EXPERIMENTAL INVESTIGATIONS Gender-Dependent Bimanual Task Performance

Summary. Background and Objective. Many studies have suggested that each hand has a different special talent; however, there is a lack of data in the area of goal-directed bimanual hand coordination and its dependence on gender. The aim of this paper was to investigate gender-dependent bimanual speed-accuracy task performance. and Methods. Twelve young and twelve healthy young females (all right-handed) performed protractile movements with both arms simultaneously by pushing joysticks toward two targets as quickly and accurately as possible.


Introduction
According to traditional views, the right hemisphere is more involved in spatial activities (e.g., producing smaller movement errors) (1), whereas the left hemisphere is more involved in temporal activities (2). The left hand (nondominant) is specialized for movement preparation and the visuospatial aspects of movement evidenced by shorter reaction time (RT) to localize targets (3). The right hand (dominant) has a specialized role in the coordinated timing of pointing actions and more effective use of limb dynamics through intersegmental coordination of reaching movement (4). Unimanual aiming movements have been reported to be planned faster and controlled more accurately for the left (or nonpreferred) hand than for the right (or preferred) hand (5,6). Thus, the RT of the left hand was shorter and the accuracy was better than that of the right hand, but the right hand performed the movement faster.
It has been established that the principles of bimanual movement cannot simply be extrapolated from the laws of single-limb movement (7)(8)(9)(10)11).
There are numerous studies analyzing reaction time, movement speed and accuracy, and their dependence on gender (12)(13)(14)(15) and the right or left hands (12,13,16). It is traditionally accepted that the female human brain is less lateralized than that of males (12). Males have been reported to have faster reaction times than females, and this drawback is not reduced by training practice (15). However, in the speed-accuracy task, women performed more slowly and accurately than men (13,14). Despite these and other fi ndings, it is still not clear what main differences exist between men and women performing bimanual task.
Although many studies have suggested that each hand has a different special talent, there is, however, a lack of studies in the area of goal-directed bimanual hand coordination and its dependence on gender. Besides, it has been determined that intraindividual variability of reaction time is a signifi cant criterion of cognitive performance capacity (17)(18)(19)(20). However, we have not found any data about the differences in intraindividual variability of reaction time, move-ment speed and accuracy during speed-accuracy tasks between male and female subjects as well as between the left and right hands performing bimanual tasks.
In this study, the following research questions were intended to answers: a) if the left hand reacts faster performing speed-accuracy unimanual task (3,5), does the same law work performing bimanual task? b) if women perform speed-accuracy unimanual task more accurately but slower than men, and their reaction time is shorter (13)(14)(15), does the same law work in the performance of bimanual task? c) do both hands work synchronically performing bimanual task? and d) are the intraindividual variability of reaction time as well as kinematic characteristic of movement of the left hand of right-handed people greater than those of the right hand performing a bimanual task? The aim of this study was to investigate dominant and nondominant hand differences in interjoint coordination during targeted rapid aiming bimanual movements. The performance of the dominant and nondominant hands during rapid aimed bimanual reaching task (50 repetitions) in 12 adult neurologically intact, right-handed men and 12 women was compared.  [4.7]) were enrolled into the study. All the subjects were righthanded. The assessment and analysis of handedness was performed by the Edinburgh Inventory (21). The subjects were informed about the course of the study. All of them were physically active, but they did not take part in any formal physical exercise or sports program.

Material and Methods
Methods. All the experiments took place from 9 AM to noon. The subjects had no physical load before the experiments. The subjects were asked to perform two tasks: a) a simple reaction task with one hand (10 repetitions with the right and left hands); and b) a triple task with the right and left hands simultaneously: react as fast as possible and move to two targets with maximal speed and maximal accuracy with both hands (bimanual symmetrical speedaccuracy task toward separate visual targets) (50 repetitions with each hand were performed). The movements were performed in the natural reaching space in an upward-forward direction, involving shoulder and elbow movements. The interval between repetitions in both tasks was 2-5 s (the interval between the tasks performed by different hands and one or both hands was 1 min). The hands were chosen randomly. Two-three days before the experiment, the subjects were introduced to both tasks (they tried to perform each task about 10-15 times).
During the study, the subjects were seated in a special chair at the table with a DPA-1 ( Fig. 1) fastened to it (22). The subject's back was straight and leant at the backrest; both arms were bent 90° at the elbow joint so that the upper arms were nestled against the sides, and the forearms rested on the DPA-1 support panel. The distance between the hands at the start place as well as the target was 20 cm. The position of DPA-1 chair was regulated so that the subject could sit comfortably and take a standard position. The distance between the computer screen and the subject's eyes was approximately 70 cm. The participant's right and left hands were fi xed to joysticks, from which the path and velocity of hand movements at the distal part of the hands were recorded. The sampling rate was 100 Hz. The handle at the end of the lever was adjusted to accommodate the participant's hand (the lever was allowed to move only in a horizontal plane). For each experiment, the reaction time (time to movement initiation from the start of motion stimulus) was measured.
The target appeared in the same place on the screen (the distance from the start zone to the target was 0.17 m) (Fig. 2). The subjects were instruct- Start Zones ed to focus on the cross in the center of the screen, react as fast as possible to the auditory and visual signal (appearance of circle 0.7 cm in radius on the screen), and push the joystick with their hand as fast as possible and with the greatest accuracy to the target and then to stop the movement. The endpoint of the movement was recorded when the center of the handle symbol stopped in the circle and stayed there for no less than 0.02 s. During each task, the subject was required to position the handle symbol 0.35 cm in radius in the start zone (the center of circle 0.1 cm in radius) on the computer screen. The program intermittently generated a sound signal and a target on the computer screen, and the subject had to react to it by pushing the handle. The measurement cycle was completed after hitting the target with the circle of the handle symbol.
On the days of the experiment, after the task was explained to the subjects, they were allowed 5 practice attempts, the results of which were not recorded. The results of each repetition were shown to the subjects on the computer screen, and they were verbally instructed to do their best. The average reaction times (simple reaction time, RTs, and complex reaction time, RTc, in the fi rst and second tasks respectively) were calculated. In the second task, the average velocity (Va), maximal velocity (Vm), time to Vm (Tv), and path of movement (S) from 50 repetitions were also calculated. Besides, the intraindividual variability (coeffi cient of variation, CV) of these variables was calculated.
Statistical Analysis. The two-way analysis of variance (ANOVA) for repeated measures was used to determine the effect of gender (female vs. male) and lateralization (right vs. left hand) on different measurements. If signifi cant effects were found, post hoc testing was performed applying paired t tests with a Bonferroni correction for multiple comparisons. Descriptive data are presented as means (SD). The level of signifi cance was set at 0.05. Based on alpha level of 0.05, the sample size (n=12), standard deviations, and the average level of variables, statistical power (SP) was calculated for all mechanical indicators. In order to evaluate the relationship between the right and left hands in different variables, the Pearson coeffi cient of correlation was calculated.

Results
No signifi cant difference (P>0.05) was found in RTs between women and men as well as between the right and left hands when performing the unimanual task (Fig. 3). However, during the bimanual task, the RTc of both the right and left hands was signifi cantly shorter in men than women (P<0.05) (Fig. 4). There was no signifi cant difference in the Va and Vm between women and men (Figs. 5 and 6). No signifi cant difference (P>0.05) was observed in the Va and Vm between the right and left hands in women; however, in men, the Va and Vm of the right hand were signifi cantly greater (P<0.05) as compared with those of the left hand (Figs. 5 and 6). The Tv (time to maximal velocity of movement) of the right hand was shorter than that of the left hand both in women and men; however, there was no signifi cant difference in the Tv between women and men (Fig. 7). Moreover, no signifi cant difference in the S between the right and left hands was found (P>0.05); however, the S of the left hand in men was shorter than that in women (Fig. 8).
No signifi cant differences in intraindividual variability (CV) of RTc, Vm, and S between women and men as well as between the right and left hands were found (P>0.05) (Fig. 9). However, the CV of Va both in women and men as well as the right and left hands was signifi cantly greater than other indices (P<0.05). Besides, the CV of the Va of the right hand was greater than that of the left hand (P<0.05). The CV of S in men and women was signifi cantly smaller than CV of Vm, Va, and RTc.       A signifi cant intraindividual cross-correlation of RTc and Vm between the right and left hands both in men and women was determined (P<0.05); however, there was no signifi cant difference between men and women (Fig. 10). Besides, intraindividual cross-correlation of S between the right and left hands was signifi cantly greater in men than women (P<0.001).

Discussion
The fi rst fi nding of our study is that there was no signifi cant difference in the reaction time during a unimanual speed-accuracy task between the left and right hands and between men and women. This contradicts to the fi ndings of Carson et al. (3) and Boulinquez and Bartélémy (5). These authors reported that the left hand reacted faster performing a unimanual speed-accuracy task. Moreover, Der and Deary established that the reaction time in men was shorter than in women (15).
The second fi nding of our study is that during a bimanual task, the reaction time of both hands was signifi cantly longer in women than men. It has been recently concluded that the two hands have different internal models and specialties: closedloop control for the right hand and open-loop control for the left hand (11). Consequently, it was suggested that during bimanual movements, both models might be used creating better control and planning, but requiring more computation time compared to the use of one hand only. This is in accord with our data showing that RTc was signifi cantly longer than RTs. However, it is rather strange why the RTc in women was longer than men as there was no signifi cant difference in the RTs between men and women.
The third fi nding of our study is that there was no signifi cant difference in the velocity during a bimanual speed-accuracy task between men and women as well as the left and right hands, while the accuracy of the left hand was signifi cantly greater in men than women. It is contrary to the data that women's reaction time is longer and speed of movement is slower, whereas their accuracy of movement is better compared to that of men performing a speed-accuracy unimanual task (13)(14)(15)23). However, women accomplished the Moberg Pick-Up Test (i.e., test for assessing hand dexterity) faster than men did, and their task performance with the dominant hand was faster than that with the nondominant hand (24). Thus, the principles of bimanual movement cannot be simply extrapolated from the laws of single-limb movement (7)(8)(9)(10)(11). Our fi ndings showed that the Tv of the right hand was signifi cantly shorter compared with the left hand in women as well as in men. Thus, their right hand movement was more ballistic than that of the left hand. It has been found that women often adopt a more feedback-based strategy to limb control, while men are more likely to rely on feedforward processes to optimize speed and accuracy (25). This is the main reason why women perform movements more accurately but slower.
The fourth fi nding is that there was no signifi cant difference in intraindividual variability (CV) in the RTc, Vm, and S between men and women as well as between the left and right hands, but variability in the Va of the right hand both in women and men was signifi cantly greater compared to that of the left hand. It is well established that intraindividual variability in the reaction time is a reliable criterion of motor planning capacity (17)(18)(19)(20). Accuracy and speed of the nondominant hand have been found to decrease more than those of the dominant hand when the complexity of movement performance increases (26). We expected that right-handed people would have more diffi culties in controlling their movements with the left hand, and thus we believed that variability in the movement performance parameters of the left hand would be greater that that of the right hand. Other studies reported that men performed a speed-accuracy task more quickly and regularly than women (27). However, no difference in performance variability was found comparing men and women in our study.
The fi fth fi nding of study is that the RTc and Vm between the left and right hands during a bimanual speed-accuracy task were more consistent than the Va. It may be assumed that control of motor planning variables (RTc) as well as Vm is more synchronized than the Va. Moreover, it was rather strange that the control of accuracy between the right and left hands was synchronized only in men. Researchers have found that both the hands were well synchronized at the goal with a high intermanual correlation in reaching the goal (28,29). Thus, it has been concluded that there is an asymmetry in the feedback requirements of the two hand systems when accuracy is critical (10). Although according to the prevailing viewpoint bimanual coordination is assigned to a single brain locus, more recent evidence points to a distributed network that governs the processes of neural synchronization and desynchronization that underlie the rich variety of coordinated functions (30).
One limitation, however, is specifi c to bimanual reaching: whereas people typically look at the target location for a reaching movement, it is possible that two objects are simultaneously fi xated.

Conclusions
Though there was no signifi cant difference in the reaction time during a unimanual speed-accuracy task between the left and right hands as well as men and women, during a bimanual task, the reaction time of both the hands was signifi cantly longer in women than men. Besides, there was no signifi cant difference in the velocity of both the hands during a bimanual speed-accuracy task between men and women, while the accuracy of the left hand was signifi cantly greater in men than women. There was no signifi cant difference in intraindividual variability (CV) in the RTc, Vm, and S between men and women as well as between the left and right hands, but variability in the Va of the right hand in both women and men was signifi cantly greater compared to that of the left hand. The RTc and Vm between the left and right hands during a bimanual speed-accuracy task were more consistent than the Va. This methodology of movement studies can be successfully applied while testing motor function in neurological patients.