Bimanual coordination encompasses a large class of situations in which the brain must simultaneously control multiple movements, such as when we use our two hands to manipulate an object or perform a task. Bimanual coordination has been one of the most widely studied problems in motor control. In my lab, we have re-evaluated this work, looking at bimanual coordination as a special case of dual-task performance. Movements of the two hands may be highly similar in terms of their underlying representations, and thus, might be expected to produce interference even after extensive practice according to the WM account described above. Indeed, dramatic interference can be observed with simple bimanual movements – consider the classic example of patting one’s head with one hand while rubbing one’s head with the other. Even simple (and more easily studied) movements, such as depressing the index finger of the left hand and the middle finger with the right hand can produce robust interference. Intriguingly, this combination of movements can be performed relatively effortlessly in real-world tasks, such as typing. However, in most experimental tasks huge costs are observed when individuals are required to make different keypresses with the two hands.
To examine this phenomenon, I investigated whether compatibility effects depended on the stimuli, the responses, or the way that individuals conceptualized their responses. Spatially compatible stimuli were paired with number stimuli to indicate keypresses with the left and right hands. Two groups of participants were used that differed only in terms of the mapping of the number stimuli; the mapping of the spatial stimuli was always the same and compatible. Although spatially compatible stimuli are thought to directly activate their corresponding responses, the pattern of bimanual interference for the spatial stimuli depended on the mapping of the number stimuli (Hazeltine, 2005). Even more striking, this effect was present even when no number stimuli were presented. These results indicate that this prevalent form of bimanual interference is mediated by central representations that are based on the way subjects conceptualize their responses rather than conflict between stimuli or between responses as has been assumed in previous theories.
Other bimanual phenomena offer clues about the structure of response selection processes as well. For example, making a pair of keypresses, one with a finger of one hand and one with a finger of the other, is usually performed more slowly than making two keypresses with the same hand (Hazeltine et al., 2007). However, as with the compatibility effects, this pattern is not observed during everyday tasks such as typing. The differences in bimanual costs may have to do with the extensive practice that typing movements receive, but they do not appear to stem from difficulty per se. The spatially compatible stimuli produce very similar reaction times to typing stimuli, yet the spatial stimuli produce robust bimanual costs and the typing stimuli do not. Bimanual costs were not related to the difficulty of the stimulus-response mapping but instead stemmed from overlap in the mappings for the responses of the two hands. We are presently using skilled typist to examine how practice changes the representations of actions. A theme of this research is that different stimuli used to signal the same responses can invoke very different response selection processes, and these processes are determined by the manner in which the task is conceptualized.
- Diedrichsen, J., Hazeltine, E., Kennerley, S. & Ivry, R. B. (2001). Absence of bimanual interference during directly-cued actions. Psychological Science, 12, 493-498.
- Kennerley, S., Diedrichsen, J., Hazeltine, E., Semjen, A., & Ivry, R. B. (2002). Callosotomy patients exhibit temporal uncoupling during continuous bimanual movements. Nature Neuroscience, 5, 376-381.
- Hazeltine, E. (2005). Response-response compatibility during bimanual movements: Evidence for the conceptual coding of action. Psychonomic Bulletin & Review, 12, 682-688.
- Albert, N., Weigelt, M., Hazeltine, E., Ivry, R. B. (2007). Target selection during bimanual reaching to direct cues is unaffected by the perceptual similarity of the targets. Journal of Experimental Psychology: Human Perception and Performance, 33, 1107-1116.
- Hazeltine, E., Weinstein, A, & Ivry, R. B. (2008). Parallel response selection after callosotomy. Journal of Cognitive Neuroscience, 20, 526-540.