The availability of visual information affects the anticipatory postural control associated to reaching movements

Purpose: During goal-directed movements, eyes, head and limb are dynamically coordinated so as to look at and reach a given target. This study examined whether changes in eye-head-arm coordination affects Anticipatory Postural Adjustments (APA) in lower limb muscles when performing an arm reaching movement, toward a target of known position. Taking into account that eye and hand movements toward a target during simple pointing tasks are driven by a common command and that APAs and prime mover recruitment are controlled by a shared motor command, our working hypothesis was that the temporal organization of APAs would have been subordinated to the control of eye movements. Methods: Standing right-handed subjects (n=10) flexed their shoulder and reached, with the index-fingertip, a target placed in front of them, at shoulders height. Four conditions were studied: 1) steadily looking at the target while reaching (Visually-Guided Reaching); 2) starting with the head flexed, looking at the floor, Look and Reach the target; 3) as in 2, but Without Reaching and 4) keeping the head flexed until having reached the target (Blind Reaching). We recorded eye, head and arm movements, EMGs from upper- and lower-limb muscles and forces exerted on the ground. Results: In Look and Reach, two coordination strategies were found: trials when gaze shift preceded the arm muscle recruitment (Look-First) and when the opposite occurred (Arm-First). A three-way ANOVA found that APAs in leg muscles and ground forces started significantly earlier in Look-First vs. Arm-First, as it was in Visually-Guided vs. Blind Reaching (p<0.0005), i.e. when a visual information was searched for or already present before starting arm movements. The time-lag from Look- to Arm-First was not different from that measured between Visually- Guided and Blind Reaching (46.4±9.9ms vs. 42.5±9.1ms, p>0.6). Moreover, Without Reaching data witnessed that the head-induced postural actions did not affect the APA onset in Look-First and Arm-First. Conclusions: In conclusion, in Look-First the CNS considers that visual information about the target will be available when approaching it, like in Visually-Guided Reaching; instead, in Arm- First, the CNS does not rely on such information, like in Blind Reaching. Thus, it is likely that the vision availability influences the programming of the postural control during arm reaching movements.