Identifying jitter outliers in single fiber electromyography: Comparison of four methods.

BACKGROUND: Little is known about how different outlier estimation methods affect cutoff limits for outliers in single fiber electromyography. METHODS: We compared in a prospective fashion the established 18th jitter value (18thjv) method to three, whole-distribution based, outlier detection methods: the interquartile range (IQR), the log-normal, and the Z-score method. The reference limits were probed in a normal cohort and in myasthenia gravis (MG) patients. RESULTS: Differences in outlier cutoff values between the different methods were in the range of 2 µs. The number of abnormal muscles according to the computed criteria was similar for all four methods in the control group. Classification metrics (sensitivity, specificity, Youden's statistic, and predictive values) were also similar among the different methods. In the MG group, however, the Z-score method failed to identify the abnormal jitter values. Accordingly, Kappa agreement was substantial to perfect (0.658 to 1) between the three methods (18thjv, IQR, and log-normal), but was equivalent to chance between the three methods and the Z-score in the MG group. CONCLUSIONS: The established 18thjv method proved largely robust when compared to whole-distribution based methods, and its use in clinical practice is justified. Simple estimation of outlier limits by adding two SDs to the mean of the data, leads to unacceptable deviations from the true cutoff values. Moreover, in a clinical scenario in which the final electrodiagnosis depends only on the number of outliers, it is meaningful to accept a tolerance zone of about 2 µs, which is the approximate variation range among the different methods.

Fast gaze reorientations by combined movements of the eye, head, trunk and lower extremities.

Large reorientations of the line of sight, involving combined rotations of the eyes, head, trunk and lower extremities, are executed either as fast single-step or as slow multiple-step gaze transfers. In order to obtain more insight into the mechanisms of gaze and multisegmental movement control, we have investigated time-optimal gaze shifts (i.e. with the instruction to move as fast as possible) during voluntary whole-body rotations to remembered targets up to 180° eccentricity performed by standing healthy humans in darkness. Fast, accurate, single-step movement patterns occurred in approximately 70 % of trials, i.e. considerably more frequently than in previous studies with the instruction to turn at freely chosen speed (30 %). Head-in-space velocity in these cases was significantly higher than during multiple-step transfers and displayed a conspicuously regular bell-shaped profile, increasing smoothly to a peak and then decreasing slowly until realignment with the target. Head-in-space acceleration was on average not different during reorientations to the different target eccentricities. In contrast, head-in-space velocity increased with target eccentricity due to the longer duration of the acceleration phase implemented during trials to more distant targets. Eye saccade amplitude approached the eye-in-orbit mechanical limit and was unrelated to eye/head velocity, duration or target eccentricity. Overall, the combined movement was stereotyped such that the first two principal components accounted for data variance almost up to gaze shift end, suggesting that the three mechanical degrees of freedom under consideration (eye-in-orbit, head-on-trunk and trunk-in-space) are on average reduced to two kinematic degrees of freedom (i.e. eye, head-in-space). Synchronous EMG activity in the anterior tibial and gastrocnemius muscles preceded the onset of eye rotation. Since the magnitude and timing of peak head-in-space velocity were scaled with target eccentricity and because head-on-trunk and trunk-in-space displacements were on average linearly correlated, we propose a separate controller for head-in-space movement, whereas the movement of the eye-in-space may be, in contrast, governed by global, i.e. gaze feedback. The rapid progression of the line of sight can be sustained, and the reactivation of the vestibulo-ocular reflex would be postponed, until gaze error approaches zero only in association with a strong head-in-space neural control signal.

Kinematic redundancy and variance of eye, head and trunk displacements during large horizontal gaze reorientations in standing humans.

Shifting the direction of the line of sight in everyday life often involves rotations not only of the eyes and head but also of the trunk. Here, we investigated covariation patterns of eye-in-orbit, head-on-trunk and trunk-in-space angular horizontal displacements during whole-body rotations to targets of up to 180 degrees eccentricity performed by standing healthy human subjects. The spatial covariation was quantified statistically across various behavioral task conditions (unpredictable, memory driven predictable, visual feedback) and constraints (accuracy) by principal components (PC) analysis. Overall, the combined movement was stereotyped such that the first two PCs accounted for essentially the whole data variance of combined gaze transfers up to about 400 ms, suggesting that the three mechanical degrees of freedom under consideration are reduced to two kinematic degrees of freedom. Moreover, quantification of segment velocity variability across repetitions showed that velocities of eye-in-space and head-in-space (i.e. 'end-point' velocity) were less variable than those of the elemental variables composing them. In contrast, three statistically significant PCs accounted for the covariation of the three segments during presumably vestibularly mediated nystagmic transfers, suggesting control by a separate driving circuit. We conclude that progression of the line of sight is initially stereotypic and fulfills criteria defining a motor synergy.

Spatial Analysis of Corneal Densitometry, Thickness Profile, and Volume Distribution After Uneventful Descemet Membrane Endothelial Keratoplasty.

PURPOSE: To present normative corneal densitometry (CD), thickness (CT), and volume (CV) after uneventful Descemet membrane endothelial keratoplasty (DMEK). METHODS: Medical records of 212 consecutive surgeries were reviewed. One hundred cases without intraoperative and postoperative complications were included (group 1). Scheimpflug CD of the optically relevant 0- to 2-mm zone and 2- to 6-mm annulus, CT at the 2-, 4-, and 6-mm rings, and CV at the 3-, 5-, and 7-mm zones were evaluated preoperatively and at 3, 6, 12, and 24 months postoperatively. The 24-month results were compared with an age-matched group of uncomplicated pseudophakic eyes (group 2; n = 20) and a group of healthy, young subjects (group 3; n = 30). RESULTS: Total CD at 0 to 2 mm was 33.9 ± 10.7 grayscale units preoperatively and decreased to 20.3 ± 4.7 at 24 months (P < 0.001), being significantly higher compared with both control groups (P ≤ 0.026). Total CD at 2 to 6 mm was 27.9 ± 8.1 preoperatively and decreased to 20.2 ± 4.6 at 24 months (P < 0.001), showing no statistically significant difference compared with group 2 (P = 0.061). At 24 months, group 1 showed a similar central CT, CT at the 2-mm and 4-mm rings compared with group 2 (P ≥ 0.440); however, CT at 6 mm increased significantly (P = 0.008). CV at 3, 5, and 7 m decreased significantly at 24 months (P < 0.001), showing no significant differences compared with both control groups (P ≥ 0.122). CONCLUSIONS: Twenty four months after uneventful DMEK, CT within the central 4-mm zone and CV at the 3-, 5-, and 7-mm zones were similar to uncomplicated pseudophakic eyes. The total CD at the 0- to 2-mm zone and midperipheral CT at the 6-mm ring were higher compared with the pseudophakic controls.

Evidence for wide range of time scales in oculomotor plant dynamics: implications for models of eye-movement control.

Oculomotor-plant dynamics are not well characterised, despite their importance for modelling eye-movement control. We analysed the time course of the globe's return after horizontal displacements in three rhesus monkeys lightly anaesthetised with ketamine. The eye-position traces were well fitted by a sum of four exponentials (time constants 0.012, 0.099, 0.46, 7.8 s). The two long time-constant terms accounted for 25% of plant compliance, and led to a model that accounted for hitherto unexplained features of ocular motoneuron firing such as (i) hysteresis, and (ii) the inability of a 2 time-constant model to fit data for both fast and slow eye-movements.

Alexander's law during high-acceleration head rotations in humans.

Alexander's law states that the amplitude of the spontaneous nystagmus grows with increasing gaze in the direction of the fast phase. Using the search-coil method we employed head impulses at various eye-in-orbit azimuth angles to test (i) whether the normal vestibulo-ocular reflex (VOR) in the behaviorally relevant high-frequency range has intrinsic properties that could account for Alexander's law and (ii) whether such properties can also be shown in patients with unilateral vestibulopathy. We showed that the gain of the VOR remained unaffected by eye-in-orbit position in contols and in patients, both on ipsilesional and contralesional stimuli. These findings suggest that eye-in-orbit position does not directly modulate the activity in VOR pathways, neither during unbalanced but reciprocal (in controls), nor during unbalanced and nonreciprocal natural vestibular stimulation (in patients).

Dynamics of primate oculomotor plant revealed by effects of abducens microstimulation.

Despite their importance for deciphering oculomotor commands, the mechanics of the extraocular muscles and orbital tissues (oculomotor plant) are poorly understood. In particular, the significance of plant nonlinearities is uncertain. Here primate plant dynamics were investigated by measuring the eye movements produced by stimulating the abducens nucleus with brief pulse trains of varying frequency. Statistical analysis of these movements indicated that the effects of stimulation lasted about 40 ms after the final pulse, after which the eye returned passively toward its position before stimulation. Behavior during the passive phase could be approximated by a linear plant model, corresponding to Voigt elements in series, with properties independent of initial eye position. In contrast, behavior during the stimulation phase revealed a sigmoidal relation between stimulation frequency and estimated steady-state tetanic tension, together with a frequency-dependent rate of tension increase, that appeared very similar to the nonlinearities previously found for isometric-force production in primate lateral rectus muscle. These results suggest that the dynamics of the oculomotor plant have an approximately linear component related to steady-state viscoelasticity and a nonlinear component related to changes in muscle activation. The latter may in part account for the nonlinear relations observed between eye-movement parameters and single-unit firing patterns in the abducens nucleus. These findings point to the importance of recruitment as a simplifying factor for motor control with nonlinear plants.