Manual asymmetries in the preparation and control of goal-directed movements

The primary purpose of this experiment was to determine if left hand reaction time advantages in manual aiming result from a right hemisphere attentional advantage or an early right hemisphere role in movement preparation. Right-handed participants were required to either make rapid goal-directed movements to small targets or simply lift their hand upon target illumination. The amount of advance information about the target for a particular trial was manipulated by precuing a subset of potential targets prior to the reaction time interval. When participants were required to make aiming movements to targets in left space, the left hand enjoyed a reaction advantage that was not present for aiming in right space: or simple finger lifts. This advantage was independent of the amount or type of advance information provided by the precue. This finding supports the movement planning hypothesis. With respect to movement execution, participants completed their aiming movements more quickly when aiming with their right hand, particularly in right space. This right hand advantage in right space was due to the time required to decelerate the movement and to make feedback-based adjustments late in the movement trajectory. (C) 2001 Academic Press.

Evidence for on-line visual guidance during saccadic gaze shifts.

Rapid orientating movements of the eyes are believed to be controlled ballistically. The mechanism underlying this control is thought to involve a comparison between the desired displacement of the eye and an estimate of its actual position (obtained from the integration of the eye velocity signal). This study shows, however, that under certain circumstances fast gaze movements may be controlled quite differently and may involve mechanisms which use visual information to guide movements prospectively. Subjects were required to make large gaze shifts in yaw towards a target whose location and motion were unknown prior to movement onset. Six of those tested demonstrated remarkable accuracy when making gaze shifts towards a target that appeared during their ongoing movement. In fact their level of accuracy was not significantly different from that shown when they performed a 'remembered' gaze shift to a known stationary target (F-3,F-15 = 0.15, p > 0.05). The lack of a stereotypical relationship between the skew of the gaze velocity profile and movement duration indicates that on-line modifications were being made. It is suggested that a fast route from the retina to the superior colliculus could account for this behaviour and that models of oculomotor control need to be updated.