Human side preferences in three different whole-body movement tasks.

Animals turn away from the hemisphere with the more active dopamine (DA) system. For humans, a similar relationship has been assumed. However, results from independent studies were obtained from different tasks and indicated different side preferences. To investigate side preferences between different tasks within the same subject, we assessed in 36 healthy research participants (20 women) (1) long-term spontaneous turning (number of 360 degree turns during 20 h), (2) veering (lateral deviations during walking blindfolded straight forward) and (3) stepping (deviations while stepping blindfolded on a given spot) behavior. We observed a left-sided preference for long-term spontaneous turning behavior and no significant side preference for veering and stepping behavior. The absence of consistent side preferences suggests that DA does not equally control lateralized whole-body movements. We propose that visual control enhanced left-sided movement preferences, probably through an enhanced contribution of the right hemisphere to visuo-spatial behavior. Recently, we reported [Neurosci. Lett. 339 (2003) 115] that levodopa supplementation decreases right-sided veering tendencies, while stepping behavior was unaffected by substance intake. We suggest that veering tendencies, which appeared equally pronounced in either direction, are under dopaminergic control as long as attention is directed towards extrapersonal space. Side preferences in lateralized whole-body movement tasks are thus neither comparable between tasks nor within subjects. We conclude that experimental parameters such as visuo-spatial control and spatial task demands (veering is directed to extrapersonal space and stepping to peripersonal space) determine whether or not the DA system is involved.