Paula Munoz

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.

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.

A comparison of dynamics in convergence and divergence eye movements

The details in timing and dynamics of inward (convergence) and outward (divergence) turning eye movements (vergence eye movements) were compared. Experiments showed that there were significant differences in the timing characteristics of convergence versus divergence movements. A model-based analysis showed that the dynamics were also different in these two vergence eye movements. Based on the assumption that these movements are driven by a neural signal composed of a pulse and a step, the gain ratio between pulse and step components was much larger than one for convergence responses, but was close to or less than one in divergence responses. These data indicate that the neural control signal for divergence is close to a simple step rather than the combined pulse-step signal in convergence.

Cortical activity in disparity vergence eye-movements: a fMRI study

Employing fMRI techniques, cortical activity related with disparity vergence eye movements has been identified. The gyri precentralis, the gyrus cinguli and the precuneus appeared to be involved in these eye movements under very different conditions. In addition, the prediction operator in vergence eye movements presented additional activity in all the previous regions plus the gyrus frontalis superior. From this preliminary study, at least at the cortical level, the vergence eye movements correspond mainly to voluntary movements and therefore employ the pathways of voluntary motor activity.

Stimulus content effect on the visual cortex with functional MRI

Employing fMRI techniques, brain activity in the visual cortex was identified related to three different visual stimuli: a vertical bar, a semi-random and a random pattern. Two main changes were observed: the intensity of the radio frequency (RF) signal employed for image reconstruction increases and the area of activity spreads with the presentation of the semi-random pattern in comparison with the presentations of the other two stimuli. These findings would suggest the type of visual information content processing occurring at the level of the calcarine fissure.