The problem of bimanual coupling: a reaction time study of simple unimanual and bimanual finger responses.

The properties of the sensorimotor system controlling finger movements were investigated in the simple uni- and bimanual reaction time (RT) paradigm, with emphasis on the problem of interhemispheric transfer of sensory and motor information. Unimanual and bimanual responses of the index fingers were elicited by stimulation of either left or right hand and resulting reaction times were compared to assess the degree of right-left differences and thus also of crossed-uncrossed differences (CUD). The response consisted of a force pulse (first dorsal interosseus muscle) which was elicited by a non-painful electrical stimulus applied to the base of the middle finger. In unimanual experiments, the population analysis showed that RTs obtained with contralateral stimuli were significantly longer (6 msec) than RTs elicited with ipsilateral stimuli. However, inter-subject differences were large and sometimes pointed in the non-expected direction (crossed < uncrossed). Statistically significant right-left differences in RT were detected in the bimanual response paradigm, but these differences occurred in both directions with the crossed RT either longer or shorter than uncrossed RT. The analysis of the correlation structure of bimanual RT suggested the presence of stimulus-related asymmetries of the hands. These observations provide some support for the notion of an additional processing time related to interhemispheric transmission of sensory and/or motor signals. In addition, it turned out that factors other than callosal transmission can also produce asymmetries in RTs of the two hands. Thus some subjects had consistent right-left differences which were unrelated to callosal transmission. Asymmetries were also introduced by changing the stimulation side. In the light of this multi-factorial influence, we argue that the underlying mechanisms leading to intermanual asymmetries in RT cannot be attributed exclusively to callosal transmission.