Availability of visual information modulates timing of anticipatory postural adjustments during arm reaching movements

This study examined whether changes in eye-head-arm coordination affect Anticipatory Postural Adjustments (APAs) in lower limbs, when performing arm reaching movements toward a target of known position. Ten standing subjects reached a target placed in front of them with their index-fingertip. Four conditions were studied: 1) steadily looking at the target while reaching (Visually Guided, VG); 2) watching the floor with the head flexed, then Look and Reach the target (L&R); 3) Looking as in 2, but without Reaching (LnoR); 4) keeping the head flexed while reaching (Blind Reaching, BR). Firstly, subjects memorized the target position. Then they performed 15 reaching trials in VG, LnoR, BR, and 60 in L&R, in random order. Recorded variables were: arm, head and eyes movements, EMG from Anterior Deltoid (AD, prime mover) and legs postural muscles, forces exerted to the ground. Data from L&R trials were classified in two groups: those in which both the head and eyes movements preceded AD-EMG (look first) and those in which the AD preceded head and eyes (arm first). APAs in leg muscles and forces to the ground were more anticipated in look first than in arm first, as it was in VG than in BR (p<0.0005), i.e. when a visual information was searched for or already present before starting the arm movement. Data from LnoR trials excluded that the head extension by itself could have caused the APAs advance, confirming that this should be ascribed to the availability of visual information.