Three-dimensional organization of vestibular related eye movements to rotational motion in pigeons

Magnetoreception: A Dynamo in the Inner Ear of Pigeons

A phylogenetically diverse set of animals are able to orient by the Earth's magnetic field, but how they do so is an open problem. A new study identifies ion channels in the avian inner ear that could detect magnetic fields via induced electric fields.

Semicircular Canal Influences on the Developmental Tuning of the Translational Vestibulo-Ocular Reflex

Vestibulo-ocular reflexes (VORs) rely on neuronal computations that transform vestibular sensory signals into spatio-temporally appropriate extraocular motor commands. The motoneuronal discharge for contractions of the superior oblique eye muscle during linear translation derives from a utricular epithelial sector that is spatially aligned with the pulling direction of this muscle. In Xenopus laevis, the alignment is gradually achieved during larval development and requires motion-related semicircular canal afferent activity. Here, we studied the origin of semicircular canal and utricular signals responsible for the establishment and maturation of the extraocular motor response vector. Experiments were conducted on semi-intact preparations of Xenopus tadpoles before and after unilateral transection of the VIIIth nerve and in preparations of animals in which semicircular canal formation was prevented on one side by the injection of hyaluronidase into the otic capsule prior to the establishment of the tubular structures. Unilateral VIIIth nerve sections revealed that the excitation underlying the contraction of the superior oblique eye muscle during horizontal linear acceleration and clockwise/counter-clockwise roll motion derives exclusively from the utricle and the posterior semicircular canal on the ipsilateral side. In contrast, the developmental constriction of the otolith response vector depends on signals from the posterior semicircular canal on the contralateral side. These latter signals suppress directionally incorrect components that derive from the utricular sector perpendicular to the superior oblique eye muscle. This directional tuning complies with a stabilization of spatially correct utricular inputs that are aligned with the extraocular motor target muscle. In addition, misaligned signals are concurrently suppressed by semicircular canal-related commissural pathways from the contralateral side and through local interneuronal inhibitory circuits within the ipsilateral vestibular nuclei.

Head Stabilization in the Pigeon: Role of Vision to Correct for Translational and Rotational Disturbances

Stabilization of the head in animals with limited capacity to move their eyes is key to maintain a stable image on the retina. In many birds, including pigeons, a prominent example for the important role of head stabilization is the characteristic head-bobbing behavior observed during walking. Multimodal sensory feedback from the eyes, the vestibular system and proprioceptors in body and neck is required to control head stabilization. Here, we trained unrestrained pigeons (Columba livia) to stand on a perch that was sinusoidally moved with a motion platform along all three translational and three rotational degrees of freedom. We varied the frequency of the perturbation and we recorded the pigeons' responses under both light and dark conditions. Head, body, and platform movements were assessed with a high-speed motion capture system and the data were used to compute gain and phase of head and body movements in response to the perturbations. Comparing responses under dark and light conditions, we estimated the contribution of visual feedback to the control of the head. Our results show that the head followed the movement of the motion platform to a large extent during translations, but it was almost perfectly stabilized against rotations. Visual feedback only improved head stabilization during translations but not during rotations. The body compensated rotations around the forward-backward and the lateral axis, but did not contribute to head stabilization during translations and rotations around the vertical axis. From the results, we conclude that head stabilization in response to translations and rotations depends on different sensory feedback and that visual feedback plays only a limited role for head stabilization during standing.

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.

Possible cues driving context-specific adaptation of optocollic reflex in pigeons (Columba livia)

Context-specific adaptation (Shelhamer M, Clendaniel R. Neurosci Lett 332: 200-204, 2002) explains that reflexive responses can be maintained with different "calibrations" for different situations (contexts). Which context cues are crucial and how they combine to evoke context-specific adaptation is not fully understood. Gaze stabilization in birds is a nice model with which to tackle that question. Previous data showed that when pigeons (Columba livia) were hung in a harness and subjected to a frontal airstream provoking a flying posture ("flying condition"), the working range of the optokinetic head response [optocollic reflex (OCR)] extended toward higher velocities compared with the "resting condition." The present study was aimed at identifying which context cues are instrumental in recalibrating the OCR. We investigated that question by using vibrating stimuli delivered during the OCR provoked by rotating the visual surroundings at different velocities. The OCR gain increase and the boost of the fast phase velocity observed during the "flying condition" were mimicked by body vibration. On the other hand, the newly emerged relationship between the fast-phase and slow-phase velocities in the "flying condition" was mimicked by head vibration. Spinal cord lesion at the lumbosacral level decreased the effects of body vibration, whereas lesions of the lumbosacral apparatus had no effect. Our data suggest a major role of muscular proprioception in the context-specific adaptation of the stabilizing behavior, while the vestibular system could contribute to the context-specific adaptation of the orienting behavior. Participation of an efferent copy of the motor command driving the flight cannot be excluded.

Spatial and temporal characteristics of vestibular convergence

In all species studied, afferents from semicircular canals and otolith organs converge on central neurons in the brainstem. However, the spatial and temporal relationships between converging inputs and how these contribute to vestibular behaviors is not well understood. In the current study, we used discrete rotational and translational motion stimuli to characterize canal- and otolith-driven response components of convergent non-eye movement (NEM) neurons in the vestibular nuclear complex of alert pigeons. When compared to afferent responses, convergent canal signals had similar gain and phase ranges but exhibited greater spatial variability in their axes of preferred rotation. Convergent otolith signals also had similar mean gain and phase values to the afferent population but were spatially well-matched with the corresponding canal signals, cell-by-cell. However, neither response component alone nor a simple linear combination of these components was sufficient to predict actual net responses during combined canal-otolith stimulation. We discuss these findings in the context of previous studies of pigeon vestibular behaviors, and we compare our findings to similar studies in other species.

State-dependent sensorimotor processing: gaze and posture stability during simulated flight in birds

Vestibular responses play an important role in maintaining gaze and posture stability during rotational motion. Previous studies suggest that these responses are state dependent, their expression varying with the environmental and locomotor conditions of the animal. In this study, we simulated an ethologically relevant state in the laboratory to study state-dependent vestibular responses in birds. We used frontal airflow to simulate gliding flight and measured pigeons' eye, head, and tail responses to rotational motion in darkness, under both head-fixed and head-free conditions. We show that both eye and head response gains are significantly higher during flight, thus enhancing gaze and head-in-space stability. We also characterize state-specific tail responses to pitch and roll rotation that would help to maintain body-in-space orientation during flight. These results demonstrate that vestibular sensorimotor processing is not fixed but depends instead on the animal's behavioral state.

Vestibulo-ocular reflex responses to a multichannel vestibular prosthesis incorporating a 3D coordinate transformation for correction of misalignment

There is no effective treatment available for individuals unable to compensate for bilateral profound loss of vestibular sensation, which causes chronic disequilibrium and blurs vision by disrupting vestibulo-ocular reflexes that normally stabilize the eyes during head movement. Previous work suggests that a multichannel vestibular prosthesis can emulate normal semicircular canals by electrically stimulating vestibular nerve branches to encode head movements detected by mutually orthogonal gyroscopes affixed to the skull. Until now, that approach has been limited by current spread resulting in distortion of the vestibular nerve activation pattern and consequent inability to accurately encode head movements throughout the full 3-dimensional (3D) range normally transduced by the labyrinths. We report that the electrically evoked 3D angular vestibulo-ocular reflex exhibits vector superposition and linearity to a sufficient degree that a multichannel vestibular prosthesis incorporating a precompensatory 3D coordinate transformation to correct misalignment can accurately emulate semicircular canals for head rotations throughout the range of 3D axes normally transduced by a healthy labyrinth.

Adaptation of the vestibulo-ocular reflex for forward-eyed foveate vision

To maintain visual fixation on a distant target during head rotation, the angular vestibulo-ocular reflex (aVOR) should rotate the eyes at the same speed as the head and in exactly the opposite direction. However, in primates for which the 3-dimensional (3D) aVOR has been extensively characterised (humans and squirrel monkeys (Saimiri sciureus)), the aVOR response to roll head rotation about the naso-occipital axis is lower than that elicited by yaw and pitch, causing errors in aVOR magnitude and direction that vary with the axis of head rotation. In other words, primates keep the central part of the retinal image on the fovea (where photoreceptor density and visual acuity are greatest) but fail to keep that image from twisting about the eyes' resting optic axes. We tested the hypothesis that aVOR direction dependence is an adaptation related to primates' frontal-eyed, foveate status through comparison with the aVOR of a lateral-eyed, afoveate mammal (Chinchilla lanigera). As chinchillas' eyes are afoveate and never align with each other, we predicted that the chinchilla aVOR would be relatively low in gain and isotropic (equal in gain for every head rotation axis). In 11 normal chinchillas, we recorded binocular 3D eye movements in darkness during static tilts, 20-100 deg s(1) whole-body sinusoidal rotations (0.5-15 Hz), and 3000 deg s(2) acceleration steps. Although the chinchilla 3D aVOR gain changed with both frequency and peak velocity over the range we examined, we consistently found that it was more nearly isotropic than the primate aVOR. Our results suggest that primates' anisotropic aVOR represents an adaptation to their forward-eyed, foveate status. In primates, yaw and pitch aVOR must be compensatory to stabilise images on both foveae, whereas roll aVOR can be under-compensatory because the brain tolerates torsion of binocular images that remain on the foveae. In contrast, the lateral-eyed chinchilla faces different adaptive demands and thus enlists a different aVOR strategy.

Transsphenoidal approaches to the pituitary: a progression in experience in a single centre

BACKGROUND

Evolving of a single centre by means of different transsphenoidal approaches during the survey of methodological advances in pituitary surgery is presented.

MATERIALS AND METHODS

Ninety-three consecutive patients with pituitary adenomas underwent transphenoidal pituitary operations at Gulhane Military Medical Academy from January 1996 to October 2007. Retrospective chart-based analysis of the surgical methods of transsphenoidal pituitary adenoma operations were done. Surgical methods were described. Outcomes and complications were presented. Attention is focused on the methodology of different surgical techniques of pituitary surgery.

FINDINGS

During the evaluation period, 12 Sublabial approaches (1996-1998), 13 transseptal transsphenoidal approaches (1999-2000), 15 endonasal transsphenoidal approaches (2000-2004), 25 endoscopy assisted endonasal approaches (2002-2006) and 28 pure endoscopic endonasal approaches (2006-2007) were performed.

CONCLUSIONS

Technologic advancements in endoscopy and gaining experience in pituitary surgery drives neurosurgeons toward less invasive approaches.

Canal and otolith contributions to compensatory tilt responses in pigeons

Gaze-stabilizing eye and head responses compensate more effectively for low-frequency rotational motion when such motion stimulates the otolith organs, as during earth-horizontal axis rotations. However, the nature of the otolith signal responsible for this improvement in performance has not been previously determined. In this study, we used combinations of earth-horizontal axis rotational and translational motion to manipulate the magnitude of net linear acceleration experienced by pigeons, under both head-fixed and head-free conditions. We show that phase enhancement of eye and head responses to low-frequency rotational motion was causally related to the magnitude of dynamic net linear acceleration and not the gravitational acceleration component. We also show that canal-driven and otolith-driven eye responses were both spatially and temporally appropriate to combine linearly, and that a simple linear model combining canal- and otolith-driven components predicted eye responses to complex motion that were consistent with our experimental observations. However, the same model did not predict the observed head responses, which were spatially but not temporally appropriate to combine according to the same linear scheme. These results suggest that distinct vestibular processing substrates exist for eye and head responses in pigeons and that these are likely different from the vestibular processing substrates observed in primates.

Recovery of gaze stability during vestibular regeneration

Many motion related behaviors, such as gaze stabilization, balance, orientation, and navigation largely depend on a properly functioning vestibular system. After vestibular insult, many of these responses are compromised but can return during the regeneration of vestibular receptors and afferents as is known to occur in birds, reptiles, and amphibians. Here we characterize gaze stability in pigeons to rotational motion during regeneration after complete bilateral vestibular loss via an ototoxic antibiotic. Immediate postlesion effects included severe head oscillations, postural ataxia, and total lack of gaze control. We found that these abnormal behaviors gradually subsided, and gaze stability slowly returned to normal function according to a temporal sequence that lasted several months. We also found that the dynamic recovery of gaze function during regeneration was not homogeneous for all types of motion. Instead high-frequency motion stability was first achieved, followed much later by slow movement stability. In addition, we found that initial gaze stability was established using almost exclusive head-response components with little eye-movement contribution. However, that trend reversed as recovery progressed so that when gaze stability was complete, the eye component had increased and the head response had decreased to levels significantly different from that observed in normal birds. This was true even though the head-fixed VOR response recovered normally. Recovery of gaze stability coincided well with the three stage temporal sequence of morphologic regeneration previously described by our laboratory.

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.

Visual fixation of a landing perch by chickens

Chickens were video recorded while making jumps or flights toward a landing perch, to test hypotheses about visual fixation behaviour. In the first experiment, varying the height above the landing perch of the food container providing the incentive to jump had no effect on head orientation, indicating that the birds were not fixating this object. In the second experiment, hens jumped over six combinations of perch height and distance, and a linear relationship was found at take-off between head orientation and the angular distance of the perch from the horizontal at the eye. This relationship is consistent with fixation of the perch by a linear combination of head and eye rotations, with the head component contributing 73% of the total response. The image of the perch is fixated 20 masculine below that of the bill tip, outside any region of the chicken retina specialised for high acuity vision. Fixation of the perch before jumping must therefore have some function other than inspection with high acuity, such as providing a constraint that enables precise visual control of trajectory and landing manoeuvres.

Vestibular gaze stabilization: different behavioral strategies for arboreal and terrestrial avians

In birds, it is thought that head movements play a major role in the reflexive stabilization of gaze and vision. In this study, we investigated the contributions of the eye and head to gaze stabilization during rotations under both head-fixed [vestibuloocular (VOR)] and head-free conditions in two avian species: pigeons and quails. These two species differ both in ocular anatomy (the pigeon has 2 distinct foveal regions), as well as in behavioral repertoires. Pigeons are arboreal, fly extended distances, and can navigate. Quails are primarily engrossed in terrestrial niches and fly only short distances. Unlike the head-fixed VOR gains that were under-compensatory for both species, gaze gains under head-free conditions were completely compensatory at high frequencies. This compensation was achieved primarily with head movements in pigeons, but with combined head and eye-in-head contributions in the quail. In contrast, eye-in-head motion, which was significantly reduced for head-free compared with head-fixed conditions, contributed very little to overall gaze stability in pigeons. These results suggest that disparity between the stabilization strategies employed by these two birds may be attributed to differences in species-specific behavior and anatomy.

Posture, head stability, and orientation recovery during vestibular regeneration in pigeons

Compensatory behavior such as oculomotor, gaze, and postural responses that occur during movement largely depend upon a functioning vestibular system. In the present study, the initial loss and subsequent recovery of postural and head stability in pigeons undergoing vestibular regeneration were examined. Adult pigeons were trained to manipulate a straight run chamber to peck an illuminated key for fluid reward. Six behavioral measures assessing performance, posture, and head stability were quantified. These included run latency, steps (walking), path negotiation (lane changes), gaze saccades, head bobs, and head shakes. Once normative values were obtained for four birds, complete lesion of all receptor cells and denervation of the epithelia in the vestibular endorgans were produced using a single intralabyrinthine application of streptomycin sulfate. Each bird was then tested at specific times during regeneration and the same behavioral measures examined. At 7 days post-streptomycin treatment (PST), all birds exhibited severe postural and head instability, with tremors, head shakes, staggering, and circling predominating. No normal trial runs, walking, gaze saccades, or head bobs were present. Many of these dysfunctions persisted through 3-4 weeks PST. Gradually, tremor and head shakes diminished and were replaced with an increasing number of normal head bobs during steps and gaze saccades. Beginning at 4 weeks PST, but largely inaccurate, was the observed initiation of directed steps, less staggering, and some successful path negotiation. As regeneration progressed, spatial orientation and navigation ability increased and, by 49 days PST, most trials were successful. By 70 days PST, all birds had recovered to pretreatment levels. Thus, it was observed that ataxia must subside, coincident with normalized head and postural stability prior to the recovery of spatial orientation and path navigation recovery. Parallels in recovery were drawn to hair cell regeneration and afferent responsiveness, as inferred from present results and those in other investigations.

Caspase inhibitors promote vestibular hair cell survival and function after aminoglycoside treatment in vivo

The sensory hair cells of the inner ear undergo apoptosis after acoustic trauma or aminoglycoside antibiotic treatment, causing permanent auditory and vestibular deficits in humans. Previous studies have demonstrated a role for caspase activation in hair cell death and ototoxic injury that can be reduced by concurrent treatment with caspase inhibitors in vitro. In this study, we examined the protective effects of caspase inhibition on hair cell death in vivo after systemic injections of aminoglycosides. In one series of experiments, chickens were implanted with osmotic pumps that administrated the pan-caspase inhibitor z-Val-Ala-Asp(Ome)-fluoromethylketone (zVAD) into inner ear fluids. One day after the surgery, the animals received a 5 d course of treatment with streptomycin, a vestibulotoxic aminoglycoside. Direct infusion of zVAD into the vestibule significantly increased hair cell survival after streptomycin treatment. A second series of experiments determined whether rescued hair cells could function as sensory receptors. Animals treated with streptomycin displayed vestibular system impairment as measured by a greatly reduced vestibulo-ocular response (VOR). In contrast, animals that received concurrent systemic administration of zVAD with streptomycin had both significantly greater hair cell survival and significantly increased VOR responses, as compared with animals treated with streptomycin alone. These findings suggest that inhibiting the activation of caspases promotes the survival of hair cells and protects against vestibular function deficits after aminoglycoside treatment.