Weihong Yuan

The influence of repetitive eye movements on vergence performance

We measured the peak velocity of convergence eye movement responses in four normal subjects before and after a large number of either repetitive vergence or repetitive saccadic eye movements. A 20% decrease in the mean value of peak velocity was observed in vergence responses after 100 repetitive step vergence eye movements. However, 100 cycles of slow sinusoidal vergence tracking did not induce any notable change in vergence dynamics. Five hundred repetitive saccadic eye movements also caused an approximately 20% decrease in peak velocity. The reduction in peak velocity was related to the number of repetitions for both vergence and saccadic fatiguing stimuli. The frequency of occurrence of double-vergences was also used as an index to monitor the influence of repetitive eye movements on convergence performance. Results showed that repetitive step convergence movements could double, or even triple, the frequency of the occurrence of double-vergence responses, while slow sinusoidal vergence tracking or repetitive saccades had no influence on the frequency of response doubles.

Dynamics of the disparity vergence step response: a model-based analysis

A new method to analyze the dynamics of vergence eye movements was developed based on a reconstruction of the presumed motor command signal. A model was used to construct equivalent motor command signals and transform an associated vergence transient response into an equivalent set of motor commands. This model represented only the motor components of the vergence system and consisted of signal generators representing the neural burst and tonic cells and a plant representing the ocular musculature and dynamics of the orbit. Through highly accurate simulations, dynamic vergence responses could be reduced to a set of five model parameters, each relating to a specific feature of the internal motor command. This dynamic analysis tool was applied to the analysis of inter-movement variability in vergence step responses. Model parameters obtained from a large number of response simulations showed that the width of the command pulse was tightly controlled while its amplitude, rising slope, and falling slope were less tightly regulated. Variation in the latter three parameters accounted for most of the movement-to-movement variability seen in vergence step responses. Unlike version movements, pulse width did not increase with increased stimulus amplitude, although the other command signal parameters were substantially influenced by stimulus amplitude.

Dynamic details of disparity convergence eye movements

AbstractClassically, the primary tool for quantifying the dynamics of vergence and other eye movements has been the main sequence. The main sequence is a plot of peak velocity versus response amplitude and is particularly useful for comparing the dynamics of a large number of eye movements over a range of response amplitudes. However, the main sequence represents only the equivalent first-order behavior of a response and does not describe its dynamics in detail. Since the main sequence is based on only two points on the dynamic trajectory, it is sensitive to measurement artifacts and noise. A new methodology is presented which quantifies the equivalent second-order dynamics of eye movements using a larger region of the transient response. These new indexes were applied to vergence eye movements and were found to differentiate between subtle, but important differences in movement dynamics.

Disparity vergence double responses processed by internal error.

Disparity vergence eye movements occasionally exhibit two high-velocity components to a single step stimulus (Alvarez, T. L., Semmlow, J. L. & Yuan, W. (1998). Journal of Neurophysiology, 79, 37-44). This research investigates the neural strategy used to trigger the second component of double high-velocity vergence eye movements. Vergence doubles evoked by an experimental protocol that induces post-movement visual error were compared to doubles that occur normally. The second component of a visually evoked response double occurred later, and with slower dynamics, than that of a naturally occurring double. These differences in timing and dynamics indicate that natural double responses are mediated, at least in part, by a mechanism other than visual feedback. The faster dynamics and timing of natural doubles suggest that an internal monitoring process triggers these movements.

Model-based analysis of dynamics in vergence adaptation

We previously proposed a model to study the dynamics of disparity vergence responses. This model was based on known physiology and consisted of pulse and step neural control processes feeding a linear second-order oculomotor plant. Here, we apply a slightly modified version of that model to analyze the influence of short-term adaptation on vergence dynamics. This analysis showed that, unlike normal vergence responses, adapted responses could not be accurately simulated without a delay between the step and pulse components. Through simulations of normal vergence and adapted vergence responses, we found a strong correlation between delay of the step signal and the size of the movement overshoot. This correlation suggests a strong interaction between neural process generating the pulse and step motor control signals.

Effects of prediction on timing and dynamics of vergence eye movements

Periodic square waves were used to generate predictable vergence eye movement responses. The timing and dynamic characteristics of vergence eye movement responses to predictable and non‐predictable stimuli were compared. Results showed significant changes in timing characteristics along with a highly characteristic anticipatory movement in the early part of predictable vergence responses. This phenomenon is similar to that seen in saccadic eye movements and appears to influence the timing and dynamics of the subsequent vergence response. A model‐based analysis of dynamics showed that the pulse width, pulse gain, and step gain of the motor command signal did not show major differences between predictable and non‐predictable response. However, other model parameters related to the acceleration of the response showed a substantial decrease when the movements were predictive.

Short-term adaptation in disparity vergence eye-movements

The high-velocity initial component of disparity vergence eye movements is modified by stimuli that generate large transient disparities. Modification was observed in all three subjects studied. After modification, the peak velocities were substantially higher than in normal baseline responses. The temporal distribution of the peak velocities for a step stimulus as a function of number of trial revealed the existence of a very rapid adaptive process in the vergence system. Plots of main sequence, however, showed that the first-order dynamic characteristics of all initial components were the same for both preand post adaptive responses. A process of recovery or de-adaptation was also observed.

A minimal model to simulate dynamics of initial vergence component

According to the dual-mode theory, the two different dynamic components seen in the step response of disparity vergence eye movements are under separate control mechanisms. The initial, fast rising component is controlled by an open loop, preprogrammed system while the later, slow component is feedback controlled. The ability to separate the two components, or to isolate the initial component, is critical to the study of both control systems. To approach this goal, a simple open-loop control model, containing four independently adjustable parameters, was designed to simulate the disparity vergence movement. This model provided a substantial simplification of the control system, yet demonstrated remarkable accuracy in its ability to simulate the dynamic details of the initial component vergence response.

Dynamic analysis of disparity vergence eye movements: beyond the main sequence

Classically, the primary tool for studying dynamics in vergence and other eye movements has been the main sequence. The main sequence is a plot of peak velocity versus response amplitude and is particularly useful for comparing a large number of eye movements over a range of amplitude magnitudes. However, the main sequence does not represent the dynamic details of a movement. A new methodology is explained which quantifies dynamics of eye movements over a larger region of the transient response. These new indices were used to differentiate between subtle, but important differences in movement dynamics.

Dynamic asymmetry in vergence eye movements: the underlying mechanism revealed by independent component analysis

The physiological motor response to double vision, vergence eye movements, shows a strong directional asymmetry: inward turning movements are faster than outward movements. Isolated neural components underlying these signals were identified using a new application of Independent Component Analysis. These components show that the direction-dependent nonlinearity is due primarily to a difference in only one of the major components that drive the vergence response: the transient component associated with neural burst cells.