Visually guided adjustments of body posture in the roll plane

Body position relative to gravity is continuously updated to prevent falls. Therefore, the brain integrates input from the otoliths, truncal graviceptors, proprioception and vision. Without visual cues estimated direction of gravity mainly depends on otolith input and becomes more variable with increasing roll-tilt. Contrary, the discrimination threshold for object orientation shows little modulation with varying roll orientation of the visual stimulus. Providing earth-stationary visual cues, this retinal input may be sufficient to perform self-adjustment tasks successfully, with resulting variability being independent of whole-body roll orientation. We compared conditions with informative (earth-fixed) and non-informative (body-fixed) visual cues. If the brain uses exclusively retinal input (if earth-stationary) to solve the task, trial-to-trial variability will be independent from the subject's roll orientation. Alternatively, central integration of both retinal (earth-fixed) and extra-retinal inputs will lead to increasing variability when roll-tilted. Subjects, seated on a motorized chair, were instructed to (1) align themselves parallel to an earth-fixed line oriented earth-vertical or roll-tilted 75° clockwise; (2) move a body-fixed line (aligned with the body-longitudinal axis or roll-tilted 75° counter-clockwise to it) by adjusting their body position until the line was perceived earth-vertical. At 75° right-ear-down position, variability increased significantly (p<0.05) compared to upright in both paradigms, suggesting that, despite the earth-stationary retinal cues, extra-retinal input is integrated. Self-adjustments in the roll-tilted position were significantly (p<0.01) more precise for earth-fixed cues than for body-fixed cues, underlining the importance of earth-stable visual cues when estimates of gravity become more variable with increasing whole-body rol

Alexander's Law in Patients with Acute Vestibular Tone Asymmetry—Evidence for Multiple Horizontal Neural Integrators

Alexander's law (AL) states that the slow-phase velocity of spontaneous nystagmus of peripheral vestibular origin is dependent on horizontal gaze position, with greater velocity when gaze is directed in the fast-phase direction. AL is thought to be a compensatory reaction resulting from adaptive changes in the horizontal ocular motor neural integrator. Until now, only horizontal eye movements have been investigated with respect to AL. Because spontaneous nystagmus usually includes vertical and torsional components, we asked whether horizontal gaze changes would have an effect on the 3D drift of spontaneous nystagmus and, thus, on the vertical/torsional neural integrator. We hypothesized that AL reduces all nystagmus components proportionally. Moreover, we questioned the classical theory of a single bilaterally organized horizontal integrator and searched for nonlinearities of AL implying a network of multiple integrators. Using dual scleral search coils, we measured AL in 17 patients with spontaneous nystagmus. Patients followed a pulsed laser dot at eye level jumping in 5° steps along the horizontal meridian between 25° right and left in otherwise complete darkness. AL was observed in 15 of 17 patients. Whereas individual patients typically showed a change of 3D-drift direction at different horizontal eye positions, the average change in direction was not different from zero. The strength of AL (= rate of change of total velocity with gaze position) correlated with nystagmus slow-phase velocity (Spearman's rho = 0.5; p < 0.05) and, on average, did not change the 3D nystagmus drift direction. In general, eye velocity did not vary linearly with eye position. Rather, there was a stronger dependence of velocity on horizontal position when subjects looked in the slow-phase direction compared to the fast-phase direction. We conclude that the theory of a simple leak of a single horizontal neural integrator is not sufficient to explain all aspects of A

Spatial orientation in patients with chronic unilateral vestibular hypofunction is ipsilesionally distorted

OBJECTIVE Acute unilateral peripheral-vestibular hypofunction (UVH) shifts the subjective visual vertical (SVV) ipsilesionally, triggering central compensation that usually eliminates shifts when upright. We hypothesized that compensation is worse when roll-tilted. METHODS We quantified SVV errors and variability in different roll-tilted positions (0°, ±45°, ±90°) in patients with chronic UVH affecting the superior branch (SVN; n=4) or the entire (CVN; n=9) vestibular nerve. RESULTS Errors in SVN and CVN were not different. When roll-tilted ipsilesionally 45° (9.6±5.4° vs. -0.2±6.4°, patients vs. controls, p<0.001) and 90° (23.5±5.7° vs. 16.8±8.8°, p=0.003), the patient's SVV was shifted significantly towards the lesioned ear. When upright, only a trend was noted (3.6±2.2° vs. 0.0±1.2°, p=0.099); for contralesional roll-tilts shifts were not different from controls. Variability was larger for CVN than SVN (p=0.046). With increasing disease-duration, adjustment errors decayed for ipsilesional roll-tilt and upright (p⩽0.025). CONCLUSIONS The reason verticality perception was distorted for ipsilesional roll-tilts, may be the insufficient integration of contralesional otolith-input. Similar errors in SVN and CVN suggest a dominant utricular role in verticality perception, albeit the sacculus may improve precision of SVV estimates. SIGNIFICANCE With deficiencies in central compensation being roll-angle dependent, extending SVV-testing to roll-tilted positions may improve identifying patients with chronic UVH

Total OH reactivity measurements using a new fast Gas Chromatographic Photo-Ionization Detector (GC-PID)

The primary and most important oxidant in the atmosphere is the hydroxyl radical (OH). Currently OH sinks, particularly gas phase reactions, are poorly constrained. One way to characterize the overall sink of OH is to measure directly the ambient loss rate of OH, the total OH reactivity. To date, direct measurements of total OH reactivity have been either performed using a Laser-Induced Fluorescence (LIF) system ("pump-and-probe" or "flow reactor") or the Comparative Reactivity Method (CRM) with a Proton-Transfer-Reaction Mass Spectrometer (PTR-MS). Both techniques require large, complex and expensive detection systems. This study presents a feasibility assessment for CRM total OH reactivity measurements using a new detector, a Gas Chromatographic Photoionization Detector (GC-PID). Such a system is smaller, more portable, less power consuming and less expensive than other total OH reactivity measurement techniques. &lt;br&gt;&lt;br&gt; Total OH reactivity is measured by the CRM using a competitive reaction between a reagent (here pyrrole) with OH alone and in the presence of atmospheric reactive molecules. The new CRM method for total OH reactivity has been tested with parallel measurements of the GC-PID and the previously validated PTR-MS as detector for the reagent pyrrole during laboratory experiments, plant chamber and boreal field studies. Excellent agreement of both detectors was found when the GC-PID was operated under optimum conditions. Time resolution (60–70 s), sensitivity (LOD 3–6 s&lt;sup&gt;−1&lt;/sup&gt;) and overall uncertainty (25% in optimum conditions) for total OH reactivity were similar to PTR-MS based total OH reactivity measurements. One drawback of the GC-PID system was the steady loss of sensitivity and accuracy during intensive measurements lasting several weeks, and a possible toluene interference. Generally, the GC-PID system has been shown to produce closely comparable results to the PTR-MS and thus in suitable environments (e.g. forests) it presents a viably economical alternative for groups interested in total OH reactivity observations

An introduction to aspect-oriented music representation

This paper introduces Aspect Oriented Music Representation (AOMR), and its application to create tools offering new kinds of flexibility to musicians. Psychologists have demonstrated the diversity of composers' approaches, but existing computer-based tools support this diversity poorly. Current tools generally require musicians to work with pre-established abstractions and operations. AOMR systematically allows composers to create and work with musical viewpoints of their choice. This paper uses simple case studies to analyse a prototype AOMR implementation and characterise the new kinds of flexibility it affords to musicians

Antihysteresis of perceived longitudinal body axis during continuous quasi-static whole-body rotation in the earth-vertical roll plane

Estimation of subjective whole-body tilt in stationary roll positions after rapid rotations shows hysteresis. We asked whether this phenomenon is also present during continuous quasi-static whole-body rotation and whether gravitational cues are a major contributing factor. Using a motorized turntable, 8 healthy subjects were rotated continuously about the earth-horizontal naso-occipital axis (earth-vertical roll plane) and the earth-vertical naso-occipital axis (earth-horizontal roll plane). In both planes, three full constant velocity rotations (2°/s) were completed in clockwise and counterclockwise directions (acceleration=0.05°/s2, velocity plateau reached after 40s). Subjects adjusted a visual line along the perceived longitudinal body axis (pLBA) every 2s. pLBA deviation from the longitudinal body axis was plotted as a function of whole-body roll position, and a sine function was fitted. At identical whole-body earth-vertical roll plane positions, pLBA differed depending on whether the position was reached by a rotation from upright or by passing through upside down. After the first 360° rotation, pLBA at upright whole-body position deviated significantly in the direction of rotation relative to pLBA prior to rotation initiation. This deviation remained unchanged after subsequent full rotations. In contrast, earth-horizontal roll plane rotations resulted in similar pLBA before and after each rotation cycle. We conclude that the deviation of pLBA in the direction of rotation during quasi-static earth-vertical roll plane rotations reflects static antihysteresis and might be a consequence of the known static hysteresis of ocular counterroll: a visual line that is perceived that earth-vertical is expected to be antihysteretic, if ocular torsion is hystereti

Apraclonidine—An eye opener

Pharmacological testing with apraclonidine eye drops induces a typical reversal of anisocoria in patients with Horner’s syndrome. Moreover, apraclonidine was observed to have an elevating effect on the upper eyelid in Horner’s syndrome as well as in healthy subjects, which is thought to be mediated by alpha-1 adrenergic receptors present in the Muller’s muscle. We aim to quantitatively investigate the effect of apraclonidine on eyelid position in patients with Horner’s syndrome compared to physiological anisocoria based on infrared video recordings from pupillometry. We included 36 patients for analysis who underwent binocular pupillometry before and after apraclonidine 1% testing for the evaluation of anisocoria. Vertical eyelid measurements were taken from infrared videos and averaged from multiple pupillometry cycles. Receiver operating characteristic curves were calculated to determine the optimal cutoff value for change in eyelid aperture pre- and post-apraclonidine. A decrease of inter-eye difference in the aperture of &gt;0.42 mm was discriminative of Horner’s syndrome compared to physiological anisocoria with a sensitivity of 80% and a specificity of 75%. Our data confirm an eyelid- elevating effect of the apraclonidine test, more pronounced in eyes with a sympathetic denervation deficit. Measuring eyelid aperture on pupillometry recordings may improve the diagnostic accuracy of apraclonidine testing in Horner’s syndrome

Visual contribution to postural stability: Interaction between target fixation or tracking and static or dynamic large-field stimulus

Stationary visual information has a stabilizing effect on posture, whereas moving visual information is destabilizing. We compared the influence of a stationary or moving fixation point to the influence of stationary or moving large-field stimulation, as well as the interaction between a fixation point and a large-field stimulus. We recorded body sway in 20 healthy subjects who were fixating a stationary or oscillating dot (vertical or horizontal motion, 1/3Hz, +/-12 degrees amplitude, distance 96cm). In addition, a large-field random dot pattern (extension: approximately 80x70 degrees ) was stationary, moving or absent. Visual fixation of a stationary dot in darkness did not reduce antero-posterior (AP) sway compared to the situation in total darkness, but slightly reduced lateral sway at frequencies below 0.5Hz. In contrast, fixating a stationary dot on a stationary large-field pattern reduced both AP and lateral body sway at all frequencies (0.1-2Hz). Ocular tracking of the oscillating dot caused a peak in body sway at 1/3Hz, i.e. the stimulus frequency, but there was no influence of large-field stimulus at this frequency. A stationary large-field pattern, however, reduced AP and lateral sway at frequencies between 0.1 and 2Hz when subjects tracked a moving dot, compared to tracking in darkness. Our results demonstrate that a stationary large-field pattern has a stabilizing effect in all conditions, independent of whether the eyes are fixing on a stationary target or tracking a moving target

Keeping an eye on serial order: Ocular movements bind space and time

The present study examined whether traveling through serially-ordered verbal memories exploits overt visuospatial attentional resources. In a three-phase behavioral study, five single-digits were presented sequentially at one spatial location in phase 1, while recognition and verbal recall were tested in phases 2 and 3, respectively. Participants' spontaneous eye movements were registered along with the verbal responses. Results showed that the search and the retrieval of serially-ordered information were mediated by spontaneous ocular movements. Specifically, recognizing middle items of the memorized sequence required longer inspection times and, importantly, a greater involvement of overt attentional resources, than recognizing the serially first-presented item and, to a lesser extent, the last-presented item. Moreover, serial order was found to be spatially encoded from left-to-right, as eye position during vocal responses deviated the more to the right, the later the serial position of the retrieved item in the sequence. These findings suggest that overt spatial attention mediates the scanning of serial order representation

Cold thermal irrigation decreases the ipsilateral gain of the vestibulo-ocular reflex

OBJECTIVES: During head rotations, neuronal firing rates increase in ipsilateral and decrease in contralateral vestibular afferents. At low accelerations, this "push-pull mechanism" is linear. At high accelerations, however, the change of firing rates is nonlinear in that the ipsilateral increase of firing rate is larger than the contralateral decrease. This mechanism of stronger ipsilateral excitation than contralateral inhibition during high-acceleration head rotation, known as Ewald's second law, is implemented within the nonlinear pathways. The authors asked whether caloric stimulation could provide an acceleration signal high enough to influence the contribution of the nonlinear pathway to the rotational vestibulo-ocular reflex gain (rVOR gain) during head impulses. DESIGN: Caloric warm (44°C) and cold (24, 27, and 30°C) water irrigations of the left ear were performed in 7 healthy human subjects with the lateral semicircular canals oriented approximately earth-vertical (head inclined 30° from supine) and earth-horizontal (head inclined 30° from upright). RESULTS: With the lateral semicircular canal oriented earth-vertical, the strongest cold caloric stimulus (24°C) significantly decreased the rVOR gain during ipsilateral head impulses, while all other irrigations, irrespective of head position, had no significant effect on rVOR gains during head impulses to either side. CONCLUSIONS: Strong caloric irrigation, which can only be achieved with cold water, reduces the rVOR gain during ipsilateral head impulses and thus demonstrates Ewald's second law in healthy subjects. This unilateral gain reduction suggests that cold-water caloric irritation shifts the set point of the nonlinear relation between head acceleration and the vestibular firing rate toward a less acceleration-sensitive zone