Spatial orientation of caloric nystagmus in semicircular canal-plugged monkeys

We studied caloric nystagmus before and after plugging all six semicircular canals to determine whether velocity storage contributed to the spatial orientation of caloric nystagmus. Monkeys were stimulated unilaterally with cold ( approximately 20 degrees C) water while upright, supine, prone, right-side down, and left-side down. The decline in the slow phase velocity vector was determined over the last 37% of the nystagmus, at a time when the response was largely due to activation of velocity storage. Before plugging, yaw components varied with the convective flow of endolymph in the lateral canals in all head orientations. Plugging blocked endolymph flow, eliminating convection currents. Despite this, caloric nystagmus was readily elicited, but the horizontal component was always toward the stimulated (ipsilateral) side, regardless of head position relative to gravity. When upright, the slow phase velocity vector was close to the yaw and spatial vertical axes. Roll components became stronger in supine and prone positions, and vertical components were enhanced in side down positions. In each case, this brought the velocity vectors toward alignment with the spatial vertical. Consistent with principles governing the orientation of velocity storage, when the yaw component of the velocity vector was positive, the cross-coupled pitch or roll components brought the vector upward in space. Conversely, when yaw eye velocity vector was downward in the head coordinate frame, i.e., negative, pitch and roll were downward in space. The data could not be modeled simply by a reduction in activity in the ipsilateral vestibular nerve, which would direct the velocity vector along the roll direction. Since there is no cross coupling from roll to yaw, velocity storage alone could not rotate the vector to fit the data. We postulated, therefore, that cooling had caused contraction of the endolymph in the plugged canals. This contraction would deflect the cupula toward the plug, simulating ampullofugal flow of endolymph. Inhibition and excitation induced by such cupula deflection fit the data well in the upright position but not in lateral or prone/supine conditions. Data fits in these positions required the addition of a spatially orientated, velocity storage component. We conclude, therefore, that three factors produce cold caloric nystagmus after canal plugging: inhibition of activity in ampullary nerves, contraction of endolymph in the stimulated canals, and orientation of eye velocity to gravity through velocity storage. Although the response to convection currents dominates the normal response to caloric stimulation, velocity storage probably also contributes to the orientation of eye velocity.     

Optimal estimator models for spatial orientation and vestibular nystagmus

Mathematical models have played an important role in research on the vestibular system over the past century, from the torsion pendulum analogies of the semicircular canal to the optimal estimator ??observer?? models of multisensory interaction and adaptation. This short review is limited to our own contributions in bringing the technology of feedback control theory to bear on the understanding of human spatial orientation, eye movements, and nystagmus, both on Earth and in space. It points to the importance of the ??internal model?? concept for treatment of the manner in which the brain constantly makes predictions about future sensory feedback, adjusts the weightings of sensors according to their signal-to-noise ratios, and adapts control according to the motion environment, and availability of sensory cues.     

Three-dimensional eye position and slow phase velocity in humans with downbeat nystagmus

Downbeat nystagmus (DN), a fixation nystagmus with the fast phases directed downward, is usually caused by cerebellar lesions, but the precise etiology is not known. A disorder of the smooth-pursuit system or of central vestibular pathways has been proposed. However, both hypotheses fail to explain why DN is usually accompanied by gaze-holding nystagmus, which implies a leaky neural velocity-to-position integrator. Because three-dimensional (3-D) analysis of nystagmus slow phases provides an excellent means for testing both hypotheses, we examined 19 patients with DN during a fixation task and compared them with healthy subjects. We show that the presentation of DN patients is not uniform; they can be grouped according to their deficits: DN with vertical integrator leakage, DN with vertical and horizontal integrator leakage, and DN without integrator leakage. The 3-D analysis of the slow phases of DN patients revealed that DN is most likely neither caused by damage to central vestibular pathways carrying semicircular canal information nor by a smooth pursuit imbalance. We propose that the observed effects can be explained by partial damage of a brain stem-cerebellar loop that augments the time constant of the neural velocity to position integrators in the brain stem and neurally adjusts the orientation of Listing's plane.     

Three-dimensional binocular kinematics of torsional vestibular nystagmus during convergence on head-fixed targets in humans.

When a human subject is oscillated about the nasooccipital axis and fixes upon targets along the horizontal head-fixed meridian, angular eye velocity includes a vertical component that increases with the horizontal eccentricity of the line-of-sight. This vertical eye movement component is necessary to prevent retinal slip. We asked whether fixation on a near head-fixed target during the same torsional vestibular stimulation would lead to differences of vertical eye movements between the right and the left eye, as the directions of the two lines-of-sight are not parallel during convergence. Healthy human subjects (n = 6) were oscillated (0.3 Hz, +/-30 degrees) about the nasooccipital axis on a three-dimensional motor-driven turntable. Binocular movements were recorded using the dual search coil technique. A head-fixed laser dot was presented 1.4 m (far head-fixed target) or 0.25 m (near head-fixed target) in front of the right eye. We found highly significant (P < 0.01) correlations (R binocular = 0.8, monocular = 0.59) between the convergence angle and the difference of the vertical eye velocity between the two eyes. The slope of the fitted linear regression between the two parameters (s = 0.45) was close to the theoretical slope necessary to prevent vertical retinal slippage (predicted s = 0.5). Covering the left eye did not significantly change the slope (s = 0.52). In addition, there was a marked gain reduction (approximately 35%) of the torsional vestibuloocular reflex (VOR) between viewing the far and the near targets, confirming earlier results by others. There was no difference in torsional gain reduction between the two eyes. Lenses of +3 dpt positioned in front of both eyes to decrease the amount of accommodation did not further change the gain of the torsional VOR. In conclusion, ocular convergence on a near head-fixed target during torsional vestibular stimulation leads to deviations in vertical angular velocity between the two eyes necessary to prevent vertical double vision. The vertical deviation velocity is mainly linked to the amount of convergence, since it also occurs during monocular viewing of the near head-fixed target. This suggests that convergence during vestibular stimulation automatically leads to an alignment of binocular rotation axes with the visual axes independent of retinal slip.     

Three-dimensional spatial tuning of neck muscle activation in humans

The complex structure of the neck musculoskeletal system poses challenges to understanding central nervous system (CNS) control strategies. Examining muscle activation patterns in relation to musculoskeletal geometry and three-dimensional mechanics may reveal organizing principles. We analyzed the spatial tuning of neck muscle electromyographic (EMG) activity while subjects generated moments in three dimensions. EMG tuning curves were characterized by their orientation (mean direction) and focus (spread of activity). For the four muscles that were studied (sternocleidomastoid, splenius capitis, semispinalis capitis and trapezius), EMG tuning curves exhibited directional preference, with consistent orientation and focus among 12 subjects. However, the directional preference (orientation) of three of the four neck muscles did not correspond to the muscle's moment arm, indicating that maximizing a muscle's mechanical advantage is not the only factor in determining muscle activation. The focus of muscle tuning did not change with moment magnitude, demonstrating that co-contraction did not increase with load. Axial rotation was found to have a strong influence on neck muscle spatial tuning. The uniform results among subjects indicate that the CNS has consistent strategies for selecting neck muscle activations to generate moments in specific directions; however, these strategies depend on three-dimensional mechanics in a complex manner. Electronic Publication     

Influence of static head position on the horizontal nystagmus evoked by caloric,rotational and optokinetic stimulation in the squirrel monkey

Summary We studied the influence of static head position on the horizontal nystagmus produced by caloric, rotational and optokinetic stimulation in alert squirrel monkeys. Caloric nystagmus is stronger for nose up (NU) than for nose down (ND) pitches; so, for example, slow-phase eye velocity is four times larger in supine than in prone positions. A similarly directed asymmetry occurs in the horizontal vestibulo-ocular (HVOR) responses to longduration, constant angular-head accelerations, but not to midband (0.1 Hz) sinusoidal head rotations. Consistent with a first-order model of the HVOR, the low-frequency or acceleration gain of the reflex (G_A) is equal to the product of the midband velocity gain (G_V) and a time constant (T_VOR). G_V is proportional to the cosine of the angle between the horizontal-canal plane and the plane of rotation, from which it is concluded that signals from the horizontal, but not from the vertical canals contribute to the HVOR. T_VOR can be as much as twice as large in NU than in ND positions. G_A is proportional to T_VOR and it, too, shows a NU-ND asymmetry. The time constant of optokinetic afternystagmus (T_OKAN) was also studied. Since T_VOR and T_OKAN are modified in similar ways by static tilts, it is concluded that head position affects the time constants by way of velocity-storage mechanisms. Evidence is presented that the position-dependent modification of velocity storage is otolith-mediated. The results are used to analyze the mechanisms of caloric nystagmus. The caloric response consists of a convective component (C_C), as originally envisioned by Bárány (1906), and a nonconvective component (N_C). C_C accounts for 75% of the caloric response in the conventional supine testing position. Both components can be affected by the position-dependent modification of T_VOR or, equivalently, of G_A. It has been suggested that two mechanisms might contribute to N_C: 1) a direct thermal effect on hair cells or afferents; or 2) a thermal expansion of labyrinthine fluids that results in a cupular displacement. Both theoretical and experimental evidence indicates that only the first of these mechanisms could result in the steady-state caloric response that is observed in the absence of convection (e.g., in spaceflight and after canal plugging) and that contributes to the prone-supine asymmetry seen in caloric testing.     

Quantitative analysis of the velocity characteristics of optokinetic nystagmus and optokinetic after-nystagmus

1. Velocity characteristics of optokinetic nystagmus (OKN) and optokinetic after-nystagmus (OKAN) induced by constant velocity full field rotation were studied in rhesus monkeys. A technique is described for estimating the dominant time constant of slow phase velocity curves and of monotonically changing data. Time constants obtained by this technique were used in formulating a model of the mechanism responsible for producing OKN and OKAN.2. Slow phase velocity of optokinetic nystagmus in response to steps in stimulus velocity was shown to be composed of two components, a rapid rise, followed by a slower rise to a steady-state value. Peak values of OKN slow phase velocity increased linearly with increases in stimulus velocity to 180 degrees /sec. Maximum slow phase eye velocities in the monkey are 2-3 times as great as in humans.3. At the onset of OKAN, slow phase velocity falls by about 10-20%, followed by a slower decline to zero. Peak OKAN slow phase velocities were linearly related to optokinetic stimulus velocities up to 90-120 degrees /sec. Above 120 degrees /sec OKAN slow phase velocity saturated although OKN slow phase velocity continued to increase.4. The charge and discharge characteristics of OKAN were studied. The OKAN mechanism charged in 5-10 sec and discharged over 20-60 sec in darkness. The time constants of decay in OKAN slow phase velocity decreased as stimulus velocities increased. They also decreased on repeated testing. In several monkeys there was a consistent difference in the rate of decay of OKAN slow phase velocity to the right and left.5. Extended visual fixation discharged the activity responsible for producing OKAN. Short fixation times caused only a partial discharge of the OKAN mechanism. Following brief periods of fixation, OKAN resumed but with depressed slow phase velocities.6. A model based on a state realisation of a peak detector was formulated which approximately reproduces the salient characteristics of OKN and OKAN. This model predicts the three dominant characteristics of OKAN: (1) charge over 5-7 sec, (2) slow discharge in darkness, and (3) rapid discharge with visual fixation. With the addition of direct fast forward pathways, it also correctly predicts the rapid and slow rise in OKN. We postulate that OKAN is produced by a central integrator which is also active during OKN. Presumably this integrator acts to maximize velocities during OKN and to smooth and stabilize ocular following during movement of the visual surround.     

Invariant geometric characteristics of spatial arm motion

This paper examines up to third-order geometric properties of wrist path and the first-order property of wrist trajectory (wrist speed) for spatial pointing movements. Previous studies report conflicting data regarding the time invariance of wrist-path shape, and most analyses are limited to the second-order geometric property (straightness, or strictly speaking, curvature). Subjects performed point-to-point reaching movements between targets whose locations ensured that the wrist paths spanned a range of lengths and lay in various portions of the arm’s spatial workspace. Movement kinematics were recorded using electromagnetic sensors located on the subject’s arm segments and thorax. Analysis revealed that wrist paths tend to lie in planes and to curve more as movement speed decreases. The orientation of the wrist-path plane depends on the reaching task but does not vary significantly with movement speed. The planarity of wrist paths indicates that the paths have close to zero torsion—a third-order geometric property. Wrist-speed profiles showed multiple peaks for sufficiently slow and long lasting movements, indicating deviation from the well-known, bell-shaped profile. These kinematic findings are discussed in light of various motor control theories.     

Central nystagmus and its interaction with optokinetic and reversive postoptokinetic forms of nystagmus

Interaction of central nystagmus (CN) with optokinetic (OKN) and reversive postoptokinetic (RPN) forms of nystagmus was studied in chronic experiments on rabbits. During interaction of CN with OKN or RPN not in the same direction, the latter are inhibited and CN is manifested during the next 5 sec. The reverse process is observed after a cessation of electrical stimulation. If CN coincides in direction with OKN and RPN, the period from the beginning of electrical stimulation until establishment of regular resultant nystagmus is considerably shortened. Furthermore, the period from the moment of cessation of electrical stimulation until establishment of regular OKN and RPN also is shortened.Translated from Fiziologicheskii Zhurnal SSSR imeni I. M. Sechenova, Vol. 62, No. 3, pp. 362–367, March, 1976.     

Temporal frequency characteristics of spatial interaction in human vision

Summary In psychophysical experiments the bright-dark contrast effects observed in a steady test-field were measured as a function of the temporal frequency of an inducing-field modulated symmetrically about the test-field luminance. The frequency-contrast functions obtained from these measurements were interpreted as reflecting the temporal frequency characteristics of the lateral pathways within the B and D systems (the on-center and off-center neurons) in human vision. Psychophysical evidence is further presented that the lateral neural pathways have lower temporal cut-off frequencies than the straight-through pathways. The results are discussed in terms of the frequency characteristics of the center and surround of the receptive fields of on-center and off-center neurons. It is doubtful, however, whether the psychophysical results can be fully explained by the properties of the single-unit receptive field mechanism.     

Behavioural characteristics of the quick phase of vestibular nystagmus before and after unilateral labyrinthectomy in guinea pig

The aim of this paper was to characterise the conditions under which the quick phase of vestibular nystagmus is generated in response to sinusoidal horizontal angular accelerations in guinea pig and to determine whether the characteristics of the quick phase are altered following unilateral vestibular deafferentation (UVD). In experiment 1, the quick-phase response to 2-Hz sinusoidal stimuli with different peak head velocities was measured before and after UVD. In experiment 2, lower-frequency stimuli with a fixed amplitude (±20°) were used to measure the eye-movement response. In experiment 1, at 2 Hz, the most noticeable difference between UVD animals and normal animals was a reduction in the number of quick phases generated, particularly when rotating towards the lesioned side: the onset of the quick phase was delayed and occurred at a lower value of peak head velocity compared to normals. However, both these measures probably reflect the depressed slow-phase eye-velocity gain rather than a change in the quick-phase mechanism itself, because if a quick phase was generated there was no difference between UVDs and normals on a variety of measures (duration, position, peak eye velocity) for a 2-Hz stimulus. For both UVD and normal animals there did appear to be a position threshold for the onset of a quick-phase eye movement (approximately ±7.5°), although it should be noted that the threshold is not an absolute value. In experiment 2, with lower-frequency stimuli, the characteristics of the quick phase itself were altered in UVD animals. At these lower accelerations, although the head velocity and average eye position for the onset of the quick phase did not differ between UVD and normal animals, there were significant differences between UVD and normal animals in the number of quick phases generated (fewer), the duration of the quick phase (longer) and the peak eye velocity of the quick phase (slower). For lower-frequency stimuli there was no evidence of a specific eye-position threshold for the generation of a quick phase, although the position at which a quick phase occurred rarely exceeded the ±7.5° value obtained in experiment 1. The behavioural data were used to produce a biologically based neural-network simulation of both the slow- and quick-phase components of the vestibulo-ocular reflex, the results of which are presented in a companion paper. Electronic Publication     

Three-dimensional canine renovascular structure and circulation visualized in situ with the dynamic spatial reconstructor

The dynamic spatial reconstructor–a unique, high speed, volume-scanning, X-ray computed tomographic imaging system–was utilized to examine canine renovascular anatomy and renal circulation in situ. In each of the four kidneys examined in this study initial scans were done during bolus injections of angiographic contrast material into the renal artery. A subsequent scan was then performed following an injection of methyl-methacrylate-based casting compound that had been contrast enhanced with ethiodol. After the scans, each kidney was removed, and its parenchyma was digested in potassium hydroxide to expose the vascular cast. Comparison of casts with their reconstructed images and with images obtained during injection of contrast material showed that interlobar arteries and occasionally arcuate arteries could be clearly detected. Although discrete vessels less than 1 mm in diameter could not be resolved, dynamic changes in parenchymal distribution of density during passage of contrast material allowed interpretation of flow through the multiple capillary beds of the kidney. Such analysis indicated that maximal density was in the outer-middle zone of the cortex throughout the duration of the scan. Analysis of artery-to-vein transit time showed arrival of contrast material in the renal vein as soon as 3 sec, and continuation for longer than 8 sec, after the renal artery bolus. In conclusion, renal circulation in the dog can be discretely visualized with the dynamic spatial reconstructor up to the level of the arcuate arteries; however, capillary flow as a whole can be followed through the cortex, and the results suggest the presence of both rapid and slow components of peritubular circulation.     

Horizontal vestibular nystagmus. II. Activity patterns of medial vestibular neurones during nystagmus

1. Identified medial vestibular neurones were studied before, during and after nystagmogenic labyrinthine stimulation in the Encéphale isolé cat. Motor discharges were simultaneously recorded from the contralateral abducens nerve. 2. 87.9% of the recorded neurones showed changes in tonic firing frequency during repetitibe labyrinthine stimulation with no nystagmic modulation in behaviour. 3. The secondary vestibular neurones projecting monosynaptically to the contralateral abducens motoneurones were included in this non-rhythmic population. 4. Only 2% of the recorded population fired rhythmically both during nystagmogenic stimulation and poststimulation nystagmus. These neurones showed plasticity in behaviour regarding the phases of nystagmus recorded from the contralateral abducens nerve. 5. The remaining 10.1% of the medial vestibular neurones were excited or inhibited by repetitive stimulation of the labyrinth but showed burst firing pattersn correlated with poststimulation nystagmic discharges.     

Ludloff截骨修复矫治拇外翻的三维力学特点

背景：目前针对Ludloff截骨三维特点的研究甚少,且结论稍有不同,导致不能准确地理解其截骨特点。 目的：分析Ludloff截骨技术修复矫治拇外翻的三维力学特点。 方法：采用长86.56 mm、直径30.65 mm的木质圆柱体,进行Ludloff截骨矫治拇外翻的模拟试验,测量并记录数据。 结果与结论：圆柱体的长度随矫形程度的增加而逐渐短缩;无论截骨面偏向跖侧、背侧或水平,截骨的结果均导致远端内旋。Ludolff截骨矫治拇外翻的结果是使截骨远端呈内旋,这对改善第一跖趾关节生物力学有积极的作用。对于有明显跖骨痛的患者可采用偏向跖侧15°~20°的截骨面;对于Ⅰ/ⅡIMA(≥30°)的患者,应谨慎使用Ludolff截骨。