Head Position Signals Research Articles

Robotic real-time translational and rotational head motion correction during frameless stereotactic radiosurgery.

To develop a control system to correct both translational and rotational head motion deviations in real-time during frameless stereotactic radiosurgery (SRS). A novel feedback control with a feed-forward algorithm was utilized to correct for the coupling of translation and rotation present in serial kinematic robotic systems. Input parameters for the algorithm include the real-time 6DOF target position, the frame pitch pivot point to target distance constant, and the translational and angular Linac beam off (gating) tolerance constants for patient safety. Testing of the algorithm was done using a 4D (XY Z + pitch) robotic stage, an infrared head position sensing unit and a control computer. The measured head position signal was processed and a resulting command was sent to the interface of a four-axis motor controller, through which four stepper motors were driven to perform motion compensation. The control of the translation of a brain target was decoupled with the control of the rotation. For a phantom study, the corrected position was within a translational displacement of 0.35 mm and a pitch displacement of 0.15° 100% of the time. For a volunteer study, the corrected position was within displacements of 0.4 mm and 0.2° over 98.5% of the time, while it was 10.7% without correction. The authors report a control design approach for both translational and rotational head motion correction. The experiments demonstrated that control performance of the 4D robotic stage meets the submillimeter and subdegree accuracy required by SRS.

TH-A-9A-09: Optimal Feedback Control with Feed Forward for a Robotic 4D Stage in Frameless Stereotactic Radiosurgery

Purpose: A novel 4D robotic system was developed to correct both translational and rotational head motion deviations during frameless image-guided stereotactic radiosurgery (SRS) procedures. The work presents a systematic approach to optimize correction performance. Methods: The system included a robotic 4D (XYZ+Pitch) stage, a head position sensing unit and a control computer. The measured head position signal was processed and a resulting command was sent to the interface of a 4-axis motor controller, through which 4 stepper motors were driven to perform motion compensation. Due to the displacement between the lesion and the pivot of the 4D stage, the rotation of the 4D stage leads to both intended rotational and undesired translational motion of the lesion. To address the issue, feedback control with feed forward was utilized in control law design, and coupling coefficients were online tuned to improve control performance. The synchronization of all 4 axes motion was found to be crucial in the decoupling control. Results: A dynamic 6D head phantom was placed on the top of the 4D stage and was used both to perform pure 4D displacements together with simulating actual framless SRS patient head motion. The 4D stage was applied to compensate such motion. The translational displacement 7 mm and pitch displacement 0.8 degree were corrected in 5 seconds without overshoot, and the steady errors were less than 0.15 mm and 0.04 degree, respectively. For a 15 minutes test with 0.25 mm and 0.07 degree target position tolerance, a subject within tolerance in 1% of the time without motion correction, while this was increased to 99.5% with motion correction. Conclusion: We report a general control design approach for both translational and rotational head motion correction. The experiments demonstrated that control performance of the 4D robotic stage meets real time sub-millimeter accuracy requirement of frameless SRS. Research was partially funded by NIH Grant T32 EB002103-21 from NIBIB. Contents are solely the responsibility of the authors and do not necessarily represent the official views of the NIBIB or NIH.

Optic flow neurons in area PEc integrate eye and head position signals

Neurons in area PEc, a visual area located in the superior parietal lobule, are activated by optic flow stimuli. An important issue is whether PEc neurons are able to integrate multimodal signals, such as those related to optic flow selectivity with those about eye and head position. The aim of this study was to assess if angle of gaze and/or head rotation modify the spatial representation of the focus of expansion (FOE), varying FOE, fixation point and head position in space. We found that the rotation of head modulated the firing activity of PEc optic flow neurons. The head position also changed the angle of gaze effect on the PEc neuronal activity. All recorded neurons showed a main interaction effect between head and eye position upon the selectivity for optic flow stimuli. These results seem to suggest that PEc optic flow neurons use different reference frames depending on the position of the eye and/or the head in space emphasizing a possible contribution of this area in guiding locomotion by integrating multiple extraretinal inputs.

The Vestibulo-Auricular Reflex

The mammalian orienting response to sounds consists of a gaze shift that can be a combination of head and eye movements. In animals with mobile pinnae, the ears also move. During head movements, vision is stabilized by compensatory rotations of the eyeball within the head because of the vestibulo-ocular reflex (VOR). While studying the gaze shifts made by cats to sounds, a previously uncharacterized compensatory movement was discovered. The pinnae exhibited short-latency, goal-directed movements that reached their target while the head was still moving. The pinnae maintained a fixed position in space by counter-rotating on the head with an equal but opposite velocity to the head movement. We call these compensatory ear movements the vestibulo-auricular reflex (VAR) because they shared many kinematic characteristics with the VOR. Control experiments ruled out efference copy of head position signals and acoustic tracking (audiokinetic) of the source as the cause of the response. The VAR may serve to stabilize the auditory world during head movements.

Integration System of Head, Eye, and Retinal Position Signals for Perceptual Direction

This study investigates the integration system of head (-to-trunk), eye (-to-head), and retinal position signals for hand pointing. In experiment 1, subjects changed their head and eye positions and pointed at a fixated visual stimulus by using an unseen pointer. In experiment 2, subjects fixated a visual stimulus and pointed at another visual stimulus. The results show that the head and eye position signals contributed linearly to perceptual direction (experiments 1 and 2), and that the coefficients of these signals decrease with peripheral vision and are smaller than the coefficient of the retinal position signal (experiment 2). These results collectively suggest that the integration algorithm of the position signals might be described by the linear summation equation and that the retinal position signal serves a more important role than the other position signals in the visual system.

Computations for geometrically accurate visually guided reaching in 3-D space

A fundamental question in neuroscience is how the brain transforms visual signals into accurate three-dimensional (3-D) reach commands, but surprisingly this has never been formally modeled. Here, we developed such a model and tested its predictions experimentally in humans. Our visuomotor transformation model used visual information about current hand and desired target positions to compute the visual (gaze-centered) desired movement vector. It then transformed these eye-centered plans into shoulder-centered motor plans using extraretinal eye and head position signals accounting for the complete 3-D eye-in-head and head-on-shoulder geometry (i.e., translation and rotation). We compared actual memory-guided reaching performance to the predictions of the model. By removing extraretinal signals (i.e., eye-head rotations and the offset between the centers of rotation of the eye and head) from the model, we developed a compensation index describing how accurately the brain performs the 3-D visuomotor transformation for different head-restrained and head-unrestrained gaze positions as well as for eye and head roll. Overall, subjects did not show errors predicted when extraretinal signals were ignored. Their reaching performance was accurate and the compensation index revealed that subjects accounted for the 3-D visuomotor transformation geometry. This was also the case for the initial portion of the movement (before proprioceptive feedback) indicating that the desired reach plan is computed in a feed-forward fashion. These findings show that the visuomotor transformation for reaching implements an internal model of the complete eye-to-shoulder linkage geometry and does not only rely on feedback control mechanisms. We discuss the relevance of this model in predicting reaching behavior in several patient groups.

Vestibular integration in human cerebral cortex contributes to spatial remapping

The process of visuo-spatial updating is crucial in guiding human behaviour. While the parietal cortex has long been considered a principal candidate for performing spatial transformations, the exact underlying mechanisms are still unclear.In this study, we investigated in a patient with a right occipito-parietal lesion the ability to update the visual space during vestibularly guided saccades. To quantify the possible deficits in visual and vestibular memory processes, we studied the subject's performance in two separate memory tasks, visual (VIS) and vestibular (VES). In the VIS task, a saccade was elicited from a central fixation point to the location of a visual memorized target and in the VEST task, the saccade was elicited after whole-body rotation to the starting position thus compensating for the rotation. Finally, in an updating task (UPD), the subject had to memorize the position of a visual target then after a whole-body rotation he had to produce a saccade to the remembered visual target location in space. Our main findings was a significant hypometria in the final eye position of both VEST and UPD saccades induced during rotation to the left (contralesional) hemispace as compared to saccades induced after right (ipsilesional) rotation. Moreover, these deficits in vestibularly guided saccades correlated with deficits in vestibulo-ocular time constant, reflecting disorders in the inertial vestibular integration path. We conclude that the occipito-parietal cortex in man can provide a first stage in visuo-spatial remapping by encoding inertial head position signals during gaze orientation.

Head position control on quasi-static read/write tester

To develop high-density magnetic recording systems, a simple feedback system without servo writing was applied to a quasi-static read/write tester in which a medium reciprocates against a stand-still head. The head position signal in cross–track direction during the scanning is fed back to the high-precision piezoelectric actuator of the media stage. The stage is controlled so as to make the head/medium alignment error zero. A contact head slider assembled on a parallel–link suspension was used to evaluate the feedback system. The tester shows an accuracy of 1.5nm in cross–track direction which is preferable for the read/write tests at future high recording densities.

Minimization of tape transient disturbances in track following tape drives with a disturbance observer

The primary purpose of the head servo in the track following tape drives is to track follow as accurately as possible during read/write operations in spite of disturbances such as tape edge movements or tape transients, internal and external vibrations transmitted to the head actuator and signal noise and dropouts. The track following and fine positioning is done using a position error signal (PES). Feedback controller design using proportional-integral-derivative (PID), lead-lag or estimator/controller techniques are used for maintaining accurate track following. This paper assumes that in addition to the PES signal, a head position signal is available for long seek, short seek, and track identification tasks, etc. In this paper, the development of a disturbance observer that supplements the nominal head servo controller in minimizing the effects of the tape transients is presented. The PES and the head position error signals are blended to provide a measure of the tape transient disturbance. This paper presents the theory of the disturbance observer and analyses to show the range of frequencies over which the disturbance observer will be effective. Finally, simulation results are presented for tape transients of various frequencies.

Resolution of Sensory Ambiguities for Gaze Stabilization Requires a Second Neural Integrator

The ability to simultaneously move in the world and maintain stable visual perception depends critically on the contribution of vestibulo-ocular reflexes (VORs) to gaze stabilization. It is traditionally believed that semicircular canal signals drive compensatory responses to rotational head disturbances (rotational VOR), whereas otolith signals compensate for translational movements [translational VOR (TVOR)]. However, a sensory ambiguity exists because otolith afferents are activated similarly during head translations and reorientations relative to gravity (i.e., tilts). Extra-otolith cues are, therefore, necessary to ensure that dynamic head tilts do not elicit a TVOR. To investigate how extra-otolith signals contribute, we characterized the temporal and viewing distance-dependent properties of a TVOR elicited in the absence of a lateral acceleration stimulus to the otoliths during combined translational/rotational motion. We show that, in addition to otolith signals, angular head position signals derived by integrating sensory canal information drive the TVOR. A physiological basis for these results is proposed in a model with two distinct integration steps. Upstream of the well known oculomotor velocity-to-position neural integrator, the model incorporates a separate integration element that could represent the "velocity storage integrator," whose functional role in the oculomotor system has so far remained controversial. We propose that a key functional purpose of the velocity storage network is to temporally integrate semicircular canal signals, so that they may be used to extract translation information from ambiguous otolith afferent signals in the natural and functionally relevant bandwidth of head movements.

Role of eye, head, and shoulder geometry in the planning of accurate arm movements.

Eye-hand coordination requires the brain to integrate visual information with the continuous changes in eye, head, and arm positions. This is a geometrically complex process because the eyes, head, and shoulder have different centers of rotation. As a result, head rotation causes the eye to translate with respect to the shoulder. The present study examines the consequences of this geometry for planning accurate arm movements in a pointing task with the head at different orientations. When asked to point at an object, subjects oriented their arm to position the fingertip on the line running from the target to the viewing eye. But this eye-target line shifts when the eyes translate with each new head orientation, thereby requiring a new arm pointing direction. We confirmed that subjects do realign their fingertip with the eye-target line during closed-loop pointing across various horizontal head orientations when gaze is on target. More importantly, subjects also showed this head-position-dependent pattern of pointing responses for the same paradigm performed in complete darkness. However, when gaze was not on target, compensation for these translations in the rotational centers partially broke down. As a result, subjects tended to overshoot the target direction relative to current gaze; perhaps explaining previously reported errors in aiming the arm to retinally peripheral targets. These results suggest that knowledge of head position signals and the resulting relative displacements in the centers of rotation of the eye and shoulder are incorporated using open-loop mechanisms for eye-hand coordination, but these translations are best calibrated for foveated, gaze-on-target movements.

Decreased vestibulo–ocular reflex gain of vestibular schwannoma patients

Objective: The vestibulo–ocular reflex (VOR) stabilizes gaze during head movements by producing compensatory eye movements. Gain of the VOR can be defined as the difference between the eye and corresponding head movement amplitudes. The objective of the study was to compare the gain of postoperative vestibular schwannoma (VS) patients with that of healthy controls. Methods: The gain of 19 vestibular schwannoma (VS) patients and 100 healthy controls was measured with a head autorotation test (HART) in the five frequency bands of 1–5 Hz. It was computed as the ratio of the amplitude of the eye position signal to the amplitude of the head position signal. The mean gain was compared between the VS patients and healthy subjects in each frequency band by using an analysis of variance with statistical significance pre-defined as P<0.05. Results: The HART was abnormal in 58% of the VS patients, whose mean gains in the five frequency bands of 1–5 Hz were 0.85, 0.79, 0.72, 0.64 and 0.60, respectively. The mean gains of the VS patients were significantly ( P<0.05) smaller than those of the controls in all the frequency bands. Conclusions: The deficit of the VOR gain seems to prevail in more than a half of postoperative VS patients, although this inaccuracy of compensatory eye movements may not lead to the occurrence of any symptoms. However, in these patients a potential threat to gaze stability exists.

A model of multiplicative neural responses in parietal cortex.

Visual responses of neurons in parietal area 7a are modulated by a combined eye and head position signal in a multiplicative manner. Neurons with multiplicative responses can act as powerful computational elements in neural networks. In the case of parietal cortex, multiplicative gain modulation appears to play a crucial role in the transformation of object locations from retinal to body-centered coordinates. It has proven difficult to uncover single-neuron mechanisms that account for neuronal multiplication. Here we show that multiplicative responses can arise in a network model through population effects. Specifically, neurons in a recurrently connected network with excitatory connections between similarly tuned neurons and inhibitory connections between differently tuned neurons can perform a product operation on additive synaptic inputs. The results suggest that parietal responses may be based on this architecture.

Head position signals used by parietal neurons to encode locations of visual stimuli.

The mechanism for object location in the environment, and the perception of the external world as stable when eyes, head and body are moved, have long been thought to be centred on the posterior parietal cortex. However, head position signals, and their integration with visual and eye position signals to form a representation of space referenced to the body, have never been examined in any area of the cortex. Here we show that the visual and saccadic activities of parietal neurons are strongly affected by head position. The eye and head position effects are equivalent for individual neurons, indicating that the modulation is a function of gaze direction, regardless of whether the eyes or head are used to direct gaze. These data are consistent with the idea that the posterior parietal cortex contains a distributed representation of space in body-centred coordinates.

Encoding of intention and spatial location in the posterior parietal cortex.

The posterior parietal cortex is functionally situated between sensory cortex and motor cortex. The responses of cells in this area are difficult to classify as strictly sensory or motor, since many have both sensory- and movement-related activities, as well as activities related to higher cognitive functions such as attention and intention. In this review we will provide evidence that the posterior parietal cortex is an interface between sensory and motor structures and performs various functions important for sensory-motor integration. The review will focus on two specific sensory-motor tasks--the formation of motor plans and the abstract representation of space. Cells in the lateral intraparietal area, a subdivision of the parietal cortex, have activity related to eye movements the animal intends to make. This finding represents the lowest stage in the sensory-motor cortical pathway in which activity related to intention has been found and may represent the cortical stage in which sensory signals go "over the hump" to become intentions and plans to make movements. The second part of the review will discuss the representation of space in the posterior parietal cortex. Encoding spatial locations is an essential step in sensory-motor transformations. Since movements are made to locations in space, these locations should be coded invariant of eye and head position or the sensory modality signaling the target for a movement. Data will be reviewed demonstrating that there exists in the posterior parietal cortex an abstract representation of space that is constructed from the integration of visual, auditory, vestibular, eye position, and proprioceptive head position signals. This representation is in the form of a population code and the above signals are not combined in a haphazard fashion. Rather, they are brought together using a specific operation to form "planar gain fields" that are the common foundation of the population code for the neural construct of space.

Coordinate transformations in the representation of spatial information

Coordinate transformations are an essential aspect of behavior. They are required because sensory information is coded in the coordinates of the sensory epithelia (e.g. retina, skin) and must be transformed to the coordinates of muscles for movement. In this review we will concentrate on recent studies of visual-motor transformations. The studies show that representations of space are distributed, being specified in the activity of many cells rather than in the activity of individual cells. Furthermore, these distributed representations appear to be derived by a specific operation, which systematically combines visual signals with eye and head position signals.

Dynamic space codes in the superior colliculus

Space coding in the superior colliculus has traditionally been viewed as a static representation by multiple, aligned, sensory and motor maps. Recent evidence has revealed that the maps are dynamic, shaped by sensory experience in developing animals, and by eye and head position signals in adults. The superior colliculus thus provides an ideal model for studying the neural mechanisms underlying developmental and real-time modifications of information representation in the brain.

Tonic cervical influences on eye nystagmus following hemilabyrinthectomy: immediate and plastic effects

In intact guinea pigs a passive horizontal rotation of the body about the fixed head induces compensatory ocular movements (cervico-ocular reflex). When the static neck deviation is maintained, a significant ocular displacement is observed. In acutely hemilabyrinthectomized animals, static body deviation towards the lesion side tonically alters eyenystagmus. It affects slow phase eye velocity and quick phase amplitude and frequency causing the eye to reach a less eccentric orbital position. Apart from such immediate influences, a plastic effect on eye nystagmus abatement is induced. In the animals restrained with no body-on-head deviation, abatement of nystagmus is delayed with respect to the animals restrained with 35° body deviation towards the lesion side. Thus the head position signal is not only a contributing factor for the correction of postural deficits but also influences the time course of the ocular balancing process following unilateral vestibular damage.

A head-positioning system using buried servos

Current head-positioning servo methods require a spatial separation between the data and servo. This separation limits the accuracy with which a head-positioning servo system can follow a data track. The buried servo techniques described here allow servo information to be recorded in the same medium area as that used for data, a single head is used for both data and servo. These techniques provide high-bandwidth continuous head-position information with a potential for a high track-density servo system. The servo pattern consists of two single-frequency signals recorded deeply into the magnetic medium below the area used for data. The servo signals are detected by synchronous detection techniques and provide a continuous head-position error signal. The servo system uses this head-position error signal to follow a given data track and to seek new data tracks. Buried servo was implemented on a flexible disk drive. The track-following system had performance advantages when compared to a sector servo scheme implemented on a similar device.

An electronic tachometer for disk file motion control

An electronic tachometer system used as a velocity transducer for the track-accessing control system of a disk file is described. Rather than employing a conventional moving magnet and fixed-coil velocity transducer, this electronic tachometer combines information from the servo head position signal and from the actuator motor current to generate an accurate measurement of actuator velocity. Transducer bandwidths of several kilohertz, together with good calibration accuracy and linearity, are obtained over the 100:1 dynamic range of 0.013 to 1.3 meters per second.