Doing different things with the two hands at the same time, like rubbing your stomach whilepadding your head, is difficult. Drawing a circle with the left hand whiledrawing a square with the right hand is nearly impossible. Studying interference between the hands illuminates the "dark side" ofbimanual coordination. While the two hands normally benefit from "talking"to each other to achieve one common goal, this cross-talk is detrimental when we want each hands to perform independent actions.By finding the mechanisms through which the hands interfere with each other whenthey move independently, researchers hope to learn how the hands interact whenthey have to move coordinated.
One task that had been used extensively to study bimanual interference is continuous circle drawing.You can perform the self-experiment by drawing circles with both hands at a pretty high speed.Most likely you will prefer to draw circles in a mirror-symmetric fashion, i.e.one hand moves clockwise, the other counter-clockwise. The spatial quality ofcircles drawn symmetrically is higher than those drawn asymmetrically, with bothhands moving clockwise or counter-clockwise (Semjen et al., 1995). Also, whenyou try to increase the speed of asymmetric movements and go as fast as youpossibly can, you will most likely show some spontaneous transitions into symmetricperformance (Kelso, 1984). These all are indicationsfor your natural preference to move the two upper limbs in mirror symmetry.
Interactions on the spinal level
One hypothesis is that these interactions are happening very close to themotor output, i.e. on the level of the spinal cord. The idea here is that phylogenicolder behaviors, such as swimming or walking, have put into place thepreference for in-phase or anti-phase movements. While most of the fibers from theleft motor cortex go to the right hand, about 10% of all cortico-spinal fibers go uncrossedto the left hand. It is possible that these projections make it easier toperform mirror-symmetric movements (Carson, 2005). An extreme example of suchcross-talk can be observed in peoplewho are born with the "congenital mirror syndrome". These individuals probablyhave many moreuncrossed fibers, and when they perform an actions with one hand, the other handshows small involuntary twitches.
However, it is unlikely that the bimanual interference in normal individualsis caused by these fibers. This conclusion can be drawn from the study ofpatients who underwent a surgical removal of the corpus callosum, a massivefiber bundle connecting the two hemispheres. These patients are actually able todraw a circle and a square at the same time (Franz et al. 1996). In the bimanualcircle drawing task, callosotomy patients do not show a preference for symmetricmovements (Kennerley et al., 2003). They exhibit something much stranger: onehand can move at a different speed than the other hand, so that the hands lapeach other. This shows that fibers connecting the two hemisphere, rather thanuncrossed cortical-spinal fibers, are the main cause for the tendency to movethe upper limbs in symmetry.
Interactions of movement parameters or goal representations?
In a series of studies, Herbert Heuer and colleagues (e.g., Heuer, 1993) haveshown that many interference effects are "transient", that is they disappearwhen subjects have enough time to mentally prepare their movements . Such findings have ledto the hypothesis that cross-talk arises through interactions during movement planning,that is, when the parameters of the movements are specified. In researchconducted at the Cognition and Action Lab at University of California, Berkeley,we have focused on the notion that interferencebetween bimanual actions can depend heavily on theway the movements are instructed, and consequently, represented by theparticipant. For example, the time to initiate movements with differentmovement parameters is greater than the time to initiate symmetricmovements, but only when the movement are instructed with symbolic cues.When movements are specified by presenting the target locations directly,the movement parameters are not part of the action representation, andconsequently no interference between the two actions arises (Diedrichsenet al., 2001).
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Figure 1. The selection of bimanual movements can either be a direct consequence of the presentation of the targets (direct cueing), or can be based on symbolic cues indicating a movement parameter (symbolic cueing). Here the indicated parameter is movement amplitude with S for a short and L for a long movements. | Figure 2. The reaction time cost in the initiation of bimanual movements is restricted to a situation, in which movements are cued symbolically. When cued directly, movement initiation does not show any cost of incongruent movements (short-long). This holds true even compared to a situation in which only one of the movements has to be initiated (Diedrichsen et al., 2001). |
Movements of different amplitudes can sometimes be initiated even more quickly than movements of same amplitudes. This is the case, when the movements of different amplitude are directed to targets of the same color, and the movements of the same amplitude are directed to targets of different colors (Diedrichsen et al.,2003a).
Thus, we think that much of bimanual interference is based on a representation ofabstract actions codes.These action codes may encompass the features of the targets of reaching movements or the intendedconsequences of actions (Hommel, 2001). This interference can be found to be abolished following callosotomy(the surgical removal of the fiber-band that connects the two cerebral hemispheres). For example, whencallosotomy patients have to produce short force pulses with their left and right hand, they are as fastto initiate force pulses of the same as different target amplitudes. In contrast, age-matched controls showa substantial reaction time cost in this situation (Diedrichsen et al.,2003b).
In agreement with thisfinding, Mechsner et al. (2001) recently reported that bimanual interactions inthe continuous circle drawing task can be dramatically altered when theperceptual consequences of such movements are manipulated. Both findings arguethat the interference in this bimanual task is based rather on a more abstractrepresentation of the action than on low-level interactions between the motorimplementation processes of the two hands.
Another dramatic demonstration of the independence of the two hands comes from a study using online adjustments during bimanual reaching movements(Diedrichsen et al. 2004).Adjustments of hand trajectory occur very quickly (under 200 ms) when a target is displaced during a reaching movement.In the bimanual case, even when both targets are displaced in differentdirections, these rapid corrections are also observed and only delayed by 20 ms compared to unimanual reaches, with little change observed in movementvelocity and accuracy. These results suggest two independent "autopilots", ensuring that eachhand reaches its target location. These corrections do not require eye movements or shifts ofvisual attention towards the displaced target(s). However, when one hand adjustsfor a target displacement to the left (dashed) or to the right (gray), the otherhand is briefly perturbed in the same direction (see Figure). While the sourceof this cross-talk remains to be determined, the main insight is that theinterference in bimanual movements is strongly reduced or absent when wedirectly reach for targets or when other external stimuli help us representthese otherwise difficult movements. Thus, it is most likely the difficulty torepresent two conflicting spatial plans that limits our ability to pad our headwhile rubbing our stomach. Put simply, we shouldn't blame the hands for alimitation in our heads.
Neural mechanisms of interference
In recent work (Diedrichsen, in press) we studied neural correlated of interference inthe bimanual reaching task in functional magnetic resonance imaging (fMRI). Participants executedsymmetric or asymmetric reaching movements either cued by the spatially bypresenting the targets directly, or symbolically by presenting letters(F for forward, S for sideways). A large reaction time cost was again only observed when symbolic cueswere in conflict (respond to FS, compared to FF). This conflict causedincreased activity in PreSMA and the cingulate gyrus, the same areas that areactive when you try to perform cognitive interference tasks, such as theStroop-test.This supports our notion that bimanual interference occurs when we try to decidewhat to do with each hand, rather than how to do it.
However, both in the symbolic and spatial cueing conditions, we observedactivity in the posterior aspect of the superior parietal lobe, an area that isresponsible for spatial planning of movements. Even though participantscan perform bimanual movements in asymmetric directions rather effortlessly, theinterference occurring during the spatial planning of the movement left a neuraltrace.
References
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