Yongqing Xiang

Head Stabilization by Vestibulocollic Reflexes During Quadrupedal Locomotion in Monkey

Little is known about the three-dimensional characteristics of vestibulocollic reflexes during natural locomotion. Here we determined how well head stability is maintained by the angular and linear vestibulocollic reflexes (aVCR, lVCR) during quadrupedal locomotion in rhesus and cynomolgus monkeys. Animals walked on a treadmill at velocities of 0.4-1.25 m/s. Head rotations were represented by Euler angles (Fick convention). The head oscillated in yaw and roll at stride frequencies (approximately 1-2 Hz) and pitched at step frequencies (approximately 2-4 Hz). Head angular accelerations (100-2,500 degrees/s2) were sufficient to have excited the aVOR to stabilize gaze. Pitch and roll head movements were <7 degrees , peak to peak, and the amplitude was unrelated to stride frequency. Yaw movements were larger due to spontaneous voluntary head shifts and were smaller at higher walking velocities. Head translations were small (< or =4 cm). Cynomolgus monkeys positioned their heads more forward in pitch than the rhesus monkeys. None of the animals maintained a forward head fixation point, indicating that the lVCR contributed little to compensatory head movements in these experiments. Significantly, aVCR gains in roll and pitch were close to unity and phases were approximately 180 degrees over the full frequency range of natural walking, which is in contrast to previous findings using anesthesia or passive trunk rotation with body restraint. We conclude that the behavioral state associated with active body motion is necessary to maintain head stability in pitch and roll over the full range of stride/step frequencies encountered during walking.

A Model of Blood Pressure, Heart Rate, and Vaso-Vagal Responses Produced by Vestibulo-Sympathetic Activation

Blood Pressure (BP), comprised of recurrent systoles and diastoles, is controlled by central mechanisms to maintain blood flow. Periodic behavior of BP was modeled to study how peak amplitudes and frequencies of the systoles are modulated by vestibular activation. The model was implemented as a relaxation oscillator, driven by a central signal related to Desired BP. Relaxation oscillations were maintained by a second order system comprising two integrators and a threshold element in the feedback loop. The output signal related to BP was generated as a nonlinear function of the derivative of the first state variable, which is a summation of an input related to Desired BP, feedback from the states, and an input from the vestibular system into one of the feedback loops. This nonlinear function was structured to best simulate the shapes of systoles and diastoles, the relationship between BP and Heart Rate (HR) as well as the amplitude modulations of BP and Pulse Pressure. Increases in threshold in one of the feedback loops produced lower frequencies of HR, but generated large pulse pressures to maintain orthostasis, without generating a VasoVagal Response (VVR). Pulse pressures were considerably smaller in the anesthetized rats than during the simulations, but simulated pulse pressures were lowered by including saturation in the feedback loop. Stochastic changes in threshold maintained the compensatory Baroreflex Sensitivity. Sudden decreases in Desired BP elicited non-compensatory VVRs with smaller pulse pressures, consistent with experimental data. The model suggests that the Vestibular Sympathetic Reflex (VSR) modulates BP and HR of an oscillating system by manipulating parameters of the baroreflex feedback and the signals that maintain the oscillations. It also shows that a VVR is generated when the vestibular input triggers a marked reduction in Desired BP.

The vasovagal response of the rat: its relation to the vestibulosympathetic reflex and to Mayer waves

Vasovagal responses (VVRs) are characterized by transient drops in blood pressure (BP) and heart rate (HR) and increased amplitude of low‐frequency oscillations in the Mayer wave frequency range. Typical VVRs were induced in anesthetized, male, Long‐Evans rats by sinusoidal galvanic vestibular stimulation (sGVS). VVRs were also produced by single sinusoids that transiently increased BP and HR, by 70‐90° nose‐up tilts, and by 60° tilts of the gravitoinertial acceleration vector using translation while rotating (TWR). The average power of the BP signal in the Mayer wave range increased substantially when tilts were >70° (0.91 g), i.e., when linear accelerations in the x–z plane were ≥0.9–1.0 g. The standard deviations of the wavelet‐filtered BP signals during tilt and TWR overlaid when they were normalized to 1 g. Thus, the amplitudes of the Mayer waves coded the magnitude of the linear acceleration ≥1 g acting on the head and body, and the average power in this frequency range was associated with the generation of VVRs. These data show that VVRs are a natural outcome of stimulation of the vestibulosympathetic reflex and are not a disease. The results also demonstrate the usefulness of the rat as a small animal model for studying human VVRs.—Cohen, B., Martinelli, G. P., Raphan, T., Schaffner, A., Xiang, Y., Holstein, G. R., Yakushin, S. B. The vasovagal response of the rat: its relation to the vestibulosympathetic reflex and to Mayer waves. FASEB J. 27, 2564–2572 (2013). www.fasebj.org

Dependence of the roll angular vestibuloocular reflex (aVOR) on gravity.

Little is known about the dependence of the roll angular vestibuloocular reflex (aVOR) on gravity or its gravity-dependent adaptive properties. To study gravity-dependent characteristics of the roll aVOR, monkeys were oscillated about a naso-occipital axis in darkness while upright or tilted. Roll aVOR gains were largest in the upright position and decreased by 7-15% as animals were tilted from the upright. Thus the unadapted roll aVOR gain has substantial gravitational dependence. Roll gains were also decreased or increased by 0.25 Hz, in- or out-of-phase rotation of the head and the visual surround while animals were prone, supine, upright, or in side-down positions. Gain changes, determined as a function of head tilt, were fit with a sinusoid; the amplitudes represented the amount of the gravity-dependent gain change, and the bias, the gravity-independent gain change. Gravity-dependent gain changes were absent or substantially smaller in roll (approximately 5%) than in yaw (25%) or pitch (17%), whereas gravity-independent gain changes were similar for roll, pitch, and yaw (approximately 20%). Thus the high-frequency roll aVOR gain has an inherent dependence on head orientation re gravity in the unadapted state, which is different from the yaw/pitch aVORs. This inherent gravitational dependence may explain why the adaptive circuits are not active when the head is tilted re gravity during roll aVOR adaptation. These behavioral differences support the idea that there is a fundamental difference in the central organization of canal-otolith convergence of the roll and yaw/pitch aVORs.

Quantification of trabecular bone mass and orientation using Gabor wavelets

Bone strength is dependent on both its mass and architecture. In this study, a tool was developed that incorporates metrics associated with both of these features. To accomplish this, textural features of trabecular bone were extracted from stained bone images using Gabor wavelets. Gabor wavelets are 2-D spatial filters that are both frequency and orientation tunable. A texture feature vector was constructed that consists of localized texture energies along different orientations at different scales. The texture feature characterizes the spatial (regional) distributions of the constituent bone lattice in terms of their size, shape and orientation. Results indicated that wavelet analysis provides the insight of the frequency composition that can be localized to the pizel level. Bone mass can be discriminated by the averaged texture energy across all orientations. Dominant bone lattice orientation can be determined by the orientation with the maximal value of the averaged texture energy across all scales. A measure of anisotropy can be quantified by the span between the maximum texture energy and the minimum texture energy. This methodology has the potential to provide a tool for quantifying both bone mass and bone structural anisotropy.

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

Abstract: 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.

Vasovagal oscillations and vasovagal responses produced by the vestibulo-sympathetic reflex in the rat.

Sinusoidal galvanic vestibular stimulation (sGVS) induces oscillations in blood pressure (BP) and heart rate (HR), i.e., vasovagal oscillations, as well as transient decreases in BP and HR, i.e., vasovagal responses, in isoflurane-anesthetized rats. We determined the characteristics of the vasovagal oscillations, assessed their role in the generation of vasovagal responses, and determined whether they could be induced by monaural as well as by binaural sGVS and by oscillation in pitch. Wavelet analyses were used to determine the power distributions of the waveforms. Monaural and binaural sGVS and pitch generated vasovagal oscillations at the frequency and at twice the frequency of stimulation. Vasovagal oscillations and vasovagal responses were maximally induced at low stimulus frequencies (0.025–0.05 Hz). The oscillations were attenuated and the responses were rarely induced at higher stimulus frequencies. Vasovagal oscillations could occur without induction of vasovagal responses, but vasovagal responses were always associated with a vasovagal oscillation. We posit that the vasovagal oscillations originate in a low frequency band that, when appropriately activated by strong sympathetic stimulation, can generate vasovagal oscillations as a precursor for vasovagal responses and syncope. We further suggest that the activity responsible for the vasovagal oscillations arises in low frequency, otolith neurons with orientation vectors close to the vertical axis of the head. These neurons are likely to provide critical input to the vestibulo-sympathetic reflex to increase BP and HR upon changes in head position relative to gravity, and to contribute to the production of vasovagal oscillations and vasovagal responses and syncope when the baroreflex is inactivated.

Image-quality enhancement of objects in turbid media by use of a combined computational–photonics approach

Ultrafast time-gated optical imaging and computational image-enhancement techniques were combined to produce a robust system for viewing objects in turbid media. Image enhancement was implemented by use of images from the early light with a histogram contrast-enhancement algorithm. Image quality was assessed by use of the contrast radius of gyration and the contrast-to-noise ratio. The technique was applied to viewing the dispersion of water droplets emanating from a jet spray and to pictures of an object embedded in turbid media. In all instances there were substantial improvements in image quality at a given time delay.

Effect of Canal Plugging on Quadrupedal Locomotion in Monkey

The vestibular system plays an important role in controling gait, but where in the labyrinths relevant activity arises is largely unknown. After the semicircular canals are plugged, low frequency (0.01–2 Hz) components of the angular vestibulo‐ocular reflex (aVOR) and angular vestibulo‐collic reflex (aVCR) are lost, but high frequency (3–20 Hz) components remain. We determined how loss of low frequency canal afference affects limb and head movements during quadrupedal locomotion. Head, body, and limb movements were recorded in three dimensions (3‐D) in a cynomolgus monkey with a motion detection system, while the animal walked on a treadmill. All six canals were plugged, reducing the canal time constants from ≈4.0 sec to ≈0.07 sec. Major changes in the control of the limbs occurred after surgery. Fore and hind limbs were held farther from the body, producing a broad‐based gait. Swing‐phase trajectories were inaccurate, and control of medial‐lateral limb movement was erratic. These changes in gait were present immediately after surgery, as well as 15 months later, when the animal had essentially recovered. Thus, control of the limbs in the horizontal plane was defective after loss of the low‐frequency semicircular canal input and never recovered. Cycle‐averaged pitch and roll head rotations, and 3‐D head translations were also significantly larger and more erratic after than before surgery. Head rotations in yaw could not be quantified due to intrusion of voluntary head turns. These findings indicate that the semicircular canals provide critical low frequency information to maximize the accuracy of stepping and stabilize the head during normal quadrupedal locomotion.

Texture-based approaches for identifying neuro-anatomical structures and electrode tracks

An automated approach to identifying electrode tracks and neuro-anatomical structures (nuclei) was developed using texture attributes of their neuro-anatomical stains. The properties that make up the texture features of the nuclei include size, shape and distribution of elemental structures. The electrode tracks are characterized by elongated darkened formations due to gliosis. Based on a Gabor wavelet transform, a texture feature vector was constructed, consisting of localized texture energies along different orientations at different scales. Stained images of brainstem sections in the vestibular nuclei were segmented using partitional clustering in feature space. A metric that computes the location of the tracks relative to the nuclei centers was then implemented. This methodology should be useful for quantifying and automating the procedure by which tracks are localized in anatomical structures.