Effects of fixation and optokinetic stimulation on vestibulo-ocular reflex suppression.

Suppression of the vestibulo-ocular reflex was assessed in normal subjects and patients with neurological disorders to determine the relative effects on suppression of a single fixation target and an optokinetic field. Subjects were rotated sinusoidally in yaw at varying frequencies of up to 0.5 Hz whilst seated in a Barany chair. A comparison was made between eye movements in darkness, those produced during fixation on a central target mounted to the chair, and eye movements during fixation on the target plus an "earth-fixed" or "chair-fixed" visual background. Presentation of a background produced only minimal effects on the suppression of the vestibulo-ocular reflex in normal subjects. In patients with impairment of fixation suppression, suppression of the vestibulo-ocular reflex was not improved after presentation of either form of optokinetic field. The results demonstrate that central fixation is the predominant requirement for suppression of the vestibulo-ocular reflex. This correlates closely with the ability to pursue. Although the optokinetic reflex generates following eye movements similar to pursuit, it cannot be used to mediate suppression of the vestibulo-ocular reflex in the absence of an intact pursuit system. The findings strengthen the view that the optokinetic reflex evolved to act in synergy with the vestibulo-ocular reflex in generating compensatory eye movements.     

Light-dependent suppression of the vestibulo-ocular reflex during development

In the fish Oreochromis mossambicus, light conditions affect the development of the roll-induced vestibuloocular reflex (rVOR). During development under continous light-dark conditions the rVOR amplitude, which is the maximum eye movement during a complete 360 degrees lateral roll, shows a secondary drop after a first peak at stage 17 by 64% (36.3 degrees at stage 17; 13.0 degrees at stage 20). This drop was shifted by 2 stages to older postembryonal stages and was 33% (29.2 degrees at stage 20; 19.5 degrees at stage 22) less pronounced in animals which were exposed to complete darkness for several days. Because the period of rVOR diminution is sensitive to light conditions, it is likely that outgrowing visual projection fibres reorganize the neuronal network underlying visual-vestibular behavior thus transiently suppressing the rVOR.     

Selective suppression of the vestibulo-ocular reflex during human locomotion

Introduction

In lower vertebrates, gaze stabilization during locomotion is at least partially driven by a direct coupling of spinal locomotor commands with extraocular motor signals. To what extent locomotor feed-forward mechanisms contribute to gaze stabilization during human locomotion is yet unknown. In principle, the feasibility of a feed-forward regulation of gaze during locomotion should critically depend on the spatiotemporal coupling between body and head kinematics and hence the internal predictability of head movements (HMP). The present study thus investigated whether changes in eye–head coordination during human locomotion can be explained by concurrent changes in HMP.

Methods

Eye and head movements were recorded at different locomotor speeds in light and darkness to obtain the gain and phase of the horizontal and vertical angular VOR (aVOR). Potential correlations between aVOR performance and HMP were analyzed in dependence of locomotor speed and gait cycle phase.

Results

Horizontal aVOR responses persisted independent of locomotor speed. In contrast, with increasing locomotor speed vertical eye–head coordination switched from a VOR-driven compensatory mode to a synergistic behavior where head and eyes move in phase. Concurrently, vertical HMP increased with faster locomotion. Furthermore, modulations in vertical aVOR gain across the gait cycle corresponded to simultaneous alterations in vertical HMP.

Conclusion

The vertical aVOR appears to be suppressed during faster walking and running, whereas at the same time, the predictability of resultant head movements increases. This suggests that during stereotyped human locomotion, internal feed-forward commands supplement or even suppress sensory feedback to mediate gaze stabilization in the vertical plane.

     

The spoon test: a valid and reliable bedside test to assess sudomotor function

Objective

To establish the validity of bedside sudomotor tests in a controlled prospective study.

Methods

Five different tests were used to assess presence of sweating at seven bilaterally symmetrical body loci, namely, examination with the unaided eye, visualization with light reflection, magnification with an ophthalmoscope, palpation of skin for slickness, and gliding the back of a spoon over the skin to detect friction. Sensitivity and specificity of these bedside tests were compared with the thermoregulatory sweat test in 130 subjects with generalized body sweating and 16 patients with complete anhidrosis.

Results

The spoon test was more sensitive in detecting sweating than the other four bedside tests, demonstrating sensitivity of about 86% at the neck, 58% at the chest, and 51% at the forehead. Specificity of the spoon test was almost 100% at all body sites except at the chest (81%) and the neck (50%).

Interpretation

With caveats related to methodology, the spoon test is a clinically practical and useful bedside screening test for the assessment of sweating, especially at the forehead and chest.

     

A simple bedside test to assess the swallowing dysfunction in Parkinson's disease

Background:

Swallowing changes are common in Parkinson''s disease (PD). Early identification is essential to avoid complications of aspiration.

Objectives:

To evaluate the swallowing ability of the PD patients and to correlate it with the indicators of disease progression.

Materials and Methods:

A total of 100 PD patients (70 males and 30 females) aged between 50 years and 70 years with varying stage, duration, and severity were enrolled in a cross-sectional study carried out between January and May 2012. A simple bedside water swallowing test was performed using standard 150 ml of water. Swallowing process was assessed under three categories-swallowing speeds (ml/s), swallowing volume (ml/swallow) and swallowing duration (s/swallow). Equal number of age and sex matched controls were also evaluated.

Results:

All of them completed the task of swallowing. A mean swallowing speed (27.48 ml/s), swallowing volume (28.5 ml/s), and swallowing duration (1.05 s/swallow) was established by the control group. The PD patients showed decreased swallowing speed (7.15 ml/s in males and 6.61 ml/s in females), decreased swallowing volume (14.59 ml/swallow and 14 ml/swallow in females), and increased swallowing duration (2.37 s/swallow and 2.42 s/swallow) which are statistically significant. There was a significant positive correlation between the severity, duration, and staging of the disease with the swallowing performance and a poor correlation between the subjective reports of dysphagia and the objective performance on water swallow test.

Conclusion:

The water swallowing test is a simple bedside test to identify the swallowing changes early in PD. It is recommended to do the test in all PD Patients to detect dysphagia early and to intervene appropriately.     

Diagnostic accuracy of acute vestibular syndrome at the bedside in a stroke unit

Acute vestibular syndrome may be due to vestibular neuritis (VN) or posterior circulation strokes. Bedside ocular motor testing performed by experts is superior to early MRI in excluding strokes. We sought to demonstrate that differentiation of strokes from VN in our stroke unit is reliable. During a prospective study at a tertiary hospital over 1 year, patients with AVS were evaluated in the emergency department (ED) and underwent admission with targeted examination: gait, gaze-holding, horizontal head impulse test (hHIT), testing for skew deviation (SD) and vertical smooth pursuit (vSP). Neuroimaging included CT, transcranial Doppler (TCD) and MRI with MR angiogram (MRA). VN was diagnosed with normal diffusion-weighted images (DWI) and absence of neurological deficits on follow-up. Acute strokes were confirmed with DWI. A total of 24 patients with AVS were enrolled and divided in two groups. In the pure vestibular group (n = 20), all VN (n = 10/10) had positive hHIT and unidirectional nystagmus, but 1 patient had SD and abnormal vertical smooth pursuit (SP). In all the strokes (n = 10/10), one of the following signs suggestive of central lesion was present: negative hHIT, central-type nystagmus, SD or abnormal vSP. Finding one of these was 100% sensitive and 90% specific for stroke. In the cochleovestibular group (n = 4) all had normal DWI, but 3 patients had central ocular motor signs (abnormal vertical SP and SD). Whilst the study is small, classification of AVS in our stroke unit is reliable. The sensitivity and specificity of bedside ocular motor testing are comparable to those previously reported by expert neuro-otologists. Acute cochleovestibular loss and normal DWI may signify a labyrinthine infarct but differentiating between different causes of inner ear dysfunction is not possible with bedside testing.     

Vestibular-nerve inputs to the vestibulo-ocular reflex: a functional-ablation study in the squirrel monkey

Advantage was taken of differences in the electrical excitability of vestibular-nerve fibers to characterize the afferent input to the canal-related vestibulo-ocular reflex. Large anodal (inhibitory) currents, when delivered to both ears, result in a selective, reversible ablation of irregular afferents. Their background discharge and responses to head rotations are temporarily abolished. The same currents have less effect on the background discharge and no effect on the rotational sensitivity of regular afferents. Eye movements were evoked by head rotations in alert monkeys. The ablating currents did not alter the ocular responses to sinusoidal head rotations in yaw or pitch planes. Responses to rapid changes in head velocity were similarly unaffected. It is concluded that irregular afferents do not make a net contribution to the reflex. Slow-phase eye movements evoked by unilateral galvanic currents are consistent with this conclusion. The results are incorporated into a systems model of the reflex. There are three conclusions from the model: (1) the signal to motoneurons consists of the sum of three components related, respectively, to head velocity, eye position, and filtered eye velocity; (2) regular afferents provide the best match to the dynamic requirements of the reflex; and (3) the central pathways responsible for all three signal components receive regular inputs.     

Diagnostic utility of the dexamethasone suppression test

This article discusses the current controversy surrounding the diagnostic utility of the Dexamethasone Suppression Test, addresses the questions raised by the recent editorial by Ross in this journal, discusses the general principles behind the development of tests, and estimates their diagnostic utility. This discussion aims to clarify some aspects of the controversy. It presents an operational analysis of the Dexamethasone Suppression Test as utilized at a state hospital. This operational analysis shows that the test may be useful in distinguishing schizophrenia from psychotic depression, and mania from schizophrenia. Furthermore, it shows that the test is not useful as a screening test. These results are compared with those obtained by other investigators. The authors further show how test results can be used rationally by clinicians by so-called threshold analysis. Clinical data from a state hospital are used to illustrate this.     

Looking with a paralysed eye: adaptive plasticity of the vestibulo-ocular reflex.

When a patient with a peripheral monocular paresis is forced to look with the paretic eye, head movements induce the sensation of an unstable visual world. The patient behaves as if he had acute bilateral labyrinthine lesions. These symptoms are due to the lack of compensatory ocular movement and the patients complain that the visual objects move in the direction opposite to the head. The patients develop ataxia, nausea, vomiting and past pointing. The symptoms, however, are transient and consistently disappear after approximately 48 hours. The central adaptation to looking and seeing with the paralysed eye is associated with a plastic change of the VOR. This plastic adaptation is probably induced by the large retinal slip produced by the lack of compensatory movement of the eye and can be studied in the normally mobile eye in the dark. The psychophysical adaptation is probably generated by an efferent copy or corollary discharge of the vestibular system to the visual system that cancels the retinal error.     

Diagnostic accuracy of the clapping test in Parkinsonian disorders

Background To determine the diagnostic value of the clapping test, which has been proposed as a reliable measure to differentiate between progressive supranuclear palsy (where performance is impaired) and Parkinson’s disease (where performance should be normal). Methods Our study group included a large cohort of consecutive outpatients including 44 patients with Parkinson's disease, 48 patients with various forms of atypical parkinsonism and 149 control subjects. All subjects performed the clapping test according to a standardized protocol. Results Clapping test performance was normal in all control subjects, and impaired in 63% of the patients with atypical parkinsonism. Unexpectedly, we also found an impaired clapping test in 29% of the patients with Parkinson's disease. Conclusion Although the proportion with an abnormal clapping test was significantly higher in atypical parkinsonism, the clapping test did not discriminate well between Parkinson's disease and atypical parkinsonism.     

Diagnostic performance of the thirty-four hour dexamethasone suppression test

The performance of the dexamethasone suppression test (DST) in the diagnostic confirmation of endogenous depression was compared according to two times of blood collection—1600 hr on day 2 (usual sample) and 0800 hr on day 3 (34 hr after dexamethasone intake)—in 14 endogenous depressives and in a control group of 17 psychiatric inpatients with other diagnoses. For the day 2 (1600 hr) sample, a 5 μg/dl cortisol concentration represented the best cut-off score, with sensitivity of 57% specificity of 88%, and diagnostic confidence of 80%. For the day 3 (0800 hr) sample, the best cut-off score was 20 μg/dl, with the same sensitivity (57%) but there was a decrease of both specificity (to 76%) and diagnostic confidence (to 67%). The mean cortisol levels were much higher on day 3 than on day 2, suggesting that the inhibitory activity of dexamethasone was no longer present.     

Adaptability of the vestibulo-ocular reflex to vision reversal in strobe reared cats

Optical reversal of vision brings about adaptive changes in the vestibulo-ocular reflex (VOR) tending to reduce retinal image slip during head movement. The present experiments investigated this form of adaptation in cats whose complement of direction sensitive central visual cells had been substantially reduced by rearing in 8 Hz stroboscopic light. Horizontal vision reversal was produced by dove prisms carried in a skull-mounted mask. A scleral eye coil was used to measure horizontal eye movements. VOR gain and phase were measured in the dark during sinusoidal rotation using test stimuli of1/8 Hz and 5- of 20°/sec velocity amplitude. Initially, strobe reared cats produced virtually normal VOR in the dark, except for slight but significant exaggeration of the normal phase advancement to be expected at1/8 Hz. Addition of their familiar strobe illumination produced almost perfect oculomotor compensation. Maintained vision reversal in both strobe and normal illumination produced similar patterns of adaptive change in normal and strobe reared subjects, i.e. all animals exhibited an initial fast, and subsequent much slower, stage of gain attenuation, with similar changes in phase. Thus, strobe rearing did not prevent the development of an essentially normal VOR, nor did it interfere significantly with the ability to adapt in response to vision reversal. Since strobe rearing depletes direction selective visual movement detectors in the cortex and superior colliculi, it is inferred that signals responsible for activating the adaptive process are probably carried mainly in the accessory optic, rather than cortical and collicular, visual system.     

Oscillopsia: visual function during motion in the absence of vestibulo-ocular reflex

OBJECTIVES—To investigate (1) the effects of lossof vestibular function on spatiotemporal vision and (2) the mechanismswhich enable labyrinthine defective (LD) patients to adapt to oscillopsia.
METHODS—Visual function and eye movements wereassessed in seven normal subjects and four LD patients with oscillopsiadue to absent vestibulo-ocular reflex. Temporal vision was assessed bymeasurement of threshold sensitivity for detection of a target whichmoved across a flickering, spatially uniform background field. Spatial vision was investigated by measurements of threshold sensitivity forthe detection of a target moving across a spatially modulated background in the form of square wave gratings. Velocity discrimination was assessed with drifting gratings. All measurements were made understatic conditions and during oscillatory movement of either the visualstimulus or the subject (1 Hz, peak velocity 50°/s).
RESULTS
TEMPORAL RESPONSES—Normal subjects and LDpatients exhibited similar responses while static and under body oscillation.
SPATIAL RESPONSES—The two groups achieved similarresults under static conditions but body oscillation reduced thresholdsensitivities and shifted the spatial response function towards lowerspatial frequencies in the LD patients only. Similar changes in thespatial responses were seen during oscillation of the visual stimulusbut these occurred in both normal subjects and LD patients.
VELOCITY DISCRIMINATION—Two LD patients achievednormal velocity discrimination but the other two showed abnormalresponses to visual stimulus movement; one displayed a loss of velocitydiscrimination during whole body oscillation, and the other mismatchedthe velocity of two moving grating stimuli.
CONCLUSIONS—The changes in the spatialresponses are attributed to the presence of retinal slip during visualstimulus motion in all subjects or body oscillation in the LD patients.It is concluded that any visual adaptation to oscillopsia achieved bythe LD patients does not influence the measured spatial responsefunctions, which arise at an early stage of visual processing. Theabnormal velocity discrimination may relate to the progressiveimprovement in oscillopsia reported by LD patients.

     

Behavior and physiology of the macaque vestibulo-ocular reflex response to sudden off-axis rotation: Computing eye translation

The vestibulo-ocular reflex (VOR) has historically been considered a computationally simple reflex: to stabilize images on the retina against imposed head rotation, the eyes must be counterrotated by an equal amount in the opposite direction. During almost any head rotation, however, the eyes are also translated. We show that the VOR compensates for 90% of this translation, and suggest a computational scheme by which this is done, based on a temporal dissection of the VOR response to sudden head rotation. An initial response that corrects only for imposed rotation is refined by a series of three temporally delayed corrections of increasing complexity. The first correction takes only head rotation and viewing distance into account; the second, head rotation, viewing distance, and otolith translation; and the third, head rotation, viewing distance, otolith translation, and translation of the eyes relative to the otoliths. Responses of type I gaze velocity Purkije (GVP) cells in the cerebellar flocculus and ventral paraflocculus of rhesus monkeys were recorded during sudden head rotation. We show that cell discharge was modulated both by axis location and by viewing distance, suggesting that GVP cells play a role in the VOR response to rotation-induced eye translation.     

Visuo-spatial neglect: a new copying test to assess perceptual parsing

A new copying test has been devised to explore different forms of perceptuo-motor impairment in visuo-spatial neglect. The stimuli are constructed so that one hypercomplex figure can be transformed into two complex figures by deletion of part of the original drawing. Task performance by five patients with left neglect after right hemisphere damage is reported. Their copies illustrate the varied spatial reference frames within which visual neglect can be manifest.     

Intersession reliability of a protocol to assess reflex activation history in the vastus medialis

The purpose of this study was to determine the reliability of a protocol to assess reflex activation history in the vastus medialis. Eight subjects reported to the laboratory on two occasions. Reflex activation history was assessed by delivering two stimuli of the same intensity 80 ms apart. The dependent variable evaluated was the percentage of the unconditioned reflex amplitude. Eight trials were elicited and averaged on each day. An intraclass correlation coefficient (ICC 2,1) was used to estimate intersession reliability. The ICC for the protocol was found to be 0.9647. The results of this investigation indicate that this technique can reliably estimate reflex activation history in the vastus medialis.     

The variable gain element of the vestibulo-ocular reflex is common to the optokinetic system of the cat

We have used two neuroanatomical tracing techniques to study the trigeminocollicular projection in the cat. In one series of experiments we injected [3H]proline into the alaminar division of the spinal trigeminal nucleus and analyzed the distribution and pattern of anterogradely transported label within the superior colliculus. These autoradiographic data reveal that the trigeminocollicular projection: (1) is primarily contralateral; (2) reaches only the rostral 60-70% of the colliculus; and (3) terminates in a discontinuous, patch-like tier within the middle of the dorsal-ventral axis of the stratum griseum intermediale (SGI). The patches of label measure approximately 330 micrometer in the medial-lateral dimension and 250 micrometer in the dorsal-ventral extent. There seems to be an alignment of the patches in the rostral-caudal direction, suggesting that the trigeminal input forms longitudinal columns in the colliculus. Anterogradely transported protein is also present within the stratum griseum profundum (SGP). In contrast to the patch-like pattern present in the SGI, label in the SGP is more diffusely distributed. In a second series of experiments we injected horseradish peroxidase-wheat germ agglutinin (HRP-WGA) into the spinal trigeminal nucleus. While the distribution and pattern of the contralateral trigeminocollicular axons is similar to that in the autoradiographic experiments, patches of anterogradely transported HRP-WGA are also present within the ipsilateral SGI and SGP. Furthermore, the HRP-WGA data reveal groups of retrogradely labeled collicular neurons which lie in intimate association with the patches of anterogradely transported tracer. Our findings are discussed in relation to other collicular afferents which terminate in a patch-like manner within the SGI. We hypothesize that the colliculus contains many vertically oriented modules. Each module consists of tectopetal axon terminals -- arranged in sandwich fashion -- and functionally related collicular (tectofugal) neurons.