Initial Head Position Research Articles

Actual patient position versus safety models: Specific Absorption Rate implications of initial head position for Ultrahigh Field Magnetic Resonance Imaging.

Specific absorption rate (SAR) relates power absorption to tissue heating, and therefore is used as a safety constraint in magnetic resonance imaging (MRI). This study investigates the implications of initial head positioning on local and whole-head SAR. A virtual body model was simulated at 161 positions inside an eight-channel parallel-transmit (pTx) array. On-axis displacements and rotations of up to 20 mm/degrees and off-axis axial/coronal translations were investigated. Single-channel, radiofrequency (RF) shimming (i.e., single-spoke pTx) and multispoke pTx pulses were designed for seven axial, five coronal and five sagittal slices at each position (the slices were consistent across all positions). Whole-head and local SAR were calculated using safety models consisting of a single (centred) body position, multiple representative positions and all simulated body positions. Positional mismatches between safety models and actual positions cause SAR underestimation. For axial imaging, the actual peak local SAR was up to 4.2-fold higher for both single-channel and 5-spoke pTx, 3.5-fold higher for 3-/4-spoke pTx, and 2-fold higher for RF shimming and 2-spoke pTx, compared with that calculated using the centred body position. For sagittal and coronal imaging, the underestimation of peak local SAR was up to 5.2-fold and 3.8-fold, respectively. Using all body positions to estimate SAR prevented SAR underestimation but yielded up to 11-fold SAR overestimation for RF shimming. Local SAR of single-channel and pTx multispoke pulses showed considerable dependence on the initial patient position. RF shimming yielded much lower sensitivity to positional mismatches for axial imaging but not for sagittal and coronal imaging. This was deemed attributable to the higher degrees-of-freedom of control offered by the investigated coil array for axial imaging. Whole-head SAR is less sensitive to positional mismatches compared with local SAR. Nevertheless, whole-head SAR increased by up to 80% for sagittal imaging. Local and whole-head SAR were observed to be more sensitive to positional mismatches in the axial plane, because of larger variations in coil-tissue proximity. Using all possible body positions in the safety model may become substantially over-conservative and limit imaging performance, especially for the RF shimming mode for axial imaging.

Human Head and Neck Kinematics during Autonomous and Human Braking in Three Initial Head Positions

Human Head and Neck Kinematics during Autonomous and Human Braking in Three Initial Head Positions

Reaching to sounds in virtual reality: A multisensory-motor approach to promote adaptation to altered auditory cues

When localising sounds in space the brain relies on internal models that specify the correspondence between the auditory input reaching the ears, initial head-position and coordinates in external space. These models can be updated throughout life, setting the basis for re-learning spatial hearing abilities in adulthood. In addition, strategic behavioural adjustments allow people to quickly adapt to atypical listening situations. Until recently, the potential role of dynamic listening, involving head-movements or reaching to sounds, have remained largely overlooked. Here, we exploited visual virtual reality (VR) and real-time kinematic tracking, to study the role of active multisensory-motor interactions when hearing individuals adapt to altered binaural cues (one ear plugged and muffed). Participants were immersed in a VR scenario showing 17 virtual speakers at ear-level. In each trial, they heard a sound delivered from a real speaker aligned with one of the virtual ones and were instructed to either reach-to-touch the perceived sound source (Reaching group), or read the label associated with the speaker (Naming group). Participants were free to move their heads during the task and received audio-visual feedback on their performance. Most importantly, they performed the task under binaural or monaural listening. Results show that both groups adapted rapidly to monaural listening, improving sound localisation performance across trials and changing their head-movement behaviour. Reaching the sounds induced faster and larger sound localisation improvements, compared to just naming its position. This benefit was linked to progressively wider head-movements to explore auditory space, selectively in the Reaching group. In conclusion, reaching to sounds in an immersive visual VR context proved most effective for adapting to altered binaural listening. Head-movements played an important role in adaptation, pointing to the importance of dynamic listening when implementing training protocols for improving spatial hearing.

The Dynamics of Prosthetically Elicited Vestibulo-Ocular Reflex Function Across Frequency and Context in the Rhesus Monkey.

Electrical vestibular neurostimulation may be a viable tool for modulating vestibular afferent input to restore vestibular function following injury or disease. To do this, such stimulators must provide afferent input that can be readily interpreted by the central nervous system to accurately represent head motion to drive reflexive behavior. Since vestibular afferents have different galvanic sensitivity, and different natural sensitivities to head rotational velocity and acceleration, and electrical stimulation produces aphysiological synchronous activation of multiple afferents, it is difficult to assign a priori an appropriate transformation between head velocity and acceleration and the properties of the electrical stimulus used to drive vestibular reflex function, i.e., biphasic pulse rate or pulse current amplitude. In order to empirically explore the nature of the transformation between vestibular prosthetic stimulation and vestibular reflex behavior, in Rhesus macaque monkeys we parametrically varied the pulse rate and current amplitude of constant rate and current amplitude pulse trains, and the modulation frequency of sinusoidally modulated pulse trains that were pulse frequency modulated (FM) or current amplitude modulated (AM). In addition, we examined the effects of differential eye position and head position on the observed eye movement responses. We conclude that there is a strong and idiosyncratic, from canal to canal, effect of modulation frequency on the observed eye velocities that are elicited by stimulation. In addition, there is a strong effect of initial eye position and initial head position on the observed responses. These are superimposed on the relationships between pulse frequency or current amplitude and eye velocity that have been shown previously.

A Neural Model of Coordinated Head and Eye Movement Control

Gaze shifts require the coordinated movement of both the eyes and the head in both animals and humanoid robots. To achieve this the brain and the robot control system needs to be able to perform complex non-linear sensory-motor transformations between many degrees of freedom and resolve the redundancy in such a system. In this article we propose a hierarchical neural network model for performing 3-D coordinated gaze shifts. The network is based on the PC/BC-DIM (Predictive Coding/Biased Competition with Divisive Input Modulation) basis function model. The proposed model consists of independent eyes and head controlled circuits with mutual interactions for the appropriate adjustment of coordination behaviour. Based on the initial eyes and head positions the network resolves redundancies involved in 3-D gaze shifts and produces accurate gaze control without any kinematic analysis or imposing any constraints. Furthermore the behaviour of the proposed model is consistent with coordinated eye and head movements observed in primates.

Comparison of the Thickness of the Neck Flexor Muscles of Subjects With and Without a Forward Head Posture on the Two Initial Head Positions During Cranio-Cervical Flexion Exercise

This study compared the effects of the initial head position (i.e., a HHP versus a relaxed head position) of subjects with and without a FHP on the thickness of the deep and superficial neck flexor muscles during CCF. The study recruited 6 subjects with a FHP and 10 subjects without a FHP. The subjects performed CCF in two different head positions: a HHP, with the head aligned so that the forehead and chin formed a horizontal line, and a relaxed head position (RHP), with the head aligned in a self-selected comfortable position. During the CCF exercise, the thickness of the longus colli (LCo) and the thickness of the sternocleidomastoid (SCM) were recorded using ultrasonography. The thickness of each muscle was measured by Image J software. The statistical analysis was performed with a two-way mixed-model analysis of variance. The thickness of the SCM differed significantly (p<.05) between the subjects with and without FHP. According to a post hoc independent t-test, the change in thickness of the SCM increased significantly during CCF in the subjects with FHP while adopting a HHP compared to that in the subjects without FHP. The change in thickness of the SCM was not significantly different between the two positions in subjects without FHP, and there was no significant change in thickness of the LCo muscle during the CCF exercise according to the initial position in both subjects with and without FHP. The results suggest that CCF should be performed in RHP to minimize contraction of the SCM in subjects with a FHP.

Effects of sniffing position for tracheal intubation: a meta-analysis of randomized controlled trials

BackgroundThe purpose of this meta-analysis was to validate the efficacy of the sniffing position in the performance of intubation with direct laryngoscopy. MethodsWe searched MEDLINE, the Cochrane Central Register of Controlled Trials, Embase, and Web of Science. Six randomized controlled trials comprising 2759 adult participants were analyzed. The DerSimonian-Laird method was used to calculate pooled relative risk (RR) and the 95% confidence interval (CI) of Cormack-Lehane classification, Intubation Difficulty Scale, success rate of the first intubation, and weighted mean difference of intubation time. ResultsCompared with the other head positions, the sniffing position did not improve glottic visualization, success rate of the first intubation, or intubation time. However, the sniffing position was significantly associated with better Intubation Difficulty Scale compared with the simple head extension position. (RR,1.28; 95% CI, 1.15-1.42; p<0.0001) ConclusionsAlthough patients do not benefit from the sniffing position in terms of glottic visualization, success rate of the first intubation, or intubation time, the sniffing position can still be recommended as the initial head position for tracheal intubation because the sniffing position provides easier intubation conditions.

Head elevation improves laryngeal exposure with direct laryngoscopy

Study objectiveThe aim of this study was to examine the effect of changing head position on the laryngeal view in the same subject. DesignProspective, randomized, crossover comparison of laryngeal views. SettingOperating suite at a university-affiliated, community hospital. PatientsOne hundred sixty-seven consenting adult patients scheduled to undergo elective surgery with general anesthesia. InterventionsAfter anesthesia induction and muscle relaxation and the head in extended position, the laryngeal view was graded in 3 different head height positions. A special inflatable pillow was placed under the subject's head before induction and was deflated to produce no head elevation or inflated to produce either 6.0cm (sniffing position), or 10.0cm elevation (elevated sniffing position) in random order. Main resultsThe incidence of difficult laryngoscopy (grade ≥3) was 8.38% with no head elevation, 2.39% in the sniffing position, and 1.19% in the elevated sniffing position. Head elevation was not associated with a worse grade in any single patient. ConclusionsSniffing position improves glottic exposure when the laryngoscopic grade is greater than 1 in the head-flat position. The elevated sniffing position improves the view to a better grade in some patients. Because head elevation was not associated with a worse grade in any subject, the elevated sniffing position should be considered as the initial head position before direct laryngoscopy when a difficult exposure is anticipated.

Effect of Initial Head Position on Range of Active Cervical Flexion

Effect of Initial Head Position on Range of Active Cervical Flexion

Influence of Static Eye and Head Position on Tone-Evoked Gaze Shifts

The auditory system represents sound-source directions initially in head-centered coordinates. To program eye-head gaze shifts to sounds, the orientation of eyes and head should be incorporated to specify the target relative to the eyes. Here we test (1) whether this transformation involves a stage in which sounds are represented in a world- or a head-centered reference frame, and (2) whether acoustic spatial updating occurs at a topographically organized motor level representing gaze shifts, or within the tonotopically organized auditory system. Human listeners generated head-unrestrained gaze shifts from a large range of initial eye and head positions toward brief broadband sound bursts, and to tones at different center frequencies, presented in the midsagittal plane. Tones were heard at a fixed illusory elevation, regardless of their actual location, that depended in an idiosyncratic way on initial head and eye position, as well as on the tone's frequency. Gaze shifts to broadband sounds were accurate, fully incorporating initial eye and head positions. The results support the hypothesis that the auditory system represents sounds in a supramodal reference frame, and that signals about eye and head orientation are incorporated at a tonotopic stage.

Human sound localization: measurements in untrained, head-unrestrained subjects using gaze as a pointer

Studies of sound localization in humans have used various behavioral measures to quantify the observers' perceptions; a non-comprehensive list includes verbal reports, head pointing, gun pointing, stylus pointing, and laser aiming. Comparison of localization performance reveals that in humans, just as in animals, different results are obtained with different experimental tasks. Accordingly, to circumvent problems associated with task selection and training, this study used gaze, an ethologically valid behavior for spatial pointing in species with a specialized area of the fovea, to measure sound localization perception of human subjects. Orienting using gaze as a pointer does not require training, preserves the natural link between perception and action, and allows for direct behavioral comparisons across species. The results revealed, unexpectedly, a large degree of variability across subjects in both accuracy and precision. The magnitude of the average angular localization errors for the most eccentric horizontal targets, however, were very similar to those documented in studies that used head pointing, whereas the magnitude of the localization errors for the frontal targets were considerably larger. In addition, an overall improvement in sound localization in the context of the memory-saccade task, as well as a lack of effect of initial eye and head position on perceived sound location were documented.

Head Movements Evoked by Electrical Stimulation in the Frontal Eye Field of the Monkey: Evidence for Independent Eye and Head Control

When the head is free to move, electrical stimulation in the frontal eye field (FEF) evokes eye and head movements. However, it is unclear whether FEF stimulation-evoked head movements contribute to shifting the line of sight, like visually guided coordinated eye-head gaze shifts. Here we investigated this issue by systematically varying initial eye (IEP) and head (IHP) positions at stimulation onset. Despite the large variability of IEP and IHP and the extent of stimulation-evoked gaze amplitudes, gaze displacement was entirely accounted for by eye (re head) displacement. Overall, the majority (3/4) of stimulation-evoked gaze shifts consisted of eye-alone movements, in which head movements were below the detection threshold. When head movements did occur, they often started late (re gaze shift onset) and coincided with rapid eye deceleration, resulting in little change in the ensuing gaze amplitudes. These head movements often reached their peak velocities over 100 ms after the end of gaze shifts, indicating that the head velocity profile was temporally dissociated from the gaze drive. Interestingly, head movements were sometimes evoked by FEF stimulation in the absence of gaze shifts, particularly when IEP was deviated contralaterally (re the stimulated side) at stimulation onset. Furthermore, head movements evoked by FEF stimulation resembled a subset of head movements occurring during visually guided gaze shifts. These unique head movements minimized the eye deviation from the center of the orbit and contributed little to gaze shifts. The results suggest that head motor control may be independent from eye control in the FEF.

Variables Contributing to the Coordination of Rapid Eye/Head Gaze Shifts

In this article results of several published studies are synthesized in order to address the neural system for the determination of eye and head movement amplitudes of horizontal eye/head gaze shifts with arbitrary initial head and eye positions. Target position, initial head position, and initial eye position span the space of physical parameters for a planned eye/head gaze saccade. The principal result is that a functional mechanism for determining the amplitudes of the component eye and head movements must use the entire space of variables. Moreover, it is shown that amplitudes cannot be determined additively by summing contributions from single variables. Many earlier models calculate amplitudes as a function of one or two variables and/or restrict consideration to best-fit linear formulae. Our analysis systematically eliminates such models as candidates for a system that can generate appropriate movements for all possible initial conditions. The results of this study are stated in terms of properties of the response system. Certain axiom sets for the intrinsic organization of the response system obey these properties. We briefly provide one example of such an axiomatic model. The results presented in this article help to characterize the actual neural system for the control of rapid eye/head gaze shifts by showing that, in order to account for behavioral data, certain physical quantities must be represented in and used by the neural system. Our theoretical analysis generates predictions and identifies gaps in the data. We suggest needed experiments.

Neck muscle responses to stimulation of monkey superior colliculus. II. Gaze shift initiation and volitional head movements.

We report neck muscle activity and head movements evoked by electrical stimulation of the superior colliculus (SC) in head-unrestrained monkeys. Recording neck electromyography (EMG) circumvents complications arising from the head's inertia and the kinetics of muscle force generation and allows precise assessment of the neuromuscular drive to the head plant. This study served two main purposes. First, we sought to test the predictions made in the companion paper of a parallel drive from the SC onto neck muscles. Low-current, long-duration stimulation evoked both neck EMG responses and head movements either without or prior to gaze shifts, testifying to a SC drive to neck muscles that is independent of gaze-shift initiation. However, gaze-shift initiation was linked to a transient additional EMG response and head acceleration, confirming the presence of a SC drive to neck muscles that is dependent on gaze-shift initiation. We forward a conceptual neural architecture and suggest that this parallel drive provides the oculomotor system with the flexibility to orient the eyes and head independently or together, depending on the behavioral context. Second, we compared the EMG responses evoked by SC stimulation to those that accompanied volitional head movements. We found characteristic features in the underlying pattern of evoked neck EMG that were not observed during volitional head movements in spite of the seemingly natural kinematics of evoked head movements. These features included reciprocal patterning of EMG activity on the agonist and antagonist muscles during stimulation, a poststimulation increase in the activity of antagonist muscles, and synchronously evoked responses on agonist and antagonist muscles regardless of initial horizontal head position. These results demonstrate that the electrically evoked SC drive to the head cannot be considered as a neural replicate of the SC drive during volitional head movements and place important new constraints on the interpretation of electrically evoked head movements.

Neck Muscles in the Rhesus Monkey. II. Electromyographic Patterns of Activation Underlying Postures and Movements

Electromyographic (EMG) activity was recorded in < or = 12 neck muscles in four alert monkeys whose heads were unrestrained to describe the spatial and temporal patterns of neck muscle activation accompanying a large range of head postures and movements. Some head postures and movements were elicited by training animals to generate gaze shifts to visual targets. Other spontaneous head movements were made during orienting, tracking, feeding, expressive, and head-shaking behaviors. These latter movements exhibited a wider range of kinematic patterns. Stable postures and small head movements of only a few degrees were associated with activation of a small number of muscles in a reproducible synergy. Additional muscles were recruited for more eccentric postures and larger movements. For head movements during trained gaze shifts, movement amplitude, velocity, and acceleration were correlated linearly and agonist muscles were recruited without antagonist muscles. Complex sequences of reciprocal bursts in agonist and antagonist muscles were observed during very brisk movements. Turning movements of similar amplitudes that began from different initial head positions were associated with systematic variations in the activities of different muscles and in the relative timings of these activities. Unique recruitment synergies were observed during feeding and head-shaking behaviors. Our results emphasize that the recruitment of a given muscle was generally ordered and consistent but that strategies for coordination among various neck muscles were often complex and appeared to depend on the specifics of musculoskeletal architecture, posture, and movement kinematics that differ substantially among species.

Standardized Initial Head Position in Cervical Range-of-Motion Assessment: Reliability and Error Analysis

Standardized Initial Head Position in Cervical Range-of-Motion Assessment: Reliability and Error Analysis

Standardized initial head position in cervical range-of-motion assessment: Reliability and error analysis

Objective: To assess the clinical reliability and precision of the OSI CA-6000 Spinal Motion Analyzer for measurement of range of motion in cervical spines of pain-free subjects by using a novel procedure designed to minimize variability and quantitatively evaluate sources of errors.Methods: Twenty asymptomatic volunteer subjects were evaluated twice by each of two trained examiners in one session. Subject position was carefully standardized. Rotation, lateral bending, and flexion-extension were evaluated in repeated movements (cycles) from extreme to extreme.Analysis: Descriptive statistics and reliability coefficients (interclass correlation coefficients [ICCs]) were calculated for all full- and half-cycle motions. Possible sources of systematic errors were evaluated, and random errors were estimated.Results: ICCs indicate that the instrument performs very reliably for rotation and lateral bending (0.93–0.97) and acceptably for flexion-extension (0.75–0.93) measurements. Differences in instrument placement, subject posture, or both in different trials correlate neither with differences in measured values nor with variances. Within-trial errors did not correlate with ranges of motion. Standardizing head position resulted in increases in reliability of from 3% to 15% for axial rotation and lateral bending but actually decreased the ICCs for flexion-extension (up to 14%) compared with data collected under a less-stringent protocol. Errors in clinical use are estimated at 4.5 degrees.Conclusions: By using our modifications to the accessories and standardization of subject position, the CA-6000 is a highly precise and reliable instrument for measuring active cervical motion about the 3 Cartesian axes. Individuals can repeat the same patterns of motion in sequential trials on the same day with very little variation. Ease of repetitious measurement without examiner intervention contributes to the instrument's ability to obtain highly reliable data. Changes in instrument placement or subject body posture between trials do not give rise to systematic errors. Design of the instrument for flexion-extension could be improved.

Point of View: The Effect of Initial Head Position on Active Cervical Axial Rotation Range of Motion in Two Age Populations

Point of View: The Effect of Initial Head Position on Active Cervical Axial Rotation Range of Motion in Two Age Populations

Compensation methods for head motion detected during PET imaging

The authors describe two methods to correct for motion artifacts in head images obtained by positron emission tomography (PET). The methods are based on six-dimensional motion data of the head that have to be acquired simultaneously during scanning. The data are supposed to represent the rotational and translational deviations of the head as a function of time, with respect to the initial head position. The first compensation method is a rebinning procedure by which the lines of response are geometrically transformed according to the current values of the motion data, assuming a cylindrical scanner geometry. An approximation of the rebinning transformations by use of large look-up tables, having the potential of on-line event processing, is presented. The second method comprises post-processing of the reconstructed images by unconstrained or constrained deconvolution of the image or image segments with kernels that are generated from the motion data. The authors use motion data that were acquired with a volunteer in supine position, immobilized by a thermoplastic head holder, to demonstrate the effects of the compensation methods. Preliminary results obtained with test data indicate that the methods have the potential to improve the resolution of PET images in cases where significant head motion has occurred, provided that the head position and orientation can be accurately measured.

Mode switching control design with initial value compensation and its application to head positioning control on magnetic disk drives

To meet an increasing demand for high performance and large-capacity in magnetic hard disk drives, both fast response and precise positioning are strongly required for the head positioning control. A mode switching control (MSC) system, which includes a track seeking controller, a track following controller, and a switching function, is widely used to meet this requirement. The issue for MSC is to confirm a design method of servo mode switching. This paper proposes the initial value compensation method (IVC). The IVC inputs certain initial values in the controller state variables at mode switching in order to improve the transient response after switching. There are two approaches to determine the values: (1) minimizing a linear-quadratic function of the plant state variables; and (2) cancelling the unfavorable poles of the transfer function between initial values and head position by relocating zeros. Some experimental results are shown to be effective.