Effects of background field-of-view and depth-plane on the oculogyral illusion.

This study examined the effects of background field-of-view and depth plane on the oculogyral illusion. Seven subjects viewed a stationary fixation stimulus during the postrotatory interval following a 45-sec constant-velocity chair rotation. The duration of the illusory movement of the fixation stimulus during the postrotatory interval was measured, along with the duration of the illusion of whole-body rotation (known as the somatogyral illusion) and the duration of the subject's slow-phase vestibular nystagmus. Subjects viewed the fixation stimulus by itself in a No-background condition or when surrounded by six background fields formed by the combination of two fields-of-view (35 degrees and 115 degrees) and three depth-planes (near, coplanar, and far). The different background fields inhibited the oculogyral illusion relative to the No-background condition but did not differ statistically from each other. The somatogyral durations better matched the oculogyral ones than did nystagmus decay, especially when a background field was present. These results suggest that the oculogyral illusion is more related to the experience of whole-body rotation than to oculomotor mechanisms and that the inhibitory effect of a background scene is only modestly affected by its field-of-view and depth plane.

Cost effectiveness of human immunodeficiency virus postexposure prophylaxis for healthcare workers.

OBJECTIVE: The United States Public Health Service (USPHS) published recommendations for human immunodeficiency virus (HIV) postexposure prophylaxis (PEP) of healthcare workers in May 1998. The aim of this study was to analyse the cost effectiveness of the USPHS PEP guidelines. DESIGN AND SETTING: This was a modelling study in the setting of the US healthcare system in 1989. The analysis was performed from the societal perspective; however, only HIV healthcare costs were considered and health-related losses of productivity were not included. METHODS: A decision tree incorporating a Markov model was created for 4 PEP strategies: the current USPHS recommendations, triple drug therapy, zidovudine monotherapy or no prophylaxis. A probabilistic sensitivity analysis using a Monte Carlo simulation was performed. Confidence intervals (CIs) around cost-effectiveness estimates were estimated by a bootstrapping method. RESULTS: The costs (in 1997 US dollars) per quality-adjusted life-year (QALY) save by each strategy were as follows: monotherapy $US688 (95% CI: $US624 to $US750); USPHS recommendations $US5211 (95% CI: $US5126 to $US5293); and triple drug therapy $US8827 (95% CI: $US8715 to $US8940). The marginal cost per year of life saved was: USPHS recommendations $US81 987 (95% CI: $US80 437 to $US83 689); triple drug therapy $US970 451 (95% CI: $US924 786 to $US 1 014 429). Sensitivity testing showed that estimates of the probability of seroconversion for each category of exposure were most influential, but did not change the order of strategies in the baseline analysis. With the prolonged HIV stage durations and increased costs associated with recent innovations in HIV therapy, the marginal cost effectiveness of the USPHS PEP strategy was decreased to $US62 497/QALY saved. All 3 intervention strategies were cost effective compared with no postexposure prophylaxis. CONCLUSIONS: Current USPHS PEP recommendations are marginally cost effective compared with monotherapy, but the additional efficacy of triple drug therapy for all risk categories is rewarded by only a small reduction in HIV infections at great expense. For the foreseeable future, assuming innovations in therapy that employ expensive drug combinations earlier in the HIV disease course to extend life expectancy and the increasing prevalence of HIV drug resistance, our model supports the use of the USPHS PEP guidelines.

Application of robust data processing methods to the analysis of eye movements.

We have developed a robust, nonlinear differentiating digital filter for the estimation of eye velocity from an eye-position signal. This filter is equally applicable to a large variety of other biomedical signals. The filter is implemented by taking an odd number of two-point differences around the point of interest and then selecting the median difference. Dividing the median difference by the time interval yields the derivative of the input signal. The Robust Differentiator (RD) is a classic order-statistic filter consisting of a bank of linear filters (the two-point differences) followed by the median operation to select one difference for the derivative calculation. The RD has no impulse response function and eliminates the "ringing" that is typical of all linear filters. The RD not only performs better than the FIR differentiating filters for impulse noise but is equally effective for both broad-band and narrow-band Gaussian noise. The RD "bandwidth" is adjustable by selecting the interval for the two-point differences.

Eye tracking performance variability in a homogeneous population.

Pursuit tracking eye movements were recorded and analyzed from a group of US Air Force Pilot Candidates (PCs). The PCs ranged in age from 21 to 27 with a median age of 23. All were college graduates and recently passed a Flying Class I physical exam. These PCs comprise a highly motivated, intelligent group of young subjects. Pursuit tracking was assessed by having the subjects track a small spot of green light moving sinusoidally in the horizontal plane at frequencies from 0.2 to 1.0 Hz in 0.2 Hz increments. Peak-to-peak target amplitude was 40 degrees. Eye movements were recorded using an infrared reflectance device. Eye movements were separated into smooth pursuit (SP) and saccadic (SA) components. Tracking performance was evaluated by computing the gain and asymmetry of the SP component and the percentage of tracking movements contributed by the SA component. Both mean values and variance of the tracking performance of the PCs were not found to be statistically different from a group consisting of both flying and nonflying Air Force personnel.

The application of smooth pursuit eye movement analysis to clinical medicine.

Pursuit tracking eye movements were analyzed from selected patients with neurological injuries and compared to the responses of 20 normal subjects. The patients/subjects tracked a small spot of light moving sinusoidally in the horizontal plane at a frequency of 0.4 Hz and a peak-to-peak amplitude of 40 degrees. Eye-movement responses were separated into a smooth-pursuit component and a saccadic component. The smooth-pursuit component was analyzed by calculating the gain, phase, and asymmetry. The saccadic component was quantified by calculating the percentage of the total tracking movement contributed by the saccadic system. The patients with smooth-pursuit impairment exhibited a higher percentage of saccadic tracking and a lower smooth pursuit gain compared to the normal subjects. One patient with a unilateral lesion exhibited significant asymmetry in the smooth-pursuit component. In this case, the direction of the asymmetry indicated the side of the lesion.

Analysis of pursuit tracking eye movements in pilots and nonfliers.

Pursuit tracking eye movements were recorded and analyzed from a group of Air Force pilots and a group of nonflying Air Force members. The tracking performance of the pilots was compared to the performance of the nonfliers. Subjects tracked a small spot of light moving sinusoidally in the horizontal plane at frequencies ranging from 0.2 to 1.0 Hz at a peak-to-peak amplitude of 40 degrees. Maximum target velocities ranged from 25 to 126 degrees/s. An adaptive nonlinear digital filter was used to separate the total tracking response (TTR) into smooth pursuit (SP) and saccadic (SA) components. Frequency domain analysis was used to relate the tracking components to the target movement. There were no statistically significant differences in tracking performance between the pilots and nonfliers. When tracking the 0.2 and 0.4 Hz targets, the TTR consisted principally of SP tracking with SA tracking representing less than 11% of the TTR. As the target velocity increased, the TTR remained adequate but the proportion of SP tracking decreased while the SA tracking increased. Over one-half of the TTR is contributed by the SA system when tracking the 1.0 Hz targets for both the pilot and nonpilot groups.

Linear systems analysis of the vestibulo-ocular reflex: clinical applications.

We used whole-body angular acceleration stimuli to estimate the transfer function of the vestibuloocular reflex in 20 normal subjects and several patients. Eye movements evoked by the stimuli were recorded and an adaptive nonlinear digital filter was used to extract the compensatory component of the eye-movement response. Frequency domain analysis of the stimulus and the compensatory component of the response was used to estimate the transfer function. The transfer function was estimated at 6 discrete frequencies (0.01 to 0.32 Hz by octaves) using individual sine-wave stimuli. Data from several patients with known lesions affecting the vestibular system were compared to the normal responses. Patients with a unilateral vestibular loss demonstrated low gains at frequencies below 0.02 Hz and large phase shifts below 0.32 Hz, but had normal responses at 0.32 Hz. Some of these same patients also exhibited a significant directional asymmetry in their responses.

Selecting a stimulus signal for linear systems analysis of the vestibulo-ocular reflex.

We evaluated 3 types of stimulus signals for use in estimating the transfer function of the vestibulo-ocular reflex. We used individual sine-wave, sum-of-sine, and pseudorandom stimuli. Five normal human subjects were tested 5 times each using each of the 3 stimulus types. Frequency domain techniques were used to estimate the transfer function at 0.01, 0.03, and 0.05 Hz. The most consistent estimates were obtained using individual sine-wave stimuli. The pseudorandom signal yielded the most variable estimates. A sum-of-sine stimulus composed of 3 sine-wave signals provided estimates slightly more variable than the individual sine-wave stimuli, but much less variable than the pseudorandom stimulus. The redundency of the sine and sum-of-sine stimuli seems to be an advantage by providing stable estimates of the transfer function in the presence of noise.

Relationships between manual reaction time and saccade latency in response to visual and auditory stimuli.

Manual reaction time (RT) responses were analyzed from seven human subjects. Responses were recorded using four kinds of target presentations: fixed visual target, moving visual target, fixed auditory target, and moving auditory target. Moving targets (moving in the horizontal plane) were presented at constant intensity and provided only a motion cue. Fixed targets "popped on" at the primary position and provided only an onset cue. RTs for the fixed and moving visual targets were 241.5 ms and 233.1 ms, respectively. The 8.4 ms (3.5%) advantage for the moving visual target over the fixed visual target was statistically significant, p less than 0.05. RT for the moving auditory target varied with target movement amplitude and ranged from 219 ms for 40 degree movements to 268 ms for 5 degree movements. For the fixed auditory target in the sagittal plane, average RT was 182.9 ms. Thus, sound-source motion detection was from 36 to 85 ms slower than sound onset detection, p less than 0.001. The RT results were compared to saccade latency measurements from an earlier study. Both RT and saccade latency showed the same dependency upon target movement amplitude. For small target displacements, saccade latencies for the moving auditory target were longer than for the moving visual target. The longer latencies for the moving auditory target are attributed to the increased processing time required to detect the sound-source motion.

Optimization of an adaptive nonlinear filter for the analysis of nystagmus.

An adaptive nonlinear digital filter has been designed for the analysis of an eye-movement signal called nystagmus. Nystagmus is a bi-phasic signal consisting of a sequence of tracking eye movements called "slow-phase" interspersed with brief, high-velocity refixation movements called "fast-phase." The objective of the analysis is to separate the nystagmus signal into its fast- and slow-phase components. Specifically, the goal is to produce an evenly sampled estimate of slow-phase velocity (SPV) and an estimate of the peak fast-phase velocity. Classically this has been done using pattern recognition methods that exploit the fact that the fast-phase is a relatively short duration, high-velocity movement compared to the slow-phase. Unfortunately, these velocity and duration differences do not reliably separate the slow- and fast-phases under all conditions, especially when the signal is noisy. We have designed and built an adaptive nonlinear digital filter that easily outperforms the more complex pattern recognition algorithms. This new filter, called an Adaptive Asymmetrically Trimmed-Mean (AATM) filter, works under the assumption that, on the average, the eyes spend more time in slow-phase than in fast-phase. Thus, in any given data segment, most of the data samples are slow-phase samples. By analyzing the amplitude distribution of the data samples in the segment we can determine which of these samples are slow-phase. We used computer generated nystagmus signals contaminated with 3 levels of noise to evaluate the filter. The filter parameters were then optimized using Monte Carlo procedures producing an extremely robust analysis method.

A new approach to the analysis of nystagmus: an application for order-statistic filters.

A computer program has been designed for the analysis of nystagmus. This program employs a class of nonlinear digital filters called order-statistic (OS) filters. Two OS filters and one linear filter are used. First, the eye-movement signal is smoothed using a predictive finite-impulse response (FIR), median hybrid filter. Then the smoothed signal is processed by a linear band-limited differentiating filter to calculate eye velocity. And finally, the slow-phase velocity (SPV) envelope is extracted from the eye-velocity signal using an adaptive asymmetrically trimmed-mean filter. This approach yields an evenly sampled SPV estimate without resorting to the various interpolation or extrapolation schemes generally used. The adaptive filter estimates SPV based on the local statistical properties of the eye-velocity signal. The adaptive strategy works under the assumption that, on the average, the eyes spend more time in slow-phase than in fast-phase. No assumptions are made about the direction of the nystagmus or the nature of the stimulus used to elicit the nystagmus. This method eliminates all the usual threshold tests and decision logic common to other nystagmus analysis programs. The robust performance of OS filters and the use of adaptive filter structures totally eliminates the need to custom "tune" the program parameters for atypical data sets.

Development of a non-linear smoothing filter for the processing of eye-movement signals.

The analysis of eye-movement (EM) signals poses problems for the designer of smoothing filters since many of the interesting types of EMs are bimodal. For example, optokinetic and/or vestibular stimulation results in an EM pattern called nystagmus consisting of alternating fast- and slow-phase components. Also, saccadic (refixation) EMs do not occur continuously, but are interspersed with periods of fixation. Conventional linear, low-pass filters (both finite impulse response (FIR) and infinite impulse response (IIR) types) smear the boundries between the fast- and slow-phases of nystagmus and the fixation and fast components of saccadic EMs. We have adapted a nonlinear smoothing filter (originally designed to optimize edge preservation in image processing applications) for the smoothing of EM signals. This filter is called a Predictive FIR-Median Hybrid (PFMH) filter. The PFMH filter operates on a moving window of data samples centered at the current point of interest. Several predictive FIR filters are applied to the "upper" and "lower" halves of the window and each are designed to predict the sample value at the center of the window. The median of these FIR filter outputs and the actual center data sample are taken as the PFMH filter output for each window position. By properly choosing the length and structure of the FIR subfilters, a PFMH filter can be designed to smooth a bimodal EM signal without blurring the boundries between the two signal components.

Saccadic eye movements in response to visual, auditory, and bisensory stimuli.

Saccadic eye movements were recorded and analyzed from eight normal human subjects. Various visual, auditory, and bisensory (visual and auditory) targets were tracked. Primary saccade latency, amplitude, duration, and peak velocity were calculated, as well as overall saccade duration (total time spent making saccades) and final eye position. Saccades made to bisensory targets employing a constant-intensity auditory component were not different from the pure visual target responses. Saccades to bisensory targets having an intermittent auditory component (with sound onset synchronous with the visual component) demonstrated a significant reduction in latency (11.3%) compared to the visual responses. The reduction occurred both for a fixed overhead sound source and for a sound source moving with the visual component. This result indicates that providing an auditory motion or localization cue alone does not reduce latency, but that a sound onset cue facilitates response time. No other response parameters were enhanced by using bisensory targets.

Computer analysis of smooth pursuit eye movements.

Smooth-pursuit eye movements are analyzed using a system analysis approach. Frequency domain methods are used to factor the smooth-pursuit response into a linear component and a nonlinear or remnant component. The linear component is that part of the tracking response that is linearly related to the target movement; the remnant is the component remaining after the linear component is subtracted out. Tracking abnormalities that are not evident by inspection of the complete eye-movement record become obvious upon inspection of the remnant.

Single sinusoids compared with a multiple-sinusoids technique for evaluating horizontal semicircular canal function.

The vestibulo-ocular reflex (VOR) of 162 normal subjects was evaluated using two rotational testing procedures. Data from 79 subjects were collected using a set of five single-frequency stimuli (0.01 to 0.16 Hz) and 83 subjects were tested using multifrequency stimuli (0.01 to 0.27 Hz). The use of multifrequency stimuli in place of single frequencies reduced testing time from 75 min to 20 min per subject. The diagnostically useful phase response of the VOR, as estimated by the two tests, were not statistically different. Equivalent phase measures may be obtained within a shorter testing period using the multifrequency procedure.