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Dickman, J. D.

Publications and source records attributed to Dickman, J. D..

18 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS2003

Histological preparation of developing vestibular otoconia for scanning electron microscopy

The unique nature of vestibular otoconia as calcium carbonate biominerals makes them particularly susceptible to chemical deformation during histological processing. We fixed and stored otoconia from all three otolith endorgans of embryonic, hatchling and adult Japanese quail in glutaraldehyde containing either phosphate or non-phosphate buffers for varying lengths of time and processed them for scanning electron microscopy. Otoconia from all age groups and otolith endorgans processed in 0.1 M phosphate buffer (pH 7.4) showed abnormal surface morphology when compared to acetone fixed controls. Otoconia processed in 0.1 M sodium cacodylate or HEPES buffered artificial endolymph (pH 7.4) showed normal morphology that was similar to controls. The degree of otoconial deformation was directly related to the time exposed to phosphate buffer. Short duration exposure produced particulate deformations while longer exposures resulted in fused otoconia that formed solid sheets. Otoconial surface deformation and fusing was independent of the glutaraldehyde component of the histological processing. These findings should help vestibular researchers to develop appropriate histological processing protocols in future studies of otoconia.

Non-NASA Center↗
NASA NTRS2003

Afferent innervation patterns of the saccule in pigeons

The innervation patterns of vestibular saccular afferents were quantitatively investigated in pigeons using biotinylated dextran amine as a neural tracer and three-dimensional computer reconstruction. Type I hair cells were found throughout a large portion of the macula, with the highest density observed in the striola. Type II hair cells were located throughout the macula, with the highest density in the extrastriola. Three classes of afferent innervation patterns were observed, including calyx, dimorph, and bouton units, with 137 afferents being anatomically reconstructed and used for quantitative comparisons. Calyx afferents were located primarily in the striola, innervated a number of type I hair cells, and had small innervation areas. Most calyx afferent terminal fields were oriented parallel to the anterior-posterior axis and the morphological polarization reversal line. Dimorph afferents were located throughout the macula, contained fewer type I hair cells in a calyceal terminal than calyx afferents and had medium sized innervation areas. Bouton afferents were restricted to the extrastriola, with multi-branching fibers and large innervation areas. Most of the dimorph and bouton afferents had innervation fields that were oriented dorso-ventrally but were parallel to the neighboring reversal line. The organizational morphology of the saccule was found to be distinctly different from that of the avian utricle or lagena otolith organs and appears to represent a receptor organ undergoing evolutionary adaptation toward sensing linear motion in terrestrial and aerial species.

NASA Discipline Neuroscience↗
NASA NTRS2000

Spatiotemporal processing of linear acceleration: primary afferent and central vestibular neuron responses

Spatiotemporal convergence and two-dimensional (2-D) neural tuning have been proposed as a major neural mechanism in the signal processing of linear acceleration. To examine this hypothesis, we studied the firing properties of primary otolith afferents and central otolith neurons that respond exclusively to horizontal linear accelerations of the head (0.16-10 Hz) in alert rhesus monkeys. Unlike primary afferents, the majority of central otolith neurons exhibited 2-D spatial tuning to linear acceleration. As a result, central otolith dynamics vary as a function of movement direction. During movement along the maximum sensitivity direction, the dynamics of all central otolith neurons differed significantly from those observed for the primary afferent population. Specifically at low frequencies (</=0.5 Hz), the firing rate of the majority of central otolith neurons peaked in phase with linear velocity, in contrast to primary afferents that peaked in phase with linear acceleration. At least three different groups of central response dynamics were described according to the properties observed for motion along the maximum sensitivity direction. "High-pass" neurons exhibited increasing gains and phase values as a function of frequency. "Flat" neurons were characterized by relatively flat gains and constant phase lags (approximately 20-55 degrees ). A few neurons ("low-pass") were characterized by decreasing gain and phase as a function of frequency. The response dynamics of central otolith neurons suggest that the approximately 90 degrees phase lags observed at low frequencies are not the result of a neural integration but rather the effect of nonminimum phase behavior, which could arise at least partly through spatiotemporal convergence. Neither afferent nor central otolith neurons discriminated between gravitational and inertial components of linear acceleration. Thus response sensitivity was indistinguishable during 0.5-Hz pitch oscillations and fore-aft movements. The fact that otolith-only central neurons with "high-pass" filter properties exhibit semicircular canal-like dynamics during head tilts might have important consequences for the conclusions of previous studies of sensory convergence and sensorimotor transformations in central vestibular neurons.

NASA Discipline Neuroscience↗
NASA NTRS2000

Primate translational vestibuloocular reflexes. IV. Changes after unilateral labyrinthectomy

The effects of unilateral labyrinthectomy on the properties of the translational vestibuloocular reflexes (trVORs) were investigated in rhesus monkeys trained to fixate near targets. Translational motion stimuli consisted of either steady-state lateral and fore-aft sinusoidal oscillations or short-lasting transient displacements. During small-amplitude, steady-state sinusoidal lateral oscillations, a small decrease in the horizontal trVOR sensitivity and its dependence on viewing distance was observed during the first week after labyrinthectomy. These deficits gradually recovered over time. In addition, the vertical response component increased, causing a tilt of the eye velocity vector toward the lesioned side. During large, transient lateral displacements, the deficits were larger and longer lasting. Responses after labyrinthectomy were asymmetric, with eye velocity during movements toward the side of the lesion being more compromised. The most profound effect of the lesions was observed during fore-aft motion. Whereas responses were kinematically appropriate for fixation away from the side of the lesion (e.g., to the left after right labyrinthectomy), horizontal responses were anticompensatory during fixation at targets located ipsilateral to the side of the lesion (e.g., for targets to the right after right labyrinthectomy). This deficit showed little recovery during the 3-mo post-labyrinthectomy testing period. These results suggest that inputs from both labyrinths are important for the proper function of the trVORs, although the details of how bilateral signals are processed and integrated remain unknown.

NASA Discipline Neuroscience↗
NASA NTRS2000

Primate translational vestibuloocular reflexes. III. Effects of bilateral labyrinthine electrical stimulation

The effects of functional, reversible ablation and potential recruitment of the most irregular otolith afferents on the dynamics and sensitivity of the translational vestibuloocular reflexes (trVORs) were investigated in rhesus monkeys trained to fixate near and far targets. Translational motion stimuli consisted of either steady-state lateral and fore-aft sinusoidal oscillations or short-lasting transient lateral head displacements. Short-duration (usually <2 s) anodal (inhibitory) and cathodal (excitatory) currents (50-100 microA) were delivered bilaterally during motion. In the presence of anodal labyrinthine stimulation, trVOR sensitivity and its dependence on viewing distance were significantly decreased. In addition, anodal currents significantly increased phase lags. During transient motion, anodal stimulation resulted in significantly lower initial eye acceleration and more sluggish responses. Cathodal currents tended to have opposite effects. The main characteristics of these results were simulated by a simple model where both regularly and irregularly discharging afferents contribute to the trVORs. Anodal labyrinthine currents also were found to decrease eye velocity during long-duration, constant velocity rotations, although results were generally more variable compared with those during translational motion.

NASA Program Biomedical Research and Countermeasur↗
NASA NTRS2000

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

During rotational motions, compensatory eye movement adjustments must continually occur in order to maintain objects of visual interest as stable images on the retina. In the present study, the three-dimensional organization of the vestibulo-ocular reflex in pigeons was quantitatively examined. Rotations about different head axes produced horizontal, vertical, and torsional eye movements, whose component magnitude was dependent upon the cosine of the stimulus axis relative to the animal's visual axis. Thus, the three-dimensional organization of the VOR in pigeons appears to be compensatory for any direction of head rotation. Frequency responses of the horizontal, vertical, and torsional slow phase components exhibited high pass filter properties with dominant time constants of approximately 3 s.

NASA Discipline Neuroscience↗
NASA NTRS1999

Three-dimensional organization of vestibular-related eye movements to off-vertical axis rotation and linear translation in pigeons

During linear accelerations, compensatory reflexes should continually occur in order to maintain objects of visual interest as stable images on the retina. In the present study, the three-dimensional organization of the vestibulo-ocular reflex in pigeons was quantitatively examined during linear accelerations produced by constant velocity off-vertical axis yaw rotations and translational motion in darkness. With off-vertical axis rotations, sinusoidally modulated eye-position and velocity responses were observed in all three components, with the vertical and torsional eye movements predominating the response. Peak torsional and vertical eye positions occurred when the head was oriented with the lateral visual axis of the right eye directed orthogonal to or aligned with the gravity vector, respectively. No steady-state horizontal nystagmus was obtained with any of the rotational velocities (8-58 degrees /s) tested. During translational motion, delivered along or perpendicular to the lateral visual axis, vertical and torsional eye movements were elicited. No significant horizontal eye movements were observed during lateral translation at frequencies up to 3 Hz. These responses suggest that, in pigeons, all linear accelerations generate eye movements that are compensatory to the direction of actual or perceived tilt of the head relative to gravity. In contrast, no translational horizontal eye movements, which are known to be compensatory to lateral translational motion in primates, were observed under the present experimental conditions.

NASA Discipline Neuroscience↗
NASA NTRS1999

Distribution and time course of hair cell regeneration in the pigeon utricle

Vestibular and cochlear regeneration following ototoxic insult from aminoglycoside antibiotics has been well documented, particularly in birds. In the present study, intraotic application of a 2 mg streptomycin paste was used to achieve complete vestibular hair cell destruction in pigeons (Columba livia) while preserving regenerative ability. Scanning electron microscopy was used to quantify hair cell density longitudinally during regeneration in three different utricular macula locations, including the striola, central and peripheral regions. The utricular epithelium was void of stereocilia (indicating hair cell loss) at 4 days after intraotic treatment with streptomycin. At 2 weeks the stereocilia began to appear randomly and mostly in an immature form. However, when present most kinocilia were polarized toward the developing striola. Initially, regeneration occurred more rapidly in the central and peripheral regions of the utricle as compared to the striola. As regeneration proceeded from 2 to 12 weeks, hair cell density in the striola region equaled the density noted in the central and peripheral regions. At 24 weeks, hair cell density of the central and peripheral regions was equal to normal values, however the striola region had a slightly greater hair cell density than that observed for normal animals.

Non-NASA Center↗
NASA NTRS1999

Functional organization of primate translational vestibulo-ocular reflexes and effects of unilateral labyrinthectomy

Translational vestibulo-ocular reflexes (trVORs) are characterized by distinct spatio-temporal properties and sensitivities that are proportional to the inverse of viewing distance. Anodal (inhibitory) labyrinthine stimulation (100 microA, < 2 s) during motion decreased the high-pass filtered dynamics, as well as horizontal trVOR sensitivity and its dependence on viewing distance. Cathodal (excitatory) currents had opposite effects. Translational VORs were also affected after unilateral labyrinthectomy. Animals lost their ability to modulate trVOR sensitivity as a function of viewing distance acutely after the lesion. These deficits partially recovered over time, albeit a significant reduction in trVOR sensitivity as a function of viewing distance remained in compensated animals. During fore-aft motion, the effects of unilateral labyrinthectomy were more dramatic. Both acute and compensated animals permanently lost their ability to modulate fore-aft trVOR responses as a function of target eccentricity. These results suggest that (1) the dynamics and viewing distance-dependent properties of the trVORs are very sensitive to changes in the resting firing rate of vestibular afferents and, consequently, vestibular nuclei neurons; (2) the most irregularly firing primary otolith afferents that are most sensitive to labyrinthine electrical stimulation might contribute to reflex dynamics and sensitivity; (3) inputs from both labyrinths are necessary for the generation of the translational VORs.

Non-NASA Center↗
NASA NTRS1999

Streptomycin ototoxicity and hair cell regeneration in the adult pigeon utricle

OBJECTIVE: The purpose of this study was to develop a technique to investigate the regeneration of utricular hair cells in the adult pigeon (Columba livia) following complete hair cell loss through administration of streptomycin. STUDY DESIGN: Experimental animal study. METHODS: Animals were divided into four groups. Group 1 received 10 to 15 days of systemic streptomycin injections. Animals in Groups 2 and 3 received a single direct placement of a 1-, 2-, 4-, or 8-mg streptomycin dose into the perilymphatic space. Animals in Groups 1 and 2 were analyzed within 1 week from injection to investigate hair cell destruction, whereas Group 3 was investigated at later dates to study hair cell recovery. Group 4 animals received a control injection of saline into the perilymphatic space. Damage and recovery were quantified by counting hair cells in isolated utricles using scanning electron microscopy. RESULTS: Although systemic injections failed to reliably achieve complete utricular hair cell destruction, a single direct placement of a 2-, 4-, or 8-mg streptomycin dose caused complete destruction within the first week. Incomplete hair cell loss was observed with the 1-mg dose. Over the long term, regeneration of the hair cells was seen with the 2-mg dose but not the 8-mg dose. Control injections of saline into the perilymphatic space caused no measurable hair cell loss. CONCLUSIONS: Direct placement of streptomycin into the perilymph is an effective, reliable method for complete destruction of utricular hair cells while preserving the regenerative potential of the neuroepithelium.

Non-NASA Center↗
NASA NTRS1997

Response properties of pigeon otolith afferents to linear acceleration

In the present study, the sensitivity to sinusoidal linear accelerations in the plane of the utricular macula was tested in afferents. The head orientation relative to the translation axis was varied in order to determine the head position that elicited the maximal and minimal responses for each afferent. The response gain and phase values obtained to 0.5-Hz and 2-Hz linear acceleration stimuli were then plotted as a function of head orientation and a modified cosine function was fit to the data. From the best-fit cosine function, the predicted head orientations that would produce the maximal and minimal response gains were estimated. The estimated maximum response gains to linear acceleration in the utricular plane for the afferents varied between 75 and 1420 spikes s-1 g-1. The mean maximal gains for all afferents to 0.5-Hz and 2-Hz sinusoidal linear acceleration stimuli were 282 and 367 spikes s-1 g-1, respectively. The minimal response gains were essentially zero for most units. The response phases always led linear acceleration and remained constant for each afferent, regardless of head orientation. These response characteristics indicate that otolith afferents are cosine tuned and behave as one-dimensional linear accelerometers. The directions of maximal sensitivity to linear acceleration for the afferents varied throughout the plane of the utricle; however, most vectors were directed out of the opposite ear near the interaural axis. The response dynamics of the afferents were tested using stimulus frequencies ranging between 0.25 Hz and 10 Hz (0.1 g peak acceleration). Across stimulus frequencies, most afferents had increasing gains and constant phase values. These dynamic properties for individual afferents were fit with a simple transfer function that included three parameters: a mechanical time constant, a gain constant, and a fractional order distributed adaptation operator.

Non-NASA Center↗
NASA NTRS1996

Spatial orientation of semicircular canals and afferent sensitivity vectors in pigeons

Rotational head motion in vertebrates is detected by the semicircular canal system, whose innervating primary afferent fibers carry information about movement in specific head planes. The semicircular canals have been qualitatively examined over a number of years, and the canal planes have been quantitatively characterized in several animal species. The present study first determined the geometric relationship between individual semicircular canals and between the canals and the stereotactic head planes in pigeons. Stereotactic measurements of multiple points along the circumference of the bony canals were taken, and the measured points fitted with a three-dimensional planar surface. Direction normals to the plane's surface were calculated and used to define angles between semicircular canal pairs. Because of the unusual shape of the anterior semicircular canals in pigeons, two planes, a major and a minor, were fitted to the canal's course. Calculated angle values for all canals indicated that the horizontal and posterior semicircular canals are nearly orthogonal, but the anterior canals have substantial deviations from orthogonality with other canal planes. Next, the responses of the afferent fibers that innervate each of the semicircular canals to 0.5 Hz sinusoidal rotation about an earth-vertical axis were obtained. The head orientation relative to the rotation axis was systematically varied so that directions of maximum sensitivity for each canal afferent could be determined. These sensitivity vectors were then compared with the canal plane direction normals. The afferents that innervated specific semicircular canals formed homogeneous clusters of sensitivity vectors in different head planes. The horizontal and posterior afferents had average sensitivity vectors that were largely co-incident with the innervated canal plane direction normals. Anterior canal afferents, however, appeared to synthesize contributions from the major and minor plane components of the bony canal structure to produce a resultant sensitivity vector that was positioned between the canal planes. Calculated angles between the average canal afferent sensitivity vectors revealed that direction orthogonality is preserved at the afferent signal level, even though deviations from canal plane orthogonality exist.

NASA Discipline Neuroscience↗
NASA NTRS1996

Differential central projections of vestibular afferents in pigeons

The question of whether a differential distribution of vestibular afferent information to central nuclear neurons is present in pigeons was studied using neural tracer compounds. Discrete tracing of afferent fibers innervating the individual semicircular canal and otolith organs was produced by sectioning individual branches of the vestibular nerve that innervate the different receptor organs and applying crystals of horseradish peroxidase, or a horseradish peroxidase/cholera toxin mixture, or a biocytin compound for neuronal uptake and transport. Afferent fibers and their terminal distributions within the brainstem and cerebellum were visualized subsequently. Discrete areas in the pigeon central nervous system that receive primary vestibular input include the superior, dorsal lateral, ventral lateral, medial, descending, and tangential vestibular nuclei; the A and B groups; the intermediate, medial, and lateral cerebellar nuclei; and the nodulus, the uvula, and the paraflocculus. Generally, the vertical canal afferents projected heavily to medial regions in the superior and descending vestibular nuclei as well as the A group. Vertical canal projections to the medial and lateral vestibular nuclei were observed but were less prominent. Horizontal canal projections to the superior and descending vestibular nuclei were much more centrally located than those of the vertical canals. A more substantial projection to the medial and lateral vestibular nuclei was seen with horizontal canal afferents compared to vertical canal fibers. Afferents innervating the utricle and saccule terminated generally in the lateral regions of all vestibular nuclei in areas that were separate from the projections of the semicircular canals. In addition, utricular fibers projected to regions in the vestibular nuclei that overlapped with the horizontal semicircular canal terminal fields, whereas saccular afferents projected to regions that received vertical canal fiber terminations. Lagenar afferents projected throughout the cochlear nuclei, to the dorsolateral regions of the cerebellar nuclei, and to lateral regions of the superior, lateral, medial, and descending vestibular nuclei.

NASA Discipline Neuroscience↗
NASA NTRS1994

Experiment K-7-31: Studies of Vestibular Primary Afferents and Eye Movements in Normal, Hypergravity and Hypogravity - Axon Cosmos Flight 2044

Fourteen days of active head movements in microgravity appear to modify the gain and neural adaptation properties of the horizontal semicircular canals in the rhesus monkey. This is the first demonstration of adaptive plasticity in the sensory receptor. Reversing prisms, for example, do not modify the gain of the primary afferent response. Pulse yaw rotation, sinusoidal rotation, and sum of sinusoidal rotation testing during the first day following recovery revealed that the gain of a sample of afferents was significantly greater than the gain derived from afferent responses obtained during pre-flight and control monkey testing. There was no strong evidence of tilt sensitivity in the sample of afferents that we tested either during the pre-flight or control tests or during the first day post-flight. Two irregular afferents tested on postflight day 2 showed changes with tilt but the responses were not systematic. The spontaneous discharge did not change following flight. Mean firing rate and coefficient of variation remained constant during the post flight tests and was near the value measured during pre flight tests. The change in gain of horizontal canal afferents might be adaptive. The animals were required to look at a target for food. This required active head and eye movements. Active head movements have been shown to be hypometric and eye movements have been shown to be hypermetric during the first few days of past Cosmos flights (see introduction). It might be that the increased gain in the horizontal semicircular canals permit accurate target acquisition during hypometric head movements by driving the eyes to greater angles for smaller angles of head movement. The mechanism by which the semicircular canals recalibrate (increase their gain) is unknown. The efferent vestibular system is a logical candidate. Horizontal nystagmus during rotation about an earth vertical axis with the horizontal semicircular canals in the plane of rotation produced the same response during postflight day 1 and post-flight day 9. But when the head was pitched down 45? the nystagmus slow phase velocity was greater and the duration was about twice during post-flight day 1. Apparently, this response involving the interaction of the horizontal and vertical semicircular canals and the otoliths did not recalibrate during post-flight day 1. The 'DC' bias of the slow phase velocity of the horizontal nystagmus during constant velocity horizontal axis rotation was roughly 4 times for one flight monkey and roughly 2 times for the other on post-flight day 1 compared to post-flight day 9. These results suggest that the otolith mediated response during constant velocity rotation also did not recalibrate on post-flight day 1.

Correia, M. J.↗
NASA NTRS1992

Changes in monkey horizontal semicircular canal afferent responses after spaceflight

Extracellular responses from single horizontal semicircular canal afferents in two rhesus monkeys were studied after recovery from a 14-day biosatellite (Cosmos 2044) orbital spaceflight. On the 1st postflight day, the mean gain for 9 different horizontal canal afferents, tested using one or several different passive yaw rotation waveforms, was nearly twice that for 20 horizontal canal afferents similarly tested during preflight and postflight control studies. Adaptation of the afferent response to passive yaw rotation on the 1st postflight day was also greater. These results suggest that at least one component of the vestibular end organ (the semicircular canals) is transiently modified after exposure to 14 days of microgravity. It is unclear whether the changes are secondary to other effects of microgravity, such as calcium loss, or an adaptive response. If the response is adaptive, then this report is the first evidence that the response of the vestibular end organ may be modified (presumably by the central nervous system via efferent connections) after prolonged unusual vestibular stimulation. If this is the case, the sites of plasticity of vestibular responses may not be exclusively within central nervous system vestibular structures, as previously believed.

Correia, M. J.↗
NASA NTRS1991

Response properties of gerbil otolith afferents to small angle pitch and roll tilts

The responses from isolated single otolith afferent fibers were obtained to small angle sinusoidal pitch and roll tilts in anesthetized gerbils. The stimulus directions that produced the maximum (response vector) and minimum response sensitivities were determined for each otolith afferent, with response vectors for the units being spread throughout the horizontal plane, similar to those reported for other species. A breadth of tuning measure was derived, with narrowly tuned neurons responding maximally to stimulation in one direction and minimally along an orthogonal ('null') direction. Most (approximately 80%) otolith afferents are narrowly tuned, however, some fibers were broadly tuned responding significantly to stimulations in any direction in the horizontal plane. The number of broadly tuned otolith afferents (approximately 20%) differs significantly from the more substantial number of broadly tuned vestibular nuclei neurons (88%) recently reported in rats.

NASA Discipline Neuroscience↗