Residual central nervous system damage due to organoarsenic poisoning.

BACKGROUND: Drinking well water contaminated with the organoarsenic compound diphenylarsinic acid (DPAA) causes central nervous system (CNS) disorders that improve within several years after last drinking such water. Subjective symptoms such as lightheadedness and dizziness appear to persist, however, suggesting CNS damage. We evaluated CNS damage due to DPAA by detecting abnormal eye movements. METHODS: Subjects comprised 29 victims of exposure to DPAA in whom this substance had been detected in the nails. Investigations were performed more than 3years following cessation of DPAA exposure. Abnormal eye movements were monitored using electronystagmography. We analysed unpaired t-test between exposure subjects who exhibited upbeat nystagmus and those who did not. Upbeat nystagmus parameters were measured, and mean values were calculated. Associations between the properties of upbeat nystagmus and maximum concentrations of DPAA among DPAA exposure were also investigated. RESULTS: Upbeat nystagmus was common among exposure victims, occurring in 23 of 29 subjects (79.0%). The subjects with upbeat nystagmus had significantly higher ratio than those without upbeat nystagmus in the points of subjective symptoms and DPAA concentration of drinking water (p<0.01). The slow-phase amplitude of upbeat nystagmus enlarged with increasing DPAA concentrations, showing a significant positive correlation (p<0.05). These findings suggest that the level of exposure to DPAA affects the properties of nystagmus. High-frequency pathological square-wave jerks (SWJ) were seen in 14 of 29 patients (48.0%), and mean SWJ frequency was 112.4±16.7/min. CONCLUSIONS: Detection of abnormal ocular movements may be useful in evaluating residual/persistent/chronic CNS damage due to organoarsenic poisoning.

Comparison of the choice effect and the distance effect in a number-comparison task by FMRI.

Behavioral and neurophysiological studies of numerical comparisons have shown a "distance effect," whereby smaller numerical distances between two digits are associated with longer response times and higher activity in the parietal region. In this experiment, we introduced a two-choice condition (between either the smaller/lower or the larger/higher of two digits) and examined its effect on brain activity by fMRI. We observed longer response times and greater activity with the choice of smaller numbers ("choice effect") in several brain regions including the right temporo-parietal region, (pre)cuneus, superior temporal sulcus, precentral gyrus, superior frontal gyrus, bilateral insula, and anterior cingulate cortex. These regions correspond to areas that have been suggested to play a role in attentional shift and response conflict. However, brain activity associated with the distance effect disappeared even though the behavioral distance effect remained. Despite the absence of the distance effect on brain activity, several areas changed activity in relation to response time, including regions that were reported to change activity in both a distance effect and a reaction-time-related manner. The result suggested that the level of task load may change the activity of regions that are responsible for magnitude detection.

Behavioral and near-infrared spectroscopy study of the effects of distance and choice in a number comparison task.

Extensive behavioral and neurophysiological numerical comparison studies have shown that response times are longer and parietal activities are stronger when the numerical distance between two digits is smaller (the distance effect). However, only a few behavioral studies have considered the effect of the choice of larger or smaller numerals in numerical comparisons. Using near-infrared spectroscopy (NIRS), we investigated the neural basis of choosing larger/smaller numerals in number comparison tasks in which subjects were required to choose a larger or smaller digit. Our results showed that choosing a smaller digit induced significantly longer response times (the choice effect) and stronger parietal activities. We also obtained significantly longer response times as the distance effect in accordance with previous works. However, NIRS data did not show any significant difference corresponding to distance effect. Our results and previous studies suggest that the parietal cortex is involved not only in measuring numerical quantities, but also in choosing a numerically larger/smaller quantity among the categories of choice. Potentials and limitations of NIRS were discussed.

Different memory types for generating saccades at different stages of learning.

It has been proposed that positional memories encoded in different types of reference frame are used for the reaching hand movement in different stages of learning. However, the types of reference frame employed for generating behavior at each stage of learning remain unclear, particularly for saccades. To examine the types of reference frame for target positions, we analyzed the saccade of monkeys performing an oculomotor task. The task required the animal to make a learning-based saccade to one of the eight landmark positions specified by the color of a fixation point. According to the color of the fixation point, the target landmark position was fixed in all experimental blocks (FIX trial) or altered to other landmark positions block by block (ALTER trial). Although the monkeys learned the target landmark position in both the FIX and the ALTER trials, once the landmarks became invisible, the success rate remained high only in the FIX trials. These results suggest that the target position was learned on the basis of the landmark positions in the early stage of learning. However, the memory of the target position in space was formed after sufficient training. When the fixation point was shifted horizontally by 5 degrees and the landmarks were invisible, the saccades in the ALTER trials were made to the normal target landmark position whereas those in the FIX trials were made to the point approximately 5 degrees shifted horizontally from the normal target landmark position. These results suggest that the target position in space was initially represented in the head-centered or world-centered reference frame and then in the eye-centered reference frame. Analysis of saccade end-points indicated that a kinematically similar saccade was generated for each FIX trial. These results showed that memories encoded by different reference frames were formed to generate a saccade while the saccade toward the same target was repeatedly executed.