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S Lebedev

Publications and source records attributed to S Lebedev.

8 recordsLinked to original sources
PubMed1997

Peak velocities of visually and nonvisually guided saccades in smooth-pursuit and saccadic tasks.

Smooth pursuit typically includes corrective catch-up saccades, but may also include such intrusive saccades away from the target as anticipatory or large overshooting saccades. We sought to differentiate catch-up from anticipatory and overshooting saccades by their peak velocities, to see whether the higher velocities of visually rather than nonvisually guided saccades in saccadic tasks may be found also in saccades in pursuit. In experiment 1, 12 subjects showed catch-up, anticipatory, and overshooting saccades to comprise 70.4% of all saccades in pursuit of periodic, 30 degrees/s constant-velocity targets. Catch-up saccades were faster than the others. Saccadic tasks were run as well, on 19 subjects, including the 12 whose pursuit data were analyzed, with target-onset, target-remaining (saccade to the remaining target when the other three extinguish), and antisaccade tasks. For 17 of the 19 subjects, antisaccade velocities were lower than for either target-onset or target-remaining tasks. Velocities for the target-remaining task were near those for target onset, indicating that target presence, not its onset, defines visually guided saccades. Error and reaction-time data suggest greater cognitive difficulty for target remaining than for target onset, so that the cognitive difficulty of typical nonvisually guided saccade tasks is not sufficient to produce their lowered velocity. To produce reliably, in each subject, catch-up and anticipatory saccades with comparable amplitude distributions, nine new subjects were asked in experiment 2 to make intentional catch-up and anticipatory saccades in pursuit, and were presented with embedded target jumps to elicit catch-up saccades, all with periodic target trajectories of 15 degrees/s and 30 degrees/s. Velocities of intentional anticipatory saccades were lower than velocities of intentional catch-up saccades, while velocities of intentional and embedded catch-up saccades were similar. Target-onset and remembered-target saccadic tasks were run, showing the expected higher velocity for the target-onset task in each subject. Both experiments demonstrate higher peak velocities for catch-up saccades than for anticipatory saccades, suggesting that cortical structures preferentially involved in nonvisually guided saccades may initiate the anticipatory and overshooting saccades in pursuit.

Calibration↗
PubMed1996

Square-root relations between main saccadic parameters.

PURPOSE: To derive and evaluate two equations in which saccade duration and peak velocity are proportional to the square root of saccade amplitude. METHODS: A population of horizontal visually guided saccades in a range of amplitudes from 1.5 degrees to 30 degrees was recorded by means of electro-oculography in eight normal adult subjects. The peak velocity-amplitude data of this population were fitted to four models: inverse linear, exponential, power law, and square root. To approximate the duration-amplitude relation, the square root was tested against the linear and power law models. For each model, the best-fit values of its parameters were estimated by the method of least squares. RESULTS: When the entire population was used, all tested models displayed comparable goodness of fit, but when different subranges of this population were used, only the square root equations appeared to be robust and acceptably accurate. CONCLUSIONS: In a restricted range of saccade amplitudes from 1.5 degrees to 30 degrees, the square root model has some advantages over the others commonly used: to express peak velocity and duration as functions of amplitude, it requires the estimation of only two parameters, whereas the others require four. Because of its robustness, this model can be used to evaluate populations of saccadic eye movements with different ranges of amplitudes. The two parameters of the model equations allow a simple and clear physical interpretation.

Adult↗
PubMed1995

Anticipatory saccades in smooth pursuit: task effects and pursuit vector after saccades.

The dramatic improvement in smooth pursuit performance seen while analyzing the pursuit target has been ascribed to attention enhancement. With a periodic constant velocity target trajectory we ran a concurrent listening condition instead, to see if this mild distraction would degrade performance. Performance improved somewhat with the listening task, suggesting that displacing attentional effort from pursuit accuracy, rather than increasing it, brings better pursuit performance. Catch-up saccades were evenly distributed across tracking, listening, and target analysis conditions, but anticipatory and overshooting saccades were almost eliminated with target analysis. Thus the poor pursuit seems to have been caused by anticipatory and overshooting saccades, produced erroneously in the attempt to perform purposive smooth pursuit. Pursuit velocity immediately following anticipatory saccades was reduced such that the target would catch up with the point of gaze when it reached the endpoint of its trajectory, indicating a predictive goal other than instantaneous target foveation and velocity match.

Adult↗
PubMed1995

Predictive human pursuit and "orbital goal" of microstimulated smooth eye movements.

1. Anticipatory saccades in smooth pursuit move the point of gaze from near the moving target to well ahead of it, interrupting accurate smooth pursuit. Their effects on the pursuit process were studied in 22 normal human subjects. We presented horizontal periodic target trajectories of 30 degrees amplitude and 30 degrees/s constant velocity or 0.4 Hz sinusoidal velocity in 40-s trials. Saccades and surrounding smooth eye movement (SEM) segments were marked and classified by computer. 2. Anticipatory saccades were often followed by slowed SEM that tended to intercept the target at the endpoint of its trajectory. This was seen in the distribution of projections of the initial 60 ms of postsaccadic SEM to the time of the trajectory endpoint. Magnitude of this SEM tended to follow a function of the time and location of the endpoint of the anticipatory saccade, decreasing as the anticipatory saccades landed closer to the trajectory endpoint. 3. The time and location of the target trajectory endpoint seemed to be the goal for this SEM. We believe this to demonstrate the predictive use of the period and amplitude of the trajectory in smooth pursuit, apart from the instantaneous velocity match of the target. 4. Gottlieb and coworkers in the frontal eye field and Ron and Robinson in the cerebellum produced SEMs in the monkey by microstimulation. At some sites in both structures, direction and velocity of the SEMs depended on the initial position of the eye in that the elicited SEMs appeared to be converging toward a common point, or "orbital goal", and the SEM velocity diminished as the gaze neared that goal.2+ Both our SEM after anticipatory saccades and microstimulated SEM in the monkey slowed as the initial position was brought closer to the inferred orbital goal. This similarity suggests that the goal-directed SEM sites in the monkey might be part of a mechanism for predictive pursuit.

Electrooculography↗
PubMedDate not supplied

Saccades in pursuit eye tracking reflect motor attention processes.

A dysfunction of pursuit eye tracking in psychiatric patients has been well documented, but its cause has not. Recent speculation implicates a pathological disinhibition of saccades in these patients. However, great variability in tracking performance was found among nine normal subjects, and tracking quality improved in each when asked to perform a simple analysis of the tracking target: saccade counts and amplitudes decreased, as did a root-mean-square (RMS) error measure of overall tracking performance. This improvement was as great at the end as at the beginning of the analysis condition, suggesting that the improvement is not due to increased attention to a novel task, but rather is the preferred mode of tracking automatically, while attending to target analysis. The anticipatory, purposive saccades produced in attending to the act of tracking thus can be studied as involuntary attention effects reflecting frontal lobe function, and used for diagnostic classification of attentional dysfunction.

Adult↗