Magnetic Head Tracker Research Articles

Experimental support that ocular tremor in Parkinson's disease does not originate from head movement.

Our recent report of ocular tremor in Parkinson's disease (PD) has raised considerable controversy as to the origin of the tremor. Using an infrared based eye tracker and a magnetic head tracker, we reported that ocular tremor was recordable in PD subjects with no apparent head tremor. However, other investigators suggest that the ocular tremor may represent either transmitted appendicular tremor or subclinical head tremor inducing the vestibulo-ocular reflex (VOR). The present study aimed to further investigate the origin of ocular tremor in PD. Eye movements were recorded in 8 PD subjects both head free, and with full head restraint by means of a head holding device and a dental impression bite plate. Head movements were recorded independently using both a high sensitivity tri-axial accelerometer and a magnetic tracking system, each synchronized to the eye tracker. Ocular tremor was observed in all 8 PD subjects and was not influenced by head free and head fixed conditions. Both magnetic tracking and accelerometer recordings supported that the ocular tremor was fully independent of head position. The present study findings support our initial findings that ocular tremor is a fundamental feature of PD unrelated to head movements. Although the utility of ocular tremor for diagnostic purposes requires validation, current findings in large cohorts of PD subjects suggest its potential as a reliable clinical biomarker.

The Vestibulo-ocular Reflex During Active Head Motion in Chiari II Malformation

Chiari type II malformation (CII) is a developmental anomaly of the cerebellum and brainstem, which are important structures for processing the vestibulo-ocular reflex (VOR). We investigated the effects of the deformity of CII on the angular VOR during active head motion. Eye and head movements were recorded using an infrared eye tracker and magnetic head tracker in 20 participants with CII [11 males, age range 8-19 years, mean (SD) 14.4 (3.2) years]. Thirty-eight age-matched healthy children and adolescents (21 males) constituted the control group. Participants were instructed to 'look' in darkness at the position of their thumb, placed 25 cm away, while they made horizontal and vertical sinusoidal head rotations at frequencies of about 0.5 Hz and 2 Hz. Parametric and non-parametric tests were used to compare the two groups. The VOR gains, the ratio of eye to head velocities, were abnormally low in two participants with CII and abnormally high in one participant with CII. The majority of participants with CII had normal VOR performance in this investigation. However, the deformity of CII can impair the active angular VOR in some patients with CII. Low gain is attributed to brainstem damage and high gain to cerebellar dysfunction.

The vestibulo-ocular reflex during active head motion in children and adolescents

Little information is available on the performance and maturity of the vestibulo-ocular reflex (VOR) in children and adolescents during active head motion as encountered during normal locomotion. We investigated the active VOR performance in children and adolescents to determine its norm and variation with age. We recorded the angular active VOR using an infrared eye tracker and a magnetic head tracker in 38 children, aged 8-19 years. Participants made sinusoidal head-on-body rotations in yaw (left-right) and pitch (up-down) at frequencies of about 0.5 and 2 Hz in the dark. Mean VOR gains, the ratios of eye to head velocities, were close to unity and were significantly higher at 2 Hz than at 0.5 Hz. VOR gains did not vary with age. The angular VOR during active head motion is functionally mature in children 8 years or older at 0.5 and 2 Hz head rotations.

Head-mounted versus Remote Eye Tracking of Radiologists Searching for Breast Cancer: A Comparison 1

We compared performance and visual search parameters of radiologists detecting masses on mammograms by using both a head-mounted (HDMT) and a remote (REM) eye tracker. Five experienced radiologists read twice a case set of 20 one-view (medial-lateral oblique) mammograms, of which 12 contained a malignant mass and eight were lesion-free. For each observer, one trial used an HDMT eye-tracking system and the other used an REM system. Trials were separated on average by 2 months. Time to hit the location of the mass, dwell, and number of fixations in the location of the mass were measured. The same parameters were measured on a per-trial basis to determine whether there were memory effects from the previous trial. Dwell times in the location of true-positive, false-positive, and false-negative results were significantly shorter (P < .05) using the HDMT (median, 0.395 seconds) than REM (median, 0.482 seconds) systems, but the number of fixations in the location of the response was smaller using the REM system (median, 4.33 versus 5.0 for the HDMT). The observed differences did not seem to be caused by a memory effect. In addition, the relative lack of head mobility using the REM system caused observers to report neck strain. Overall, radiologists' visual search behavior was very similar using both types of eye-tracking device. However, because the REM system did not contain a magnetic head tracker, radiologists were allowed very limited head movements when using it, which made them uncomfortable during the experiment.

Tracking the line of primary gaze in a walking simulator: Modeling and calibration

This article describes a system for tracking the line of primary gaze (LoPG) of participants as they view a large projection screen. Using a magnetic head tracker and a tracking algorithm, we find the onscreen location at which a participant is pointing a head-mounted crosshair. The algorithm presented for tracking the LoPG uses a polynomial function to correct for distortion in magnetic tracker readings, a geometric model for computing LoPG from corrected tracker measurements, and a method for finding the intersection of the LoPG with the screen. Calibration techniques for the above methods are presented. The results of two experiments validating the algorithm and calibration methods are also reported. Experiments showed an improvement in accuracy of LoPG tracking provided by each of the two presented calibration steps, yielding errors in LoPG measurements of less than 2 degrees over a wide range of head positions. Source code for the described algorithms can be downloaded from the Psychonomic Society Web archive, http://www.psychonomic.org/archive/.

Eyetracker analysis of fixation points using an IR HUD in an automobile

This paper describes the experimental apparatus and analysis techniques now in place at the US Army Tank-automotive and Armaments COMmand Visual Perception Lab in Warren, Michigan, and the results of some preliminary trials using these methods. The tools will be used to develop and analyse experiments to better understand how drivers use various displays and viewing systems intended to aid drivers in poor visibility conditions, particularly at night. Visible and infrared road scene imagery obtained from the field were displayed simultaneously and subject eye fixation and point of gaze measured using a magnetic head tracker and near infrared eyetracker. Results were analysed to determine if personal differences between the various subjects caused them to prefer one display to another. The goal was to see if people prefer to use the heads up display or the direct view for driving at night.

A virtual cockpit for a distributed interactive simulation

We developed the virtual cockpit as an inexpensive flight simulator, using off-the-shelf equipment. This system functions as a component of a distributed interactive simulation. Any flight simulator has three principal tasks: image display, image generation, and flight dynamics. We built the flight simulator, the virtual cockpit, using a head-mounted display (HMD) to display the out-the-window imagery and the cockpit interior. The virtual cockpit consists of a Silicon Graphics workstation and supporting hardware components, such as an HMD and position tracker. The software falls into three areas by function: flight dynamics and cockpit instruments; network interface (either SimNet or DIS PDUs); and display of the out-the-window view. The virtual cockpit developed uses a multiprocessor Silicon Graphics Iris (the 4D/440VGXT) connected to a Polhemus Laboratories fiber-optic head-mounted display, a Polhemus magnetic head tracker and a hands-on-throttle-and-stick (HOTAS) by Thrustmaster. The software uses the AT and T C++ translator and the Silicon Graphics C compiler running under Irix 4.0.5. a Unix operating system. We use Software Systems' MultiGen to create the geometric models and the terrain database.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

A Prototype Visual and Audio Display

A display is described that provides a three-dimensional perspective view with spatially correlated audio. The system is developed around an optics device that projects a three-dimensional perspective view from a CRT to a concave mirror that focuses the energy at an image plane above the mirror. The result is that the objects displayed on the CRT appear to be floating in space. The directional audio is provided from an audio localization cue synthesizer that encodes pinna filtering and an interaural time delay onto an input audio signal. A magnetic head tracker is used to keep the audio images stable. The optical system is presented along with the graphics methods that were used to generate the visual cues. Then the audio localization cue synthesizer is described with emphasis on using this device to provide spatialized audio for use in synthetic environments.