Pre-pubertal and adolescent germ cell neoplasms in Taiwan: time trends and geographic variation.

Evidence from our previous study suggested that the incidence of germ cell neoplasms in children and adolescents is increasing. The objectives of this analysis were to quantify this trend in patients aged 0-9 and 10-19 years (pre-pubertal and adolescent groups, respectively) and compare rates in Taiwan according to geographic distribution. Germ cell neoplasm frequencies among 1267 patients aged 0-19 years spanning 1995-2009 were obtained from the population-based Taiwan Cancer Registry. The incidence patterns according to sex, age, disease subgroup, and geographic distribution were analyzed. The incidence rates in the pre-pubertal and adolescent groups were 10.58 and 16.06 per million person-years, respectively. The overall rates increased significantly by 3.2% annually in the adolescent group during the 15-year study period, and increased only among the males. In contrast, no change in trend was observed in the pre-pubertal group. Subgroup analysis showed significant upward trends in the incidence rates of intracranial/intraspinal and testicular germ cell tumors (GCTs) in the adolescent males and extracranial/extragonadal GCTs in the pre-pubertal boys. The most striking differences between the study population and white Americans were that the rates of testicular GCTs were 5-fold higher and 4-fold lower in the Taiwanese pre-pubertal and adolescent groups, respectively. Significantly higher rates were found in Hualien and Chiayi Counties compared with the other areas of Taiwan. The upward trend of testicular GCTs in the adolescent males is consistent with findings from Western countries. The underlying causes that led to the high rate of testicular GCTs in the pre-pubertal boys and significantly higher rates in specific counties warrant further investigation.

Dynamic asymmetries in disparity convergence eye movements.

Vergence eye movements have traditionally been considered the product of a single neural control center and are usually studied by combining the movements of each eye into a single 'vergence' response. In the present experiment, disparity-driven eye movements were produced by symmetrical step stimuli, and the dynamic properties of each eye movement were analyzed separately. Although the final positions of the two eyes were symmetrical, large dynamic asymmetries often occurred. The timing between the two eyes showed fair synchrony as they attained maximum velocity at approximately the same time. Since the final static positions were symmetrical, asymmetries occurring during the initial dynamic component must necessarily be compensated by offsetting asymmetries in the latter portion of the response.

Initial component control in disparity vergence: a model-based study.

The dual-mode theory for the control of disparity-vergence eye movements states that two components control the response to a step change in disparity. The initial component uses a motor preprogram to drive the eyes to an approximate final position. This initial component is followed by activation of a late component operating under visual feedback control that reduces residual disparity to within fusional limits. A quantitative model based on a pulse-step controller, similar to that postulated for saccadic eye movements, has been developed to represent the initial component. This model, an adaptation of one developed by Zee et al. [1], provides accurate simulations of isolated initial component movements and is compatible with the known underlying neurophysiology and existing neurophysiological data. The model has been employed to investigate the difference in dynamics between convergent and divergent movements. Results indicate that the pulse-control component active in convergence is reduced or absent from the control signals of divergence movements. This suggests somewhat different control structures of convergence versus divergence, and is consistent with other directional asymmetries seen in horizontal vergence.

Vergence eye movements under natural viewing conditions.

PURPOSE: To determine whether there are any fundamental differences in vergence dynamics under different viewing conditions, both in instrument space and free space. METHODS: Symmetric vergence responses were measured for a variety of conditions under the traditional instrument space as well as the more natural free space viewing environment. Vergence eye movements were recorded objectively in three subjects using the infrared reflection technique. Within each environment, four conditions were tested: aperiodic self-initiated voluntary gaze shifts between two simultaneously viewed targets; periodic (0.33 Hz) voluntary gaze shifts between the same two targets; gaze shifts in total darkness to the near target following initial far target fixation in the light; and gaze shifts in total darkness to the two remembered target positions. In addition, an experiment was performed in instrument space using randomized step changes of target disparity, in which the responses served as the standard for comparison. For all conditions, target disparities ranged from 0.5 degrees to 10 degrees. The peak velocity of each vergence response was calculated and plotted versus its amplitude. RESULTS: It was found that the data for all conditions tested fell within the standard "main sequence" cluster, indicating similarity in dynamics and thus similarity in the motoneuronal controller signal. Also, the data from investigators who claimed differences in dynamics were also typically found to fall within the normal cluster. CONCLUSIONS: This indicates that the vergence motoneuronal controller signal produced the same dynamics for a particular response amplitude, independent of both viewing environment and test condition.

Disparity vergence eye movements exhibit preprogrammed motor control.

To determine the control strategy used to mediate disparity vergence eye movements, an optical technique was employed which eliminated the visual feedback normally associated with a vergence movement. The resulting "open-loop" vergence responses from four subjects exhibited multiple step-like movements. This finding augments previous demonstrations of multiple-step tracking strategies in vergence eye movements and supports the theory that the initial portion of the vergence response is under preprogrammed control. Additional evidence for preprogramming is given by the similarity of the initial dynamics between open- and closed-loop responses as evidenced by similar ratios of peak velocity to amplitude (i.e. "main sequence" plots).

Initial control component in disparity vergence eye movements.

Recent experimental evidence indicates that a portion of the oculomotor response to disparity stimulation is functionally open-loop; that is, the response occurs without the aid of visual feedback. To investigate the stimulus features that elicit or influence this dynamic movement, convergence responses to a step, a step followed by target disappearance, and a pulse followed by target disappearance were obtained from four subjects using infrared oculography. The target was a thin vertical line (0.25 degrees) either 2 or 10 degrees in height. Stimuli having different amplitudes (1, 2, 4 and 8 degrees) and disappearance times (50, 100 and 200 ms) were selected randomly along with occasional divergent stimuli to minimize prediction and voluntary vergence. Experiments showed that the dynamic characteristics of the initial portion of the response were essentially the same, even when the target disappeared before the movement took place. The magnitude of the initial response depended on the stimulus amplitude, but was not influenced by either stimulus duration or target height. For example, stimulus durations as short as 50 ms elicited responses similar to those caused by standard steps. The initial response was shown to be active over a well-defined time period of about 200 ms, after which the response appears to be mediated by a visually-guided control component. These results support the recently developed dual-mode theory of vergence control in which an initial preprogrammed (open-loop) control component is followed by a feedback (closed-loop) controlled component which reduces any remaining disparity.