projections in pigeons. After injections of the retrograde tracer cholera toxin subunit B (CTB) into either LM or nBOR, retrograde labeling in the telencephalon was restricted to the hyperpallium apicale (HA) of the Wulst. From the LM injections, retrograde labeling appeared as a discrete band of cells restricted to the lateral edge of HA. From the nBOR injections, the retrograde labeling was more distributed in HA, generally dorsal and dorso-medial to the LM-projecting neurons. In the anterograde experiments, biotinylated dextran amine (BDA) was injected into HA and individual axons were reconstructed to terminal fields in the LM and nBOR. Those fibers projecting to the nBOR also innervated the adjacent ventral tegmental area. However, tracing of BDA-labeled axons revealed no evidence that individual neurons project to both LM and nBOR. In summary, our results suggest that the nBOR and LM receive input from different areas of the Wulst. We discuss how these projections may transmit visual and/or somatosensory information to the nBOR and LM.

Spatiotemporal tuning of optic flow inputs to the vestibulocerebellum in pigeons: differences between mossy and climbing fiber pathways

The pretectum, accessory optic system (AOS), and vestibulocerebellum (VbC) have been implicated in the analysis of optic flow and generation of the optokinetic response. Recently, using drifting sine-wave gratings as stimuli, it has been shown that pretectal and AOS neurons exhibit spatiotemporal tuning. In this respect, there are two groups: fast neurons, which prefer low spatial frequency (SF) and high temporal frequency (TF) gratings, and slow neurons, which prefer high SF-low TF gratings. In pigeons, there are two pathways from the pretectum and AOS to the VbC: a climbing fiber (CF) pathway to Purkinje cells (P cells) via the inferior olive and a direct mossy fiber (MF) pathway to the granular layer (GL). In the present study, we assessed spatiotemporal tuning in the VbC of ketamine-anesthetized pigeons using standard extracellular techniques. Recordings were made from 17 optic-flow-sensitive units in the GL, presumably granule cells or MF rosettes, and the complex spike activity (CSA) of 39 P-cells, which reflects CF input. Based on spatiotemporal tuning to gratings moving in the preferred direction, eight GL units were classified as fast units, with a primary response to low SF-high TF gratings (mean = 0.13 cpd/8.24 Hz), whereas nine were slow units preferring high SF-low TF gratings (mean = 0.68 cpd/0.30 Hz). CSA was almost exclusively tuned to slow gratings (mean = 0.67 cpd/0.35 Hz). We conclude that MF input to the VbC is from both fast and slow cells in the AOS and pretectum, whereas the CF input is primarily tuned to slow gratings.

Neural mechanism underlying complex receptive field properties of motion-sensitive interneurons

In many species, neurons responding to visual motion at higher processing stages are often specifically tuned to particular flow fields; however, the neural circuitry that leads to this selectivity is not yet understood. Here we have studied this problem in 'vertical system' (VS) cells of the blowfly lobula plate. These neurons possess distinctive local preferred directions in different parts of their receptive field. Dual recordings from pairs of VS cells show that they are electrically coupled. This coupling is responsible for the elongated horizontal extent of their receptive fields. VS cells with a lateral receptive field have additional connections to a VS cell with a frontal receptive field and to the horizontal system, tuning these cells to rotational flow fields. In summary, the receptive field of these cells consists of two components: one that they receive from local motion detectors on their dendrite, and one that they import from other large-field neurons.

Telencephalic Input to the Pretectum of Pigeons: An Electrophysiological and Pharmacological Inactivation Study

The pretectal nucleus lentiformis mesencephali (LM) and the nucleus of the basal optic root (nBOR) of the avian accessory optic system (AOS) are retinal-recipient visual nuclei involved in the analysis of optic flow that results from self-motion, and in the generation of the optokinetic response. Neurons in these nuclei show direction selectivity in response to large-field motion and are tuned in the spatiotemporal domain. In addition to retinal afferentation, both the nBOR and LM receive afferents from the Wulst, which is thought to be the avian homolog of the primary visual cortex. We examined the effects of Wulst electrical stimulation on the activity of LM neurons and recorded the directional and spatiotemporal tuning of LM neurons in pigeons before, during, and after the Wulst was temporarily inactivated by lidocaine injection. In response to Wulst electrical stimulation, LM neurons showed either short-latency excitation followed by longer-latency inhibition (W+ cells), or only a longer-latency inhibition (W– cells). The average response latencies for W+ and W– cells were 13.5 and 28.3 ms, respectively. The effects of Wulst stimulation did not correlate with either the directional or spatiotemporal tuning of the LM neurons. Injection of lidocaine into the nBOR reduced the longer-latency oscillations of W+ and W– cells. When the Wulst was temporarily inactivated by lidocaine neither the directional nor spatiotemporal response properties of LM neurons were affected. The possible functions of the projection from the Wulst to the LM are discussed.