Baclofen, motion sickness susceptibility and the neural basis for velocity storage

Reduction of the dominant time constant (T(VOR)) of the angular vestibulo-ocular reflex (aVOR) by habituation is associated with a decrease in motion sickness susceptibility. Baclofen, a GABA(b) agonist, reduces the time constant of the velocity storage integrator in the aVOR in a dose-dependent manner. The high frequency aVOR gain is unaltered by baclofen. Here we demonstrate that the reduction in T(VOR) produced by oral administration of 20 mg of baclofen causes a significant reduction in motion sickness susceptibility, tested with roll while rotating (RWR). These data show that motion sickness susceptibility can be pharmacologically manipulated with a GABA(b) agonist and support our conclusion that motion sickness is generated through velocity storage. We also show how baclofen acts on velocity storage at the neural level. A vestibular-plus-saccade (VPS) neuron was recorded in the rostral medial vestibular nucleus (rMVN) of a cynomolgus monkey, an area where we postulate that velocity storage is generated. The cell had a time constant during steps of velocity that was close to that of the T(VOR). After parenteral administration of baclofen, there was a similar decrease in the time constants of the VPS neuron and the T(VOR). This is the first demonstration of the concurrence of unit and aVOR time constants before and after baclofen. The data support the hypothesis that the velocity storage integrator is generated through activity of vestibular-only (VO) and VPS neurons in rMVN and suggest that GABA(b) synapses on VO and VPS neurons are likely to be involved in the baclofen-induced reduction in motion sickness susceptibility.

The systematic collection and characterisation of herbaceous forage species for recharge and discharge environments in southern Australia

Australian temperate pasture Genetic Resource Centres (GRCs) co-ordinated a major program to introduce and screen herbaceous forage species. This program aims to expand the environments where such species could reduce recharge and manage discharge for the control of dryland salinity in southern Australia. The sustainability of agriculture, in Australia especially, depends on continued access to new germplasm to enable plant breeders to continue crop and forage improvement. The GRCs supported the selection and identification of promising new legume, grass and herb species as part of a national pasture evaluation program. In total, 671 species and 21 non-species-specific genera were identified as having potential to increase water use profitability of recharge lands and to improve the productivity of saline lands across a diverse range of agricultural environments in southern Australia. Through a series of activities, 201 of these species, representing legumes, herbs and grasses were identified as promising. These were then disseminated for evaluation in a range of environments across southern Australia. The progress of selected species was monitored and germplasm of the most promising 11 species and three leguminous genera was targeted for intensive acquisition and characterisation as the basis for selection and breeding. In addition to the identification and dissemination of promising species of immediate potential, a comprehensive collection of 544 native and exotic, wild and cultivated pasture species was conserved and is now available to service future plant improvement programs.

Rotation Axes of the Head During Positioning, Head Shaking, and Locomotion

Static head orientations obey Donders’ law and are postulated to be rotations constrained by a Fick gimbal. Head oscillations can be voluntary or generated during natural locomotion. Whether the rotation axes of the voluntary oscillations or during locomotion are constrained by the same gimbal is unknown and is the subject of this study. Head orientation was monitored with an Optotrak (Northern Digital). Human subjects viewed visual targets wearing pin-hole goggles to achieve static head positions with the eyes centered in the orbit. Incremental rotation axes were determined for pitch and yaw by computing the velocity vectors during head oscillation and during locomotion at 1.5 m/s on a treadmill. Static head orientation could be described by a generalization of the Fick gimbal by having the axis of the second rotation rotate by a fraction, k, of the angle of the first rotation without a third rotation. We have designated this as a k-gimbal system. Incremental rotation axes for both pitch and yaw oscillations were functions of the pitch but not the yaw head positions. The pivot point for head oscillations was close to the midpoint of the interaural line. During locomotion, however, the pivot point was considerably lower. These findings are well explained by an implementation of the k-gimbal model, which has a rotation axis superimposed on a Fick-gimbal system. This could be realized physiologically by the head interface with the dens and occipital condyles during head oscillation with a contribution of the lower spine to pitch during locomotion.

Vertical (Z-axis) acceleration alters the ocular response to linear acceleration in the rabbit

Whether ocular orientation to gravity is produced solely by linear acceleration in the horizontal plane of the head or depends on both horizontal and vertical components of the acceleration of gravity is controversial. Here, we compared orienting eye movements of rabbits during head tilt to those produced by centrifugation that generated centripetal acceleration along the naso-occipital (X-), bitemporal (Y-) and vertical (Z-) axes in a constant gravitational field. Sensitivities of ocular counter-pitch and vergence during pitch tilts were approximately 25 degrees /g and approximately 26 degrees /g, respectively, and of ocular counter-roll during roll tilts was approximately 20 degrees /g. During X-axis centripetal acceleration with 1 g of gravity along the Z-axis, pitch and vergence sensitivities were reduced to approximately 13 degrees /g and approximately 16 degrees /g. Similarly, Y-axis acceleration with 1g along the Z-axis reduced the roll sensitivity to approximately 16 degrees /g. Modulation of Z-axis centripetal acceleration caused sensitivities to drop by approximately 6 degrees /g in pitch, approximately 2 degrees /g in vergence, and approximately 5 degrees /g in roll. Thus, the constant 1g acceleration along the Z-axis reduced the sensitivity of ocular orientation to linear accelerations in the horizontal plane. Orienting responses were also modulated by varying the head Z-axis acceleration; the sensitivity of response to Z-axis acceleration was linearly related to the response to static tilt. Although the sign of the Z-axis modulation is opposite in the lateral-eyed rabbit from that in frontal-eyed species, these data provide evidence that the brain uses both the horizontal and the vertical components of acceleration from the otolith organs to determine the magnitude of ocular orientation in response to linear acceleration.

Longitudinally preserved psychomotor performance in long-term asymptomatic HIV-infected individuals

BACKGROUND

Recent case reports have suggested that some asymptomatic HIV-infected individuals can develop CNS disturbances despite intact immunologic functioning and long-term suppression of plasma HIV concentrations to undetectable levels. This possibility has not yet been systematically studied longitudinally.

METHODS

Using longitudinal data from the Multicenter AIDS Cohort Study, we investigated neuropsychological performance in long-term asymptomatic HIV-infected men who have sex with men. Performance over a 5-year period on the Symbol Digit Modalities test and the Trail Making Tests were compared in three HIV-positive asymptomatic groups [defined as 1) highly active antiretroviral therapy (HAART) treated with undetectable viral loads (n = 83), 2) AIDS-free for more than 15 years without HAART (n = 29), and 3) absence of clinical AIDS or CD4(+) lymphocyte count below 200 cells/muL at the beginning and end of the study period (n = 233)] and in HIV-negative controls (n = 237). Data were analyzed using linear mixed models and proportional odds logistic regression modeling with generalized estimating equations.

RESULTS

There was no evidence of performance differences or performance declines over the 5-year period of study in any of the three long-term asymptomatic groups as compared with the HIV-negative group in the Symbol Digit Modalities test or the Trail Making Tests. Performance decrements were, however, observed with increasing age in each of the tests administered, demonstrating that performance declines could be detected by these methods.

CONCLUSIONS

Regardless of how long-term asymptomatic status was defined immunologically or virologically, neuropsychological test performances remained stable. These findings suggest that psychomotor speed is preserved over many years in HIV-infected individuals with controlled HIV viremia.

[Skull vibration induced nystagmus test]

OBJECTIVES

To establish during a consensus meeting the fundamental basis, the validity criteria, the main indications and results of the skull vibration induced nystagmus test (SVINT) which explores the vestibule high frequencies.

MATERIAL AND METHODS

The SVINT is applied on the mastoid process (right and left sides) at 100 Hz during 10 seconds on a sitting upright subject. Total unilateral peripheral lesions (tUVL: operated vestibular shwannomas, vestibular neurectomies) and partial unilateral peripheral lesions (pUVL: preoperative neuromas, Meniere's disease, vestibular neuritis, chemical labyrinthectomies) were studied. Thirty-six patients had brainstem lesions and 173 normal subjects were used as controls.

RESULTS

The SVINT is considered positive when the application of the vibrator produces a reproducible sustained nystagmus always beating in the same direction following several trials in various stimulation topographies (on the right and left mastoid). The skull vibratory nystagmus (SVN) begins and ends with the stimulation; the direction of the nystagmus has no secondary reversal. The slow phase velocity (SPV) is>2 degrees /second. In tUVL the SVINT always reveals a lesional nystagmus beating toward the safe side at all frequencies. The mean SVN SPV is 10.8 degrees /s+/-7.5 SD (N=45). The mastoid site was more efficient than the cervical or vertex sites. Mastoïd stimulation efficiency is not correlated with the side of stimulation. The SVN SPV is correlated with the total caloric efficiency on the healthy ear. In pUVL the SVINT is positive in 71 to 76% of cases; the mean SVN. SPV (6.7 degrees /s+/-4.7 SD)(N=30) is significantly lower than in tUVL (P=0.0004). SVINT is positive in 6 to 10% of the normal population, 31% of brain stem lesions and negative in total bilateral vestibular peripheral lesions.

CONCLUSIONS

SVINT is an effective, rapid and non invasive test used to detect vestibular asymmetry between 20 to 150 Hz stimulation. This test used in important cohorts of patients during the ten last years has demonstrated no observable adverse effect. SVINT complements other tests which evaluate lower frequencies (caloric test: 0,003 Hz) and the medium frequencies (Head-Shaking-Test (HST): 2 Hz; the head impulse test (HIT): 6 Hz). SVINT is useful in the diagnosis of labyrinthine hydrops or detection of acoustic neuromas. It is useful when the caloric test can not be practised because of middle ear problems. SVINT has its limits: in pUVL, the nystagmus direction is not always specific of the pathologic side and can change with the stimulus frequency. This test does not precisely point out the level of the lesion on the vestibular pathway.

Labyrinthine lesions and motion sickness susceptibility

The angular vestibulo-ocular reflex (aVOR) has a fast pathway, which mediates compensatory eye movements, and a slow (velocity storage) pathway, which determines its low frequency characteristics and orients eye velocity toward gravity. We have proposed that motion sickness is generated through velocity storage, when its orientation vector, which lies close to the gravitational vertical, is misaligned with eye velocity during head motion. The duration of the misalignment, determined by the dominant time constant of velocity storage, causes the buildup of motion sickness. To test this hypothesis, we studied bilateral labyrinthine-defective subjects with short vestibular time constants but normal aVOR gains for their motion sickness susceptibility. Time constants and gains were taken from rotational responses. Motion sickness was generated by rolling the head while rotating, and susceptibility was assessed by the number of head movements made before reaching intolerable levels of nausea. More head movements signified lower motion sickness susceptibility. Labyrinthine-defective subjects made more head movements on their first exposure to roll while rotating than normals (39.8 +/- 7.2 vs 13.7 +/- 5.5; P < 0.0001). Normals were tested eight times, which habituated their time constants and reduced their motion sickness susceptibility. Combining data from all subjects, there was a strong inverse relationship between time constants and number of head movements (r = 0.94), but none between motion sickness susceptibility and aVOR gains. This provides further evidence that motion sickness is generated through velocity storage, not the direct pathway, and suggests that motion sickness susceptibility can be reduced by reducing the aVOR time constant.

Isolation of Trichophyton violaceum and Trichophyton soudanense in Baltimore, Maryland

Tinea capitis is of public health importance because of its transmissibility. Trichophyton violaceum and Trichophyton soudanense, which are common causes of tinea capitis in parts of Africa and West Asia, have only rarely been reported to cause dermatophytoses in the United States. We identified 24 patients with 25 positive cultures for T. violaceum or T. soudanense that were processed in a single hospital laboratory in Baltimore, Maryland, between 1 January 2000 and 30 June 2006. Most patients for whom clinical information was available had tinea capitis. There was a marked increase in the isolation of these organisms between the period from 2000 to 2002 and the period from 2003 to 2006, possibly associated with changes in immigration to the Baltimore metropolitan area. The changing epidemiology of this transmissible fungal infection not only is of public health interest as an example of the introduction of a "new" pathogen to an area where it traditionally was not endemic but also is of clinical and microbiological importance given reports suggesting an increasing incidence of tinea capitis in some areas and increasing clinical failure rates of current therapies.

Modeling Gravity-Dependent Plasticity of the Angular Vestibuloocular Reflex With a Physiologically Based Neural Network

A neural network model was developed to explain the gravity-dependent properties of gain adaptation of the angular vestibuloocular reflex (aVOR). Gain changes are maximal at the head orientation where the gain is adapted and decrease as the head is tilted away from that position and can be described by the sum of gravity-independent and gravity-dependent components. The adaptation process was modeled by modifying the weights and bias values of a three-dimensional physiologically based neural network of canal–otolith-convergent neurons that drive the aVOR. Model parameters were trained using experimental vertical aVOR gain values. The learning rule aimed to reduce the error between eye velocities obtained from experimental gain values and model output in the position of adaptation. Although the model was trained only at specific head positions, the model predicted the experimental data at all head positions in three dimensions. Altering the relative learning rates of the weights and bias improved the model-data fits. Model predictions in three dimensions compared favorably with those of a double-sinusoid function, which is a fit that minimized the mean square error at every head position and served as the standard by which we compared the model predictions. The model supports the hypothesis that gravity-dependent adaptation of the aVOR is realized in three dimensions by a direct otolith input to canal–otolith neurons, whose canal sensitivities are adapted by the visual-vestibular mismatch. The adaptation is tuned by how the weights from otolith input to the canal–otolith-convergent neurons are adapted for a given head orientation.

Dynamics of quadrupedal locomotion of monkeys: implications for central control

We characterized the three-dimensional kinematics and dynamics of quadrupedal gait of young adult rhesus and cynomolgus monkeys while they walked with diagonal and lateral gaits at 0.4-1.0 m/s on a treadmill. Rigid bodies on the wrist, ankle, and back were monitored by an optical motion detection system (Optotrak). Kinematic data could be normalized using characteristic stride length, reducing variance due to different gait styles, to emphasize common characteristics of swing and stance parameters among animals. Mean swing phase durations fell as walking speed increased, but the swing phase durations increased at each walking velocity as a linear function of increases in amplitude, thereby following a main sequence relationship. The phase plane trajectories of the swing phases, i.e., plots of the relation of the rising and falling limb velocity to limb position in the sagittal (X-Z) plane, had unique dynamic characteristics. Trajectories were separable at each walking velocity and increases in swing amplitude were linearly related to peak swing velocities, thus comprising main sequences. We infer that the swing phase dynamics are set by central neural mechanisms at the onset of the swing phases according to the intended amplitude, which in turn is based on the walking velocity in the stance phases. From the many dynamic similarities between swing phases and rapid eye movements, we further suggest that the swing phases may be generated by neural mechanisms similar to those that produce saccades and quick phases of nystagmus from a signal related to sensed or desired walking velocity.

The role of gravity in adaptation of the vertical angular vestibulo-ocular reflex

The gain of the vertical angular vestibulo-ocular reflex (aVOR) was adapted in side-down and prone positions in two monkeys and tested in four planes: left-/right-side down; forward/backward; and two intermediate planes that lie approximately in the planes of the vertical semicircular canal pairs, left anterior/right posterior (LA/RP) and right anterior/left posterior (RA/LP). Gain changes, expressed as a percent of preadapted values, were plotted as a function of head orientation in the planes of tilt, and fitted with sinusoids to obtain the gravity-dependent (amplitude) and gravity-independent (bias) components of adaptation. Gravity-dependent gain changes were always maximal when tested in a plane that included the head orientation in which the aVOR gain had been adapted. Changes were minimal when the head was tilted in a plane orthogonal to the plane of adaptation, and were smaller but still significant when tested in the two intermediate planes. Gravity-independent VOR gain changes were uniform over all planes of head tilt. Thus, the gravity-dependent and gravity-independent components could be separated experimentally. The aVOR gain changes from the head tilts in different directions were utilized to reconstruct the gain changes in three dimensions. They formed a continuous surface, which peaked in and around the position of adaptation. These studies support the postulate that gain adaptation has both gravity-independent and gravity-dependent components, and further show that these gain changes have a three-dimensional structure. These results are similar to those in humans, indicating that the gravity-dependent adaptation of the aVOR is likely to be a common phenomenon across species.

Three-dimensional kinematics and dynamics of the foot during walking: a model of central control mechanisms

The foot is a critical interface between the body and supporting surface during walking, but there is no coherent framework on which to model the dynamics of the stance and swing phases. To establish this framework, we studied the rotational and translational dynamics of foot movement in three dimensions with a motion detection system (OPTOTRAK), while subjects walked on a treadmill. Positions, velocities, and durations were normalized to leg-length and gravity. Foot position and rotation at toe-off were closely related to walking velocity. Foot pitch at toe clearance increased with walking velocity, but the medial-lateral and vertical toe positions were unaltered. Phase-plane trajectories along the fore-aft direction, i.e., plots of toe velocity versus position, were circular during the swing phases, with radii proportional to walking velocity. Peak forward, lateral, and upward velocities were linearly related to corresponding excursions, forming main sequences. A second order model predicted the changes in toe position and velocity, and the approximately hyperbolic decrements in duration as a function of walking velocity. The model indicates that the foot is controlled in an overdamped manner during the stance phase and as a feedback-controlled undamped pendulum during the swing. The data and model suggest that the state of the foot at toe-off, set by walking velocity during the stance phase, determines the dynamics of the swing phase. Thus, in addition to determining locomotion kinematics, walking velocity plays a critical role in determining the phase-plane trajectories and main sequence relationships of foot movements during the swing phases.

HIV neuropathy natural history cohort study: assessment measures and risk factors

BACKGROUND

Distal sensory polyneuropathy (DSP) is the most common neurologic complication of human immunodeficiency virus (HIV) infection. Risk factors for DSP have not been adequately defined in the era of highly active antiretroviral therapy.

METHODS

The authors evaluated 101 subjects with advanced HIV infection over 48 weeks. Assessments included a brief peripheral neuropathy (PN) screen (BPNS), neurologic examination, nerve conduction studies, quantitative sensory testing (QST), and skin biopsies with quantitation of epidermal nerve fiber density. Data were summed into a Total Neuropathy Score (TNS). The presence, severity, and progression of DSP were related to clinical and laboratory results.

RESULTS

The mean TNS (range 0 to 36) was 8.9, with 38% of subjects classified as PN-free, 10% classified as having asymptomatic DSP, and 52% classified as having symptomatic DSP. Progression in TNS from baseline to week 48 occurred only in the PN-free group at baseline (mean TNS change = 1.16 +/- 2.76, p = 0.03). Factors associated with progression in TNS were lower current TNS, distal epidermal denervation, and white race. As compared with the TNS diagnosis of PN at baseline, the BPNS had a sensitivity of 34.9% and a specificity of 89.5%.

CONCLUSIONS

In this cohort of advanced human immunodeficiency virus (HIV)-infected subjects, distal sensory polyneuropathy was common and relatively stable over 48 weeks. Previously established risk factors, including CD4 cell count, plasma HIV RNA, and use of dideoxynucleoside antiretrovirals were not predictive of the progression of distal sensory polyneuropathy (DSP). Distal epidermal denervation was associated with worsening of DSP. As compared with the Total Neuropathy Score, the brief peripheral neuropathy screen had relatively low sensitivity and high specificity for the diagnosis of DSP.

Measles-associated encephalopathy in children with renal transplants

Two children, boys of 8 and 13 years, presented with measles-associated encephalopathy several years after kidney transplantation for congenital nephrotic syndrome. In the absence of prior clinical measles, the neurological symptoms initially eluded diagnosis, but retrospective analysis of stored samples facilitated the diagnosis of measles-associated encephalopathy without recourse to biopsy of deep cerebral lesions. Each had received a single dose of measles mumps and rubella vaccine before 12 months of age. Prior vaccination, reduction of immunosuppression and treatment with intravenous immunoglobulin and ribavirin may have contributed to their survival. Persistent measles virus RNA shedding, present in one child, was not controlled by treatment with i.v. ribavirin. Two years later, both patients continue to have functioning allografts with only minimal immunosuppression. These cases illustrate the difficulty in diagnosing measles-associated encephalopathy in the immunocompromised host, even in the era of molecular diagnostics, and highlight the renewed threat of neurological disease in communities with incomplete herd immunity.