Visually induced cross-axis postsaccadic eye drift

Eye movement abnormalities in essential tremor

Essential tremor (ET) is the most prevalent movement disorder, characterized mainly by an action tremor of the arms. Only a few studies published as yet have assessed oculomotor abnormalities in ET and their results are unequivocal. The aim of this study was to assess the oculomotor abnormalities in ET patients compared with the control group and to find the relationship between oculomotor abnormalities and clinical features of ET patients. We studied 50 ET patients and 42 matched by age and gender healthy controls. Saccadometer Advanced (Ober Consulting, Poland) was used to investigate reflexive, pace-induced and cued saccades and conventional electrooculography for evaluation of smooth pursuit and fixation. The severity of the tremor was assessed by the Clinical Rating Scale for Tremor. Significant differences between ET patients and controls were found for the incidence of reflexive saccades dysmetria and deficit of smooth pursuit. Reflexive saccades dysmetria was more frequent in patients in the second and third phase of ET compared to the first phase. The reflexive saccades latency increase was correlated with severity of the tremor. In conclusion, oculomotor abnormalities were significantly more common in ET patients than in healthy subjects. The most common oculomotor disturbances in ET were reflexive saccades dysmetria and slowing of smooth pursuit. The frequency of reflexive saccades dysmetria increased with progression of ET. The reflexive saccades latency increase was related to the severity of tremor.

Cross-coupled eye movement supports neural origin of pattern strabismus

PURPOSE

Pattern strabismus describes vertically incomitant horizontal strabismus. Conventional theories emphasized the role of orbital etiologies, such as abnormal fundus torsion and misaligned orbital pulleys as a cause of the pattern strabismus. Experiments in animal models, however, suggested the role of abnormal cross-connections between the neural circuits. We quantitatively assessed eye movements in patients with pattern strabismus with a goal to delineate the role of neural circuits versus orbital etiologies.

METHODS

We measured saccadic eye movements with high-precision video-oculography in 14 subjects with pattern strabismus, 5 with comitant strabismus, and 15 healthy controls. We assessed change in eye position in the direction orthogonal to that of the desired eye movement (cross-coupled responses). We used fundus photography to quantify the fundus torsion.

RESULTS

We found cross-coupling of saccades in all patients with pattern strabismus. The cross-coupled responses were in the same direction in both eyes, but larger in the nonviewing eye. All patients had clinically apparent inferior oblique overaction with abnormal excylotorsion. There was no correlation between the amount of the fundus torsion or the grade of oblique overaction and the severity of cross-coupling. The disconjugacy in the saccade direction and amplitude in pattern strabismics did not have characteristics predicted by clinically apparent inferior oblique overaction.

CONCLUSIONS

Our results validated primate models of pattern strabismus in human patients. We found no correlation between ocular torsion or oblique overaction and cross-coupling. Therefore, we could not ascribe cross-coupling exclusively to the orbital etiology. Patients with pattern strabismus could have abnormalities in the saccade generators.

The nonlinearity of passive extraocular muscles

Passive extraocular muscles (EOMs), like most biological tissues, are hyperelastic, that is, their stiffness increases as they are stretched. It has always been assumed, and in a few occasions argued, that this is their only nonlinearity and that it can be ignored in central gaze. However, using novel measurement techniques in anesthetized paralyzed monkeys, we have recently demonstrated that EOMs are characterized by another prominent nonlinearity: the forces induced by sequences of stretches do not sum. Thus, superposition, a central tenet of linear and quasi-linear models, does not hold in passive EOMs. Here, we outline the implications of this finding, especially in light of the common assumption that it is easier for the brain to control a linear than a nonlinear plant. We argue against this common belief: the specific nonlinearity of passive EOMs may actually make it easier for the brain to control the plant than if muscles were linear.

A simple control law generates Listing's positions in a detailed model of the extraocular muscle system

The neural commands for maintaining static Listing's positions were identified using a detailed model of extraocular muscle based on Miller and Shamaeva (Orbit 1.5 gaze mechanics simulation 1995). The commands were approximately separable, suggesting a simple control law whereby independent horizontal and vertical commands are combined to generate tertiary positions. Tests showed that this control law (i) generated Listing' s positions to reasonable accuracy over+/-30 deg, provided pulleys were represented in the model; (ii) if driven by retinal coordinates, produced errors close to the theoretical minimum for a commutative system. The proposed commands appear consistent with electrophysiological evidence.

Disconjugate adaptation of the vertical oculomotor system

Conjugate post-saccadic eye drift can be induced in normal humans if a visual pattern is made to drift after every saccade. This study examines the ability of normal humans to create disconjugate vertical post-saccadic drift. Identical fuseable patterns were presented dichoptically, one to each eye. At the end of each vertical saccade one pattern drifted up and the other down, by 5% of the saccade amplitude. Five subjects were trained for 2-3 hr. Eye movements were recorded with eye coils. Normal vertical saccades along the midline were remarkably conjugate and post-saccadic drift was minimal. Training produced only small disconjugate post-saccadic drift (0.14 deg) but substantial saccade amplitude disconjugacy (0.70 deg). For several subjects, the induced disconjugacies persisted even for saccades in the dark indicating that adaptive changes occurred in the binocular coordination of vertical saccades. Apparently vertical disparate post-saccadic retinal slip is not sufficient to stimulate significantly the saccade pulse-step matching mechanism which is believed to control post-saccadic eye drift. The changes we observed aimed to reduce position disparity and not retinal slip in each eye.

Adaptation of vertical eye alignment in relation to head tilt

Binocular visual feedback is used to continually calibrate binocular eye alignment so that the retinal images of the two eyes remain in correspondence. Past experiments have shown that vertical eye alignment (measured as vertical phoria) can be altered by training to disparities that vary as a function of orbital eye position. The present experiments demonstrate that vertical eye alignment can also be trained to differ with head position when eye position (with respect to the orbit) is held constant. Changes in head position were about either an earth-vertical or earth-horizontal axis to distinguish otolith-ocular related adaptation from cervical-ocular related adaptation. Changes in head position were implemented by either by rotating the whole body (WB) or by rotating the head with the body stationary (HO). Following training, adaptation of eye alignment was observed in all cases of rotation about an earth-horizontal axis and for HO pitch rotations about an earth-vertical axis. The results illustrate the ability of the oculomotor system to compensate for imbalances in otolith-ocular pathways.