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Department of Psychology, University of Western Ontario, London, Ontario, Canada
Submitted 28 March 2005; accepted in final form 17 May 2005
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ABSTRACT |
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INTRODUCTION |
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commented that two main themes have emerged (at the behavioral level) in the study of bimanual coordination in humans. First, when performing simultaneous symmetrical movements, the control of the two hands appears to be very similar. Second, when performing simultaneous asymmetrical movements, obvious interference arises between the control of the two hands. This interference often results in the supposedly different movements of each hand becoming similar and synchronized. The symmetry of movements is of course only one aspect that could differ across different bimanual tasks. The more general notion is that when different movements are required to be produced simultaneously, interference arises between those movements. Such interference can be related to the timing or the spatial aspects of movements. In the timing domain, for example, when simultaneous movements of different amplitudes have to be produced, the time taken for the movements tends to become similar even though differences exist when the same movements are performed alone (e.g., Kelso et al. 1979). Likewise, in the spatial domain, clear assimilation effects are observed when participants are asked to draw a circle with one hand, while simultaneously drawing a line with the other (Franz et al. 1991).
Much research has focused on trying to determine the source of such interference and why the brain finds it easier to produce and control symmetrical, identical movements. An important objective is to identify the various task demands and characteristics that modulate the ability to coordinate the hands in space and time. In the periodic bimanual coordination tasks that have been extensively studied in the laboratory, mirror symmetrical movements display both motor and spatial compatibility. For example, in a bimanual task involving movements made in the horizontal plane, mirror symmetry means two simultaneous, identical movements made toward the midline. In this situation, the hands move in the same direction with respect to the midline, and the movements are produced by the simultaneous activation of homologous muscles. Thus the fact that symmetrical bimanual movements are easier to perform could reflect a preference for the recruitment of homologous muscles (e.g., Riek et al. 1992) or the correspondence between the required movement directions of the two hands (e.g., Baldiserra et al. 1982). Even though the suggestion of a directional preference for bimanual movements has been made, the locus of this kind of effect has not yet been identified (and is likely to change across different task sets). That is, it is not entirely clear whether the preference for moving the limbs in the same (i.e., symmetrical) direction is a result of processing related to motor output or to the processing of sensory feedback about the state of the moving limbs. It is probable that many different aspects of a coordination task contribute to its difficulty. Indeed, it has previously been suggested that interference (or cross-talk) can arise at both the programming level and the execution level (Martiniuk and Mackenzie 1980; Martiniuk et al. 1984; Spijkers and Heuer 1995). The term "programming level" has typically been used to refer to the processes that specify particular parameters of a to-be-produced movement, such as amplitude and direction (Heuer et al. 2001). In fast, discrete tasks, it is possible that once specified, such signals remain relatively unchanged. Interference between such programming signals could be due to interactions between different neuronal populations that underlie movements made in particular directions or of different amplitudes (e.g., Laquaniti 1996; Laquaniti et al. 1995). The term "execution level" has typically been used to refer to motor outflow or efferent signals. Such signals are thought to be absent prior to movement initiation but evolve during the execution process (for details, see Spijkers and Heuer 2004). It has previously been suggested that interference between such execution signals might originate from uncrossed fibers in the pyramidal tract (e.g., Preilowski 1975).
Evidence for cross-talk at the level of motor programming comes from an experiment by Spijkers et al. (1997). In that study, subjects were required to make bimanual reversal movements of different amplitudes. The amplitudes were either 10 or 20 cm, and subjects were instructed by presenting a pair of precues at a variable time before an imperative cue to move. The precues comprised the German word for "short" in the case of the 10-cm amplitude movements and for "long" in the case of the 20-cm amplitude movements. Subjects performed bimanual reversal movements that were either identically precued (i.e., short-short) or not (i.e., short-long). The results showed that reaction time (RT) was longer when two different amplitude movements had to be performed at once as compared with when two same amplitude movements had to be performed at once. Furthermore, this interference effect seemed to be transient in nature because as the time between presentation of the precue and the imperative stimulus to move was increased from 0 to 750 ms, the increase in RT diminished. The authors suggested that this transient cross-talk occurs at the level of motor programming not the level of motor execution, arguing that execution-related interference would not be sensitive to differences in the time allowed for programming the responses. Hence, the more time allowed for a response to be prepared, the less evident is the interference between competing processes. However, there are at least two possible explanations for the results of Spijkers et al. On the one hand, the source of bimanual interference in a task such as theirs might indeed be the programming of the different movements. On the other hand, the source of the effect might be more related to the processing of incongruent visual cues. Because much of the work in the area of bimanual coordination uses continuous or oscillatory tasks, rather little is known about the locus of interference effects in noncontinuous tasks (Obhi 2004). However, recently, a study was conducted to further investigate this issue. In this study, Diedrichsen, Hazeltine, Kennerly, and Ivry (2001) used a bimanual task that was very similar to that used by Spijkers et al. First the authors demonstrated exactly the same effect as reported by Spijkers et al.; namely an RT interference effect for different bimanual movements when those movements were symbolically cued. However, and critically, these authors included an additional condition in their experiment in which the movements were directly cued by the visual presentation of the movement targets themselves and not by the presentation of symbolic cues. In such a condition, there is presumably no process whereby symbolic cues must be translated into appropriate movements. Interestingly, in this condition, the interference between the two different movements was completely abolished. The authors interpreted their results as showing that perceptual processing of symbolic cues is the dominant factor in the production of bimanual interference. In a second experiment, a similar result was also found for movements made in different directions. Similar results were obtained in another recent study (Hazeltine et al. 2003).
The aim of the present experiment was to further investigate the sources of interference in a symbolically cued bimanual RT task. In particular, we aimed to examine whether interference in a symbolically cued bimanual task can be accounted for purely by perceptual factors related to the processing of visual cues or whether processes related to motor preparation and execution also play a role. The experimental task required subjects to respond to visually presented cues with button presses. The required responses were varied in their spatial and motor compatibility, resulting in four sets of four experimental conditions of interest in which the two hands made the same movements (i.e., flexions or extensions) in the same directions (upward or downward), the same movements in different directions, different movements in the same direction, or different movements in different directions. Using this paradigm, it is possible to determine the roles of the spatial congruence of the movements and the programming- or execution-related motor congruence of the movements. If only spatial congruence contributes to interference in symbolically cued tasks, then interference should be present only when spatially incompatible movements are required, regardless of the motor relationship between the hands. In contrast, if both spatial and motor congruence are important, then both factors should contribute to bimanual interference effects.
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METHODS |
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Twelve right-handed participants from the undergraduate psychology program at the University of Western Ontario gave their informed consent and took part in the experiment, which was conducted in accordance with local ethical guidelines. Participants received academic credits for their participation in the experiment.
Apparatus, stimuli, and procedure
Participants were seated at a desk on which was placed a computer with two pairs of button boxes positioned below it as in Fig. 1. The required movements of the index fingers of the two hands were systematically varied in their spatial and motor congruence. Spatial congruence (movement direction) was manipulated by presenting pairs of letters, with one letter appearing on either side of a fixation cross on the computer screen. The letters specified the movement direction for each corresponding hand (i.e., the letter on the right of the fixation cross instructed a right hand movement and the letter on the left of the fixation cross instructed a left hand movement). The letter "D" instructed participants to make a downward response, and the letter "U" instructed participants to make an upward response. In this way, a spatially compatible movement combination was one in which both letters were the same (e.g., "D + D"). Motor congruence was manipulated by changing the position of the hands between palm-up and -down orientations. In this way, a motorically compatible movement combination was one in which homologous muscles were recruited simultaneously (e.g., in the case where both palms face down, both index fingers can flex downward or extend upward). Prior to each experimental block, participants positioned their index fingers in the gaps between the top and bottom buttons, which were then adjusted such that the top of the participant's finger nail was lightly touching the top button and the pad of their index finger was lightly touching the bottom button (or the other way around in the case of a palm-up posture). In this way, movement amplitude was equalized for both fingers and for both flexions and extensions. Furthermore, this setup placed minimal movement demands on participants as only very small movements were required to depress the buttons.
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For both bi- and unimanual movements, a quasi-mixed design was employed in which 40 experimental trials of each movement condition were randomly presented in blocks of 160 trials. For example, in the bimanual block in which participants oriented their hands with both palms facing down, 40 trials were collected using the stimulus set "D + D" (meaning flex both fingers down), 40 trials were collected using the stimulus set "U + D" (meaning extend left finger up and flex right finger down), 40 trials were collected using the stimulus set "U + U" (meaning extend both fingers up), and 40 trials were collected using the stimulus set "D + U" (meaning flex left finger down and extend right finger up). The orientation of the hands was held constant within a particular block and changed before the start of the next block. Specifically, for a particular block, the hands were placed in a left and right palm-down configuration, a left palm-up, right palm-down configuration, a left palm-up, right palm up configuration, or a right palm-up, left palm down configuration. In the case of unimanual trials, again taking the example in which both palms were facing down, participants underwent 40 trials in which the following stimulus sets were randomly presented: "T + D" (meaning flex right finger down, while keeping left hand still), "U + T" (meaning extend left finger up, while keeping right hand still), "T + U" (meaning extend right finger up, while keeping left hand still), and "D + T" (meaning flex left finger down, while keeping right hand still). In this way, for each of the four possible hand configurations RT data from bimanual and unimanual trials was obtained. The order of blocks was pseudo-randomly varied across subjects to avoid order effects, and subjects were given a short break between blocks to rest their fingers.
To reduce learning effects, the first 20 trials from all uni- and bimanual conditions were discarded, and only trials from the second half of each set of 40 trials were analyzed. Custom software and i/o hardware was used to acquire the RT data with a resolution of 1 ms. After the experiment, the trials were sorted into the four sets of four experimental conditions in which the same movements were made in the same direction (motor and spatial congruence; MCSC), the same movements were made in different directions (motor congruence and spatial incongruence; MCSI), different movements were made in the same direction (motor incongruence and spatial congruence; MISC), and different movements were made in different directions (motor and spatial incongruence; MISI). These conditions are illustrated in Table 1. The RT data were trimmed between 75 and 1,000 ms in the case of unimanual RT and 75 and 1,500 ms in the case of bimanual RT and stored on a PC for later analysis. The trimming procedure resulted in very few trials being excluded from the analysis (<10 trials across all subjects).
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RESULTS |
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The RT data were collapsed across the four experimental sets in Table 1 to form one set of data for each experimental condition (i.e., 1 set for each of; MCSC, MCSI, MISC, MISI). For both the bi- and unimanual movements from each experimental condition, the average RT and the range of RTs were calculated. This data is shown in Table 2.
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In the present experiment, participants performed bi- and unimanual finger movements in response to visual cues. As previously mentioned, this yielded four sets of four experimental conditions in which the left and right index fingers responded in the same direction with the same movements, different directions with the same movements, the same direction with different movements, or different directions with different movements.
RT data were recorded and stored for off-line analysis. In the analysis, instead of using raw bimanual RTs as a dependent measure, we calculated an index of performance that took into account the RTs of the unimanual control conditions. This was done to avoid any differences in basic unimanual motor performance from contributing to the results. Specifically, for each individual subject for each of the bimanual conditions, we calculated a dependent measure representing the average change in RT between the bimanual condition and the unimanual (control) conditions. This change in RT was given by
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RT is the average change in RT compared with the unimanual equivalent movements. RTRHB is the reaction time of the right hand in that bimanual condition, RTRHU is the reaction time of the same right-hand movement but in the unimanual condition (in which the same hand configuration was employed but only 1 response occurred), RTLHB is the reaction time of the left hand in that bimanual condition, RTLHU is the reaction time of the same left-hand movements but in the unimanual condition (in which the same hand configuration was employed but only 1 response occurred). All RTs are reported in milliseconds. In addition, the same formula was used to calculate a dependent measure reflecting the difference in the SDs of the RT between bi- and unimanual conditions. In this case, each mean RT measure in the formula was replaced with its own SD.
Main effects of motor and spatial factors on the mean change in RT (RT)
To establish the effects of motor and spatial congruence on the change in RT (RT), we collapsed the data across the four sets of four bimanual conditions to obtain a mean change in RT (RT) for MCSC, MCSI, MISC, and MISI conditions. We then subjected the data to a 2 (motor congruence or incongruence) x 2 (spatial congruence or incongruence) repeated-measures ANOVA. This test revealed that there was a significant main effect of spatial congruence on bimanual performance [F(1,11) = 35.593, P < 0.0001]. To determine the specific source(s) of the main effect of spatial congruence, follow-up corrected pairwise comparisons were conducted. These comparisons showed that, in particular, the MCSI condition produced larger changes in RT than either the MISC or the MCSC condition (P = 0.009 and P < 0.0001 respectively). Furthermore, the condition in which both motor and spatial incongruence existed (MISI) yielded significantly greater changes in RT than the condition in which both motor congruence and spatial congruence existed (MCSC; P < 0.0001). There was however, no significant difference between the condition in which both motor and spatial incongruence were present (MISI) and the MCSI condition. In contrast, there was no significant main effect of motor congruence [F(1,11) = 2.150, not significant] and no significant interaction [F(1,11) = 3.846, not significant].
Main effects of motor and spatial factors on the variability of the change in RT (RT)
A 2 x 2 repeated-measures ANOVA was conducted to establish the effects of motor and spatial congruence or incongruence on the variability of the change in RT (RT). There was a significant main effect of spatial congruence [F(1,11) = 24.605, P < 0.0001]. Corrected follow-up tests showed that the variability of RT was greater in the MCSI condition compared with the MCSC condition (P = 0.020) and the MISC condition (P = 0.019) but not the MISI condition. There was no significant main effect of motor congruence [F(1,11) = 0.011, not significant] and no significant interaction [F(1,11) = 1.139, not significant].
Comparison of change in RT (RT) to theoretical RT change of zero
DESCRIPTION OF ANALYSIS OF CHANGE IN RT (RT) FROM UNI- TO BIMANUAL CONDITIONS VERSUS THEORETICAL RT CHANGE OF ZERO.
So far we have examined the differences between the four experimental conditions and determined that the effects of spatial incongruence on RT are greater than the effects of motor incongruence on both the mean and the variability of the RT. However, we were also interested in examining the data with respect to how much the change in RT differed from a theoretical situation in which there is no RT difference between uni- and bimanual movements. The logic for this analysis is based on the idea that, because there is an RT cost associated with performing two movements instead of one (e.g., Corcos 1984; Ohtsuki 1994), the amount by which the bimanual RT differs from the unimanual RTs of its constituents offers a measure of the difficulty of a particular movement combination. In theory, if the RT of a bimanual movement combination is given by the average of its unimanual constituents, the average change in RT between identical uni- and bimanual movements should be zero. Therefore the amount by which the change in RT between uni- and bimanual conditions differs from zero is a valid measure of the processing load of a particular bimanual movement combination. To determine the difficulty of each bimanual movement pair, we conducted a series of Bonferroni-corrected pairwise comparisons in which we compared the change in average RT between bi- and unimanual conditions to zero. The difference in the amount of time required to initiate movements in a bimanual as opposed to unimanual condition is reflected in our dependent measure of RT. In our experiment, the only thing that differed between the uni- and bimanual conditions was the number of responses to be made (i.e., 1 or 2). The conditions were the same in terms of the configuration of the hands, the probability of each stimulus set appearing and the complexity of the stimuli. As such, the amount by which the RT change (RT) is different from zero reflects the difficulty of a particular movement combination. As in the main ANOVA presented in the preceding text, for the present analysis, the data were collapsed across the four sets of four bimanual conditions. Thereafter the difference between the mean change in RT (RT) for each of the conditions (MCSC, MCSI, MISC, and MISI) was contrasted with a theoretical difference of zero.
When two movements that were motorically and spatially congruent were performed (MCSC), the RT change was not significantly different from zero (P = 0.557). In contrast, in all the other bimanual conditions in which the fingers were required to make different movements, the RT change was significantly different from zero. Specifically, the RT change in the condition in which there was MCSI was significantly greater than zero (126.0 ± 67.9 ms, P < 0.0001) as was the RT change in the condition in which there was both MISI (109.8 ± 55.1 ms, P < 0.0001). These results reinforce the main effect of spatial congruence presented in the previous section. Importantly however, the RT change when the fingers had to make movements that were MISC was also significantly different from zero, suggesting at least some role for motor congruence in the performance of a symbolically cued bimanual RT task (54.0 ± 45.8 ms, P = 0.018).
Finally, the variability of the RT change was not significantly different from zero for any of the four experimental conditions (all tests P > 0.05).
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DISCUSSION |
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) in symbolically cued RT tasks. Indeed, although both the motor and spatial incongruence of movements resulted in changes in RT that were significantly different from zero, on average the RT difference between bimanual spatially incongruent movements and their unimanual constituents was more than twice that of motorically incongruent movements. Hence, it appears that the sensorimotor system finds it harder to prepare and produce spatially incongruent movements than to prepare and produce motorically incongruent movements. Furthermore, the effects of motor and spatial incongruence on the change in RT are not additive in the situation in which both types of incongruence exist. That is, in the condition in which there was both motor and spatial incongruence, the change in RT was not given by the sum of the RT changes for motor and spatial incongruence. In fact, the size of the RT change in the situation in which motor and spatial incongruence were present was very similar (i.e., not significantly different) to the magnitude of the change in RT when only spatial incongruence was present, further suggesting a dominant role for spatial constraints in these types of task. This finding suggests that the processing of spatial and motor aspects of to-be-produced movements may occur in parallel.
In the study by Spijkers et al. (1997), participants had to make bimanual reversal movements of different amplitudes that were instructed by precues that were presented at various times prior to the presentation of an imperative stimulus to move. The main result from that study was the reduction in bimanual interference effects (as indexed by the difference in RT between same amplitude and different amplitude movements) with increasing time between presentation of the precues and presentation of the imperative stimulus. The authors included a unimanual control condition in which the same stimulus sets were used but only one response was required. The meaning of the second stimulus was, however, different in the unimanual condition and served to indicate whether the response should be prepared or not (i.e., 2nd stimulus served as a go/no go signal). In the unimanual condition, a clear precue effect was also observed in which the RT decreased with increasing precue-imperative stimulus intervals. This suggests a clear role for stimulus processing mechanisms in experiments that employ symbolic cues to instruct movements. In the Spijkers et al. study, the same-different interference effect in the bimanual condition was more pronounced than in the equivalent unimanual condition, a result that was rightly interpreted as evidence that nonstimulus processing factors also contribute to the bimanual interference effect. The results from the present study are consistent with the idea that bimanual interference arises at the levels of stimulus processing and motor preparation. Hence, our results provide support for both the suggestion of a stimulus-processing locus of interference effects in symbolically cued tasks (Diedrichsen et al. 2001; Hazeltine et al. 2003) and the suggestion that interference arises between the structures involved in motor programming (Heuer 1986, 1993; Spijkers et al. 1997). The fact that in our experiment, motor incongruence also caused an increase in the RT change that was significantly different from zero further suggests that execution-related cross-talk also contributes to bimanual RT effects in symbolically cued tasks.
Although the present study demonstrates a dominance of spatial constraints over motor constraints on bimanual performance, it cannot distinguish between the possible sources of the spatial interference effect. For example, it is still unclear whether spatial interference arises at the level of stimulus processing or response preparation. In our task, participants had to respond to letters that appeared on either side of a fixation cross on a computer screen. The interference in such a task could occur at the stage of identifying and translating the stimuli into the appropriate movements; a process that could involve stimulus-stimulus congruence effects in which RT increases when different stimuli are presented together (De Houwer 2003). Alternatively, interference could arise at the stage of programming the actual responses during which cross-talk could occur between different neuronal populations that specify movements in each spatial direction (e.g., Georgeopolous 1993; Tipper et al. 1992).
In contrast to the spatial interference effect, the (smaller) motor interference effect cannot be due to the identification of different stimuli or the translation of different stimuli into particular movements because the stimuli used were identical and the movements only differed in terms of the muscles producing them. The motor interference effect must instead be due to system limitations related to the simultaneous activation of flexor and extensor muscles. This finding is important as it confirms the existence of constraints related to the programming and output of motor signals. Hence the present results do not support recent suggestions that the stability (or ease) of bimanual movements is completely determined by perceptual processes (e.g., Mechsner 2004; Mechsner et al. 2001). Indeed, flexion and extension movements and the neuromuscular processes that enable them have been extensively studied by Carson and colleagues (e.g., Carson 1996; Carson and Riek 1998). It is known that flexors are stronger than extensors and fewer motor units need to be innervated to produce a given level of force (Vallbo and Wessberg 1993). In the present study though, basic neuromuscular differences between flexion movements and extension movements cannot have contributed to the results because our dependent measure removed such differences by subtracting the RTs of the basic unimanual constituent movements from the corresponding bimanual RTs. It has also been demonstrated that when a particular movement is made, there is a phasic modulation of the homologous motor pathways in the opposite limb (Carson 2004). This modulation has been termed "cross-facilitation" and is very likely the mechanism that accounts for the fact that simultaneous recruitment of homologous muscles can occur faster than simultaneous recruitment of different muscles. In the present study though, this is only true for movements made in response to congruent visual stimuli. That is, when identical movements have to be made in response to incongruent visual stimuli instructing different directions for movement, the apparent preference for simultaneous activation of homologous muscles is overridden by spatial incongruence.
The question remaining to be answered relates to the source of the spatial interference effects observed in the present study. As previously mentioned, one possible level of processing at which such spatial interference might occur is the stimulus processing stage. In this stage, a stimulus is identified and then translated into the appropriate movement. At the stimulus identification level, stimulus-stimulus congruence effects could account for the observed interference. Such effects have been previously observed in the context of psychological refractory period experiments, in which responses made to briefly presented sequential stimuli are initiated faster when the stimuli that instructed them belonged to the same category (Logan and Schulkind 2000). Indeed, it is well known that processing is faster when a display consists of identical elements than when it does not (Posner 1978). Such an account could be considered as a bottom-up interference effect in which two simultaneously presented but different stimuli negatively interfere with each other and exert effects on the subsequent latency of the responses to them.
Indeed, evidence from a previous study by Obhi and Haggard (2004) is consistent with the idea that stimulus processing might be the major source of spatial interference in symbolically cued bimanual RT tasks. In their experiment, a similar movement task was employed as was used in the present study. However, in that task, responses were not symbolically cued, and participants simply responded with a particular bimanual combination of movements (upward or downward flexions or extensions of the index fingers) in response to an auditory stimulus. Hence there was no stimulus identification stage or stimulus translation stage in which a visual cue had to be processed to access a particular response. Critically, in their experiment, the main factor that affected bimanual performance was the motor congruence of the responses. The spatial relationship between the responding fingers seemed to play little if any role at all. Taken in conjunction with the present results this suggests that the source of spatial interference in bimanual RT tasks is most probably localized to the level of stimulus identification and translation. This claim is further bolstered by the fact that, when identical movements are made in response to incongruent cues (MCSI condition), performance is not significantly different from when different movements are made in response to incongruent cues (MISI condition).
Despite the fact that the likely source of spatial congruence effects in the present study is the stimulus-processing stage, it also important to consider the possibility that spatial interference might be due to cross-talk between different neuronal populations in the motor cortex. That is, we cannot be sure from the present study alone whether the spatial congruence effect is solely due to stimulus-processing mechanisms. It has previously been reported that there are groups of neurons in motor cortex that code for specific muscle activation and other groups of neurons that code for particular movement directions in external space. Furthermore, these direction-sensitive neurons have been shown to fire regardless of the muscles underlying the movements (Kakei et al. 1999). However, evidence against this possibility comes from the results of other studies that showed that when bimanual movements were made in different directions in response to direct cues, interference effects were completely abolished as compared with the situation when movements were symbolically cued (Deidrichsen et al. 2001; Hazeltine et al. 2003). If interference was taking place between neuronal populations in motor cortex, then presumably this would have been present both in the directly and the symbolically cued conditions. The fact that such interference was absent in the directly cued conditions supports the notion of a stimulus processing interpretation of spatial interference effects in symbolically cued bimanual RT tasks.
It is important to note the task specific nature of constraints in bimanual coordination. Clearly, the results from the initial study by Obhi and Haggard (2004) suggested a dominant role for motor congruence in the performance of rapid discrete bimanual actions. However, by changing the task to include a stimulus identification and translation stage, a very different picture emerges. Indeed, the idea of multiple task-dependent constraints is not new and has been repeatedly emphasized by prominent authors in the field of motor control and bimanual coordination (e.g., Cardoso de Oliveira 2004; Carson 2004; Carson and Kelso 2004; Heuer 2004).
In sum, we have shown that bimanual performance in a symbolically cued RT task is influenced by both the motor and the spatial demands of the specific task being performed. When flexion and extension movements have to be made together in time, initiation of the movements is slower than when flexion and flexion (or extension and extension) movements have to be made together in time. However, even when the same movements are made by both effectors, if they are made in different spatial directions, the RT cost is more than twice that of the situation in which different movements are made in the same direction. This strongly suggests that, in symbolically cued bimanual RT tasks, the sensorimotor system finds it more difficult to respond to incongruent stimuli than to produce bimanual responses that require the activation of different muscles. It must be emphasized that this may not be the case across different sets of bimanual tasks in which movement other than finger flexion and extension are required. It is probable that the source of the motor congruence effects in the present study is cross-facilitation in the corticospinal pathways. Last, the spatial congruence effects in the present study are in line with previous studies employing symbolically cued tasks and are likely due to difficulties associated with identifying incongruent stimuli and translating them into the appropriate movements.
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GRANTS |
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Present address and address for reprint requests and other correspondence: S. S. Obhi, Dept. of Psychology, Wilfrid Laurier University, Waterloo, Ontario, Canada (E-mail: sobhi{at}wlu.ca)
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ABSTRACT
INTRODUCTION
