THE ORGANIZATION OF PATTERNS OF MULTILIMB COORDINATION AS REVEALED THROUGH REACTION-TIME MEASURES

Simple visual reaction time (RT) during the performance of sagittal mo vements of the upper and/or lower limbs was investigated. Experiment 1 demonstrated that RTs increased when more limbs were to be moved simu ltaneously. This effect was more apparent for the upper than for the l ower limbs. Experiment 2 allowed a separation of RT into premotor time (PMT) and motor time (MOT) components through analysis of electromyog raphic activity, and showed that these longer response delays were ass ociated with increased PMTs. This suggests that the time required for the central organization of movements increased as more limbs were to be controlled simultaneously. Compared to single-limb performance cond itions, the increases in RT were much larger in the upper limbs (up to 16%) than in the lower limbs (up to 5%) when limb segments were added . During single-limb conditions, RTs in the upper limbs tended to be s maller than in the lower limbs, in accordance with efferent nerve cond uction time estimates. Conversely, the lower limb(s) was (were) initia ted before the upper limb(s) when both effector types were moved simul taneously. This pattern of activation is reminiscent of the organizati on of postural control during upright standing, where goal-directed ar m activity is preceded by (bilateral) leg activity to anticipate for t he upcoming postural destabilization. Finally, hemifield manipulations in experiment 2 revealed faster RTs and PMTs for stimuli presented in the right visual field in comparison with the left field. This advant age was evident for ipsilateral as well as contralateral responses and supports the pre-eminence of the left hemisphere in the complex organ ization of gross motor responses.