Impairments of movement execution in unilateral neglect: a kinematic analysis of directional bradykinesia.

A kinematic analysis was performed on goal-directed movements made by 14 patients with right hemisphere damage and left unilateral neglect (seven mild, seven severe), and 14 healthy controls. Leftward and rightward pen strokes of varying extents were made to targets of varying size on a centrally located digitising tablet. While mild unilateral neglect patients performed like controls, patients with severe unilateral neglect were slower to initiate leftward than rightward strokes and were slow and inefficient in movement execution. Leftward strokes made by severe unilateral neglect patients were characterised by prolonged movement time, lower peak velocity and departed from optimal bell-shaped velocity profiles. Their leftward strokes also showed prolonged accelerative phases, implying difficulties in force production, while the high proportion of their total movement time spent in decelerating with rightward strokes suggested an abnormal emphasis on terminal visual guidance. Leftward strokes made by these patients also contained more submovements than rightward strokes, suggesting poor force control. An impaired internal representation of the location of left-sided targets and desired movement trajectories in severe unilateral neglect causes breakdown in the temporal control of goal-directed movements.