Eye-movement responses to disparity vergence stimuli with artificial monocular scotomas.

The effects of artificial monocular scotomas on eye-movement responses to horizontal disparity vergence stimuli were studied in six subjects with normal binocular vision. Subjects viewed stereoscopic 1.5 degrees horizontal step disparity vergence stimuli through liquid crystal shutter glasses. The central portion of the stimulus presented to the right eye was removed to simulate monocular artificial scotomas of variable diameters (2 degrees to 10 degrees ). Eye movements were recorded with a binocular head-mounted eye tracker. Responses included pure vergence, vergence followed by saccades, and pure saccadic eye movements. The rate of responses with saccadic eye movements increased with the diameter of the artificial scotoma (p < 0.0001); there was an increase in the rate of responses starting with saccades (p < 0.0001), as well as an increase in the rate of saccades after initial vergence responses (p < 0.01). The probability of saccades after initial vergence responses was affected by the open-loop gain of the vergence response (p < 0.001). The open-loop gain decreased with increased diameters of the artificial scotomas (p < 0.0001). As the diameter of the artificial scotomas increased, the amplitude of the initial vergence eye-movement responses decreased, and the prevalence of saccadic eye movements and asymmetric vergence increased. The effects of the diameter of artificial monocular scotomas on eye-movement responses in subjects with normal binocular vision are consistent with the effects of diameter of suppression scotomas on eye-movement responses to disparity vergence stimuli in patients with infantile esotropia.

Saccades in children with spina bifida and Chiari type II malformation.

BACKGROUND: Saccades are essential for optimal visual function. Chiari type II malformation (CII) is a congenital anomaly of the cerebellum and brainstem, associated with spina bifida. OBJECTIVE: To investigate the effects of CII on saccades and correlate saccadic parameters with brain MRI measurements. METHODS: Saccades were recorded in 21 participants with CII, aged 8 to 19, using an infrared eye tracker. Thirty-nine typically developing children served as controls. Participants made saccades to horizontal and vertical target steps. Nineteen participants with CII had MRI. Regression analyses were used to investigate the effects of spinal lesion level, number of shunt revisions, presence of nystagmus, and midsagittal MRI measurements on saccades. RESULTS: Saccadic amplitude gains, asymptotic peak velocities, and latencies did not differ between the control and CII groups (p > 0.01). No significant differences were found between saccadic gains, asymptotic peak velocities or latencies, and spinal lesion level, number of shunt revisions, presence of nystagmus, or MRI measurements. CONCLUSIONS: Saccades were normal in most participants with Chiari II malformation (CII). Neural coding of saccades is robust and is typically not affected by the anatomic deformity of CII.

Optimal structure for automatic processing of DNA sequences.

The faithful recovery of the base sequence in automatic DeoxyriboNucleic Acid (DNA) sequencing fundamentally depends on the underlying statistics of the DNA electrophoresis time series. Current DNA sequencing algorithms are heuristic in nature and modest in their use of statistical information. In this paper, a formal statistical model of the DNA time series is presented and then used to construct the optimal maximum-likelihood (ML) processor. The DNA-ML algorithm that is derived in this paper features Kalman prediction of peak locations, peak parameter estimation, whitened waveform comparison and multiple hypothesis processing using the M-algorithm. Properties of the algorithm are examined using both simulated and real data. Model parameters of critical importance and their impact on different types of error mechanisms, such as insertions and deletions, are pointed out. The statistical model of the DNA time-series and the structure of the DNA-ML algorithm provides a basis for future investigation and refinement of DNA sequencing techniques.

Models of local behavior of DNA electrophoresis peak parameters.

Many base calling algorithms implicitly or explicitly rely on predictions of local sequence parameters such as amplitude, peak time and peak width. For example, an algorithm may search for the next peak about a predicted peak time formed by adding the mean peak separation to the last position measurement. In this paper, covariance models are presented which characterize the dependence of peak parameters on those of other peaks. Based on experimental measurements, the model features an exponential decay in peak time jitter covariance with respect to base separation. Both peak amplitude and peak width are modelled as being uncorrelated with those of adjacent bases. In the model, linear expressions are given to describe the growth in peak time jitter and peak width as a function of base position while other parameters, such as amplitude variance, are modeled by constants. Together, these results form a simple model which may be used in the derivation of new sequencing algorithms or in simulations for the testing of such algorithms. We suggest that the correlation of the peak times is related to the Kuhn length of the single-stranded DNA fragments.

Electrophysiological evidence of cortical fusion in children with early-onset esotropia.

PURPOSE: To investigate sensory fusion responses in infants and children with early-onset esotropia to gain insights into the sequence of events that leads to strabismus. METHODS: Sensory fusion was tested by measuring visual evoked potential (VEP) responses to dynamic random dot correlograms (DRDCs) in a group of children (n = 23) with early-onset esotropia. Thirteen children were tested before surgical alignment, and 13 children were tested after surgical alignment (three children were tested before and after surgery). If the angle of strabismus was larger than 5 prism diopters, it was corrected with Fresnel prisms (Fresnel Prism and Lens, Scottsdale, AZ). RESULTS: Five (38%) of the 13 children who were tested before surgery showed detectable VEP responses to correlogram stimuli compared with 11 (85%) of the 13 children who were tested after surgical alignment. There were no significant statistical differences between VEP responses to DRDCs from the postsurgery group and VEP responses from an age-matched control group with normal binocular vision. CONCLUSIONS: The presence of cortical sensory fusion in children with early-onset esotropia suggests that a congenital defect of sensory fusion cannot be the root cause of esotropia in most children. The data suggest that sensory fusion, when measured by VEP responses to DRDCs, is more robust than stereopsis to abnormal binocular experience and support the notion that pathways processing correlated/anticorrelated stimuli may not completely overlap with pathways processing disparity information.

Cortical binocularity and monocular optokinetic asymmetry in early-onset esotropia.

PURPOSE: To investigate the correlation between directional asymmetry in ocular responses to monocularly viewed optokinetic stimuli (monocular optokinetic nystagmus, MOKN) and sensory fusion in infants and toddlers with early-onset esotropia. METHODS: Subjects were 14 infants and toddlers with early-onset esotropia (7-26 months old; median, 10 months), and 16 with no esotropia (6-22 months; median, 11 months) who provided control data. Monocular optokinetic nystagmus in response to a 30 degrees/sec square-wave grating (0.25 cycles/degree) was measured by electro-oculogram. Sensory fusion was assessed with visual evoked potentials (VEPs) to random-dot correlograms after correction of the strabismus angle with Fresnel prisms. RESULTS: All subjects with early-onset esotropia had MOKN with a faster slow-phase component for temporal-to-nasalward (TN) than nasal-to-temporalward (NT) motion. Ninety-three percent of subjects had MOKN asymmetry higher than the 95th percentile of the control group. Of subjects who cooperated with VEP fusion testing, 5 subjects with early-onset esotropia (45%) and 11 control subjects (92%) showed evidence of sensory fusion. CONCLUSIONS: Symmetrical MOKN did not develop in infants and toddlers with early-onset esotropia. This deficit existed in most infants who showed sensory- cortical fusion. These results are consistent with the belief that optokinetic nystagmus asymmetry may not be associated with a deficit in the cortical fusion facility, but rather with deficits in binocular pathways projecting to MOKN control centers. These deficits may be associated with abnormal processing subsequent to sensory fusion or with abnormal processing in motion pathways, which run parallel to sensory fusion pathways.

Oculomotor response to linear acceleration as induced by counter-rotation in supine subjects.

BACKGROUND: Horizontal nystagmus occurs in response to sinusoidal linear accelerations directed along an upright subject's Y (interaural) axis, and is proposed to be mediated by an utricular otolith mechanism. HYPOTHESIS: The otolith organs, composed of the utricles and saccules, provide a unique set of signals for any linear acceleration in 3-dimensional space. A supine subject under alternate changing directions of linear acceleration as induced by counter-rotation will receive alternate stimulation along the Y as well as the Z (dorsoventral) axis. We hypothesized that alternate horizontal and vertical nystagmus would be elicited as a result of the changing direction of linear acceleration. METHODS: A group of eight subjects in the supine position were exposed to counter-rotation at 0.16, 0.25, and 0.33 Hz. Vertical and horizontal eye movements were recorded simultaneously using the El-Mar eye and head tracking system. RESULTS: Horizontal nystagmus was observed in all supine subjects. The direction of the slow phase of nystagmus changed with directional changes in linear acceleration. Reversals in the direction of eye movements lagged behind the reversals in the direction of the acceleration. However, only two subjects exhibited alternating horizontal and vertical nystagmus as a result of changing axis of linear acceleration, from "along the Y axis" to "along the Z axis." CONCLUSION: We propose that the nystagmus induced in the supine subject was provoked by linear acceleration and largely an otolith-mediated reflex. The lack of vertical response could be due to the relative paucity of vestibular afferents information along the dorsoventral axis.

Vestibulo-ocular reflex deficits to rapid head turns following intratympanic gentamicin instillation.

The response of the vestibulo-ocular reflex following unilateral vestibular deafferentation by gentamicin ablation was studied using transient stimuli. The response to these rapid passive head turns showed a strong asymmetry with permanent, reduced gains toward the side of lesion. These gain reductions have large variation (gains of 0.26 to 0.83), which may result from preferential sparing of regularly firing afferent fibers following gentamicin ablation. Based on the size and nature of the nonlinearity, an explanation based on Ewald's second law was discounted.

Vertical eye position control in darkness: orbital position and body orientation interact to modulate drift velocity.

How stable is vertical eye-in-head position control in darkness when no visual targets are present? We evaluated this while varying both body-in-space orientation and eye-in-orbit position in six subjects who were free from oculomotor/vestibular disease. Vertical eye movements were monitored using a CCD-video tracking system, and results were confirmed on one subject with the magnetic search coil. Three body orientations were used: (1) seated upright; (2) supine; and (3) prone. In each of these body orientations starting eye-in-orbit position was varied in quasi-random order from -20 to +20 deg, while vertical eye drift was monitored for a 90 sec period at each position. Subjects were instructed to hold their eyes as steady as possible. The relationship between body orientation/eye position and vertical eye drift velocity was examined using a linear regression technique. In contrast to prior clinical reports, normals exhibit a vertical nystagmus/drift in darkness. Moreover, slow-phase eye velocity was found to be dependent on eye-in-orbit position in the upright and supine body orientations. This pattern of eye drift mirrors Alexander's Law, with significantly increased drift velocities when subjects looked in the direction of their re-centering saccades (P < 0.05 or better). Body-in-space orientation also modulated the eye drift velocity, with significant differences in rate of eye drift (P < 0.05 or better) between extremes of body orientation (supine and prone) for five out of six subjects. The stability of the vertical oculomotor control system in the absence of visual input is strongly affected by body-in-space orientation and eye-in-orbit position: manipulating either of these variables results in non-random patterns of drift. These results are discussed using a multiple-input model of vertical eye-in-head position control.

Predictive head movement tracking using a Kalman filter.

The use of head movements in control applications leaves the hands free for other tasks and utilizes the mobility of the head to acquire and track targets over a wide field of view. We present the results of applying a Kalman filter to generate prediction estimates for tracking head positions. A simple kinematics approach based on the assumption of a piecewise constant acceleration process is suggested and is shown to track head positions with an rms error under 2 degrees for head movements with accelerations smaller than 3000 degrees /s. To account for the wide range of head dynamic characteristics, an adaptive approach with input estimation is developed. The performance of the Kalman filter is compared to that based on a simple polynomial predictor.

Combined head and eye tracking system for dynamic testing of the vestibular system.

We present a combined head-eye tracking system suitable for use with free head movement during natural activities. This system provides an integrated head and eye position measurement while allowing for a large range of head movement (approx 1.8 m of head translation is tolerated). Six degrees of freedom of head motion and two degrees of freedom of eye motion are measured by the system. The system was designed to be useful for the evaluation of the vestibulo-ocular reflex (VOR). The VOR generates compensatory eye movements in order to stabilize gaze during linear or rotational motion of the head. Current clinical and basic research evaluation of the VOR has used a restricted range of head motion, mainly low-frequency, yaw rotation. An integrated eye-head tracking system such as the one presented here allows the VOR response to linear and angular head motion to be studied in a more physiologically relevant manner. Two examples of the utility of the integrated head and eye tracking system in evaluating the vestibular response to linear and angular motion are presented.

A new VEP system for studying binocular single vision in human infants.

Visual evoked potentials (VEPs), that provide unequivocal objective evidence of cortical binocularity have been recorded from adults and young infants using a new VEP system developed for this purpose. The system uses alternating field stereoscopy (AFS) to present separate visual stimuli to each eye. With this system, the binocular image pairs to the right and left eyes alternate at a high rate on a single video monitor. The subject wears spectacles incorporating light-scattering liquid crystal lenses which alternate electronically between opaque and clear modes in synchrony with the video monitor. To detect cortical binocularity, the system analyzes VEP activity mathematically and identifies significant responses at test frequencies reflecting binocular cortical interactions exclusively. Three types of binocular stimuli were presented: (1) dynamic random dot correlograms (correlograms); (2) dynamic random dot stereograms (stereograms); and (3) dichoptic checkerboard stimuli. The correlograms are generated when moving random dot patterns presented to each eye alternate between two phases, correlated and anticorrelated. With the stereograms, portions of random dot patterns presented to each eye are shifted horizontally relative to each other at a fixed rate, alternately producing crossed and uncrossed binocular disparities. Subjectively, these patterns appear to shift in depth. Dichoptic checkerboard stimuli are regular checkerboard patterns which reverse at different rates (frequencies) for each eye. Binocular VEPs are generated due to cortical interactions at the difference (beat) frequency. Using this VEP system, we have recorded binocular VEPs from 10 normal adults and more than 40 infant subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Stability of fixation in healthy subjects during automated perimetry.

The stability of fixation of 12 healthy young subjects was assessed with a high-resolution eye tracker during automated perimetry. All the subjects exhibited eye movements and fixation shifts during the examination. The average shift in eye position between successive 1-minute intervals ranged from 0.41 degrees to 2.11 degrees, and the average standard deviation of eye position around the average of each interval ranged from 0.56 degrees to 2.55 degrees. In nine of the subjects the eye remained within 3 degrees of centre for 96% or more of the stimuli. The remaining three subjects experienced difficulty maintaining central fixation: the eye was more than 3 degrees away from centre for 28% or more of the stimuli. In the latter subjects the ability to repeatedly stimulate the intended retinal test locations was substantially reduced. The authors discuss the effects of unstable fixation on short-term fluctuation and on fixation loss.

End-point nystagmus and ocular drift: an experimental and theoretical study.

End-point nystagmus (EPN) and ocular drift during eccentric fixation were investigated in five normal subjects. All had EPN during prolonged eccentric fixation, two had sustained EPN within a few seconds of fixating an eccentric target. Our data show that the occurrence of EPN was determined by the velocity of slow phase ocular drift. Drift velocities greater than 1 deg/sec elicited sustained EPN or fatigue EPN while drift velocities lower than 0.3 deg/sec did not elicit EPN. Slow phase drift velocity increased with eccentricity and fixation duration and decreased with visual feedback. Computer simulations and theoretical analysis suggest that the reduction in drift eye movement velocity during fixation of a visible target, as compared to drift eye movement velocity during attempted fixation of a remembered target in darkness, is mainly due to the smooth pursuit system.

Eye movement perimetry in glaucoma.

Present-day computerized perimetry is often inaccurate and unreliable owing to the need to maintain central fixation over long periods while repressing the normal response to presentation of peripheral stimuli. We tested a new method of perimetry that does not require prolonged central fixation. During this test eye movements were encouraged on presentation of a peripheral target. Twenty-three eyes were studied with an Octopus perimeter, with a technician monitoring eye movements. The sensitivity was 100% and the specificity 23%. The low specificity was due to the technician's inability to accurately monitor small eye movements in the central 6 degrees field. If small eye movements are monitored accurately with an eye tracker, eye movement perimetry could become an alternative method to standard perimetry.

Power spectra for ocular drift and tremor.

Eye position during fixation has been measured without contact and the power spectrum of drift and tremor eye movements has been estimated. In the 0-40 Hz frequency range power declines with frequency roughly as 1/f2. In the 40-100 Hz frequency range the dominant spectral component is a broad spectral peak with a peak amplitude of about 6 arc sec. The shape of the broad spectral peak is dependent on the fixation direction. A model that explains qualitatively the shape of the power spectra function is presented. It is suggested that tremor eye movements are by-products of the clock-like firing of motor neurons.

Precise non-contacting measurement of eye movements using the corneal reflex.

Using a real-time, analog, zero-crossing technique, it is possible to locate the centre of the corneal reflex of a collimated infra-red beam accurately. Performance at tracking horizontal eye movements in this manner is similar to contact lens techniques. System noise is less than 30 are sec; dynamic range is 30 arc sec to 36 deg; linearity is 2%; data are currently sampled at 1 kHz; velocity resolution, for a velocity bandwidth of 125 Hz, is 2 deg/sec. With automatic acquisition of the corneal reflex, the subject can be set up for measurement within a 1 min time period. Recordings of residual eye movements of fixation and of a wide range of saccades are comparable with previous findings. Post-saccadic oscillations can be resolved for all sizes of saccades. The amplitude-peak-velocity characteristic of saccades is illustrated.