Saccades, vergence and combined eye movements in a young subject with Congenital Central Hypoventilation Syndrome (CCHS)

Speed and accuracy of saccades, vergence and combined eye movements in subjects with strabismus before and after eye surgery

The purpose of the study was to examine spatio-temporal characteristics of horizontal eye movements in the natural space (saccade, vergence and combined movements) in young subjects with early onset convergent or divergent strabismus. Nine young subjects (8-20 years old) were tested: three with divergent strabismus, six with convergent strabismus. A standard paradigm was used to elicit pure horizontal saccades at far and at close viewing distance, pure vergence along the median plane (convergence and divergence) and saccades combined with vergence movements. Horizontal eye movements from both eyes were recorded by a photoelectric device. Eye movements were recorded before surgery, and, for the majority of the subjects, two times after surgery. Before surgery the accuracy of convergence and divergence movements in their pure or combined form was poor with respect to normal values. The mean velocity of convergence was also abnormally slow. Strabismus surgery improved significantly the accuracy of these types of eye movements. The speed of pure convergence and of divergence combined movements increased significantly after surgery. We concluded that poor vergence eye movement's performance, particularly those found for convergence in strabismic subjects could be due to impairment in the central structures related to sensory disparity inputs. Adaptive mechanisms promoted by the realignment of the eyes could be at the origin of the improvement in the vergence performances observed in our subjects after strabismus eye surgery.

Latency detection in motor responses: a model-based approach with genetic algorithm optimization

The latency of a response is one of the most frequently reported parameters when describing the characteristics of a motor system. Such measurement provides important information both to the basic researcher investigating the neural circuitry of the underlying physiological system and to the clinician gathering information for diagnosing a patient. Our concern here is that when the latency of a response is determined on experimentally recorded data by using the most commonly referenced techniques to find the onset of a motor response, the resulting figure encompasses both the neural processing time and the dynamics of the system producing the response (e.g., the musculoskeletal apparatus). Therefore, the resulting latency measurement cumulates information relative to two substantially different sources and thus having different implications. The goal of our study is that of suggesting a technique allowing the separation of the relative contributions of neural transmission and processing time from that of the dynamics of the motor system. This is accomplished by using a technique based on fitting a model to the experimentally recorded response, thus allowing to exploit as much as is known with regards to the dynamics of the studied motor system (e.g., model order and constraints on the values of the model parameters). The optimization of the model parameters for fitting the experimental data is carried out using a real-valued genetic algorithm, allowing to avoid trapping in local, suboptimal minima. The use of this approach allows to estimate the pure delay in the response introduced by neural processing more accurately than the traditional latency detection techniques based on adaptive thresholds.

[Ondine's Curse and rare oculomotor abnormalities: a case report]

Ondine's Curse or congenital central hypoventilation syndrome (CCHS) is a neurocristopathy (failure of migration or differentiation of neural crest-derived precursor cells) and is characterized by hypoventilation or apnea, which is most pronounced during sleep, with no other abnormalities of the neuro-respiratory system. Because of respiratory distress soon after birth, patients must be intubated and ventilated for a long time. This disorder may be associated with other symptoms of neurocristopathy (Hirschsprung disease, neuroblastoma, neuroganglioma) and other abnormalities of the autonomic nervous system (vasomotor dysfunctions or ophthalmic abnormalities: abnormal pupils, insufficient convergence, strabismus, or ptosis). We report the original case of a CCHS patient who presented with alternative ptosis of both the right and left eyes and esotropia. The ocular findings should lead to earlier diagnosis and speedier adequate treatment.

Binocular coordination of saccades in children with vertigo: dependency on the vergence state

The present study examines the quality of binocular coordination of saccades at far and near distance in 15 children with symptoms of vertigo headache and equilibrium disorders; these children show normal vestibular function but abnormal convergence eye movements (e.g., long time preparation, slow execution and poor accuracy, see ). The results show normal binocular saccade coordination at far distance, but large abnormal disconjugacy for saccades at near distance. During combined saccade-vergence movements (studied in six of these children), convergence remains abnormally slow. This supports the interpretation according to which poor binocular yoking of the saccades is linked to the reduced ability to produce fast convergence during the saccade; a learning mechanism based on rapid vergence would help to reduce the abducting-adducting asymmetry of the saccades. An alternative interpretation would be reduced learning ability for monocular adjustment of the saccade signals.

Influence of gap and overlap paradigms on saccade latencies and vergence eye movements in seven-year-old children

The latency of eye movements is influenced by the fixation task; when the fixation stimulus is switched off before the target presentation (gap paradigm) the latency becomes short and express movements occur. In contrast, when the fixation stimulus remains on when the target appears (overlap paradigm), eye movement latency is longer. Several previous studies have shown increased rates of express saccades in children; however the presence of an express type of latency for vergence and combined movements in children has never been explored. The present study examines the effects of the gap and the overlap paradigms on horizontal saccades at far (150 cm) and at close (20 cm) viewing distances, on vergence along the median plane, and on saccades combined with convergence or divergence in 15 normal seven-year-old children. The results show that the gap paradigm produced shorter latency for all eye movements than the overlap paradigm, but the difference was only significant for saccades at close viewing distances, for divergence (pure and combined), and for saccades combined with vergence. The gap paradigm produced significantly higher rates of express latencies for saccades at close viewing distances, for divergence, and for saccades combined with divergence; in contrast, the frequencies of express latencies for saccades at far viewing distances and for convergence (pure or combined) were similar in the gap and the overlap paradigms. Interestingly, the rate of anticipatory latencies (<80 ms) was particularly high for divergence in the gap paradigm. Our collective findings suggest that the initiation of saccades at close viewing distances and of divergence is more reflexive, particularly in the gap paradigm. The finding of frequent anticipatory divergence that occurs at similar rates for seven-year-old children (this study) and for adults (Coubard et al., 2004, Exp Brain Res 154:368-381) indicates that predictive initiation of divergence is dominant.

Latency of saccades, vergence and combined saccade vergence movements in a subject with manifest latent nystagmus

PURPOSE

To study eye movements in 3D space - saccades, vergence and combined eye movements - in a subject with Manifest Latent Nystagmus (MLN).

METHODS

A 13-year-old girl diagnosed with MLN participated in this study. Saccades, pure vergence along the median plane and combined saccade-vergence movements were recorded under both binocular and monocular viewing. Horizontal eye movements from both eyes were recorded simultaneously with a photoelectric device (Bouis Oculometer).

RESULTS

The recordings of saccades, vergence or both components of combined movements show that such movements have a staircase trajectory. The consequence of this staircase behavior is that the time for approaching the target is prolonged. The latency of all types of eye movements is extended when the subject is viewing binocularly, while latency values are normal when viewing monocularly; the difference between the viewing conditions is significant for vergence.

CONCLUSION

The normal latency values under monocular viewing are attributed to facilitation of eye movement initiation by the increased nystagmus.