ANALYSIS OF ACTION TREMOR AND IMPAIRED CONTROL OF MOVEMENT VELOCITY IN MULTIPLE-SCLEROSIS DURING VISUALLY GUIDED WRIST-TRACKING TASKS

We investigated the relationship between action tremor (AT) and impair ed control of movement velocity (MV) in visually guided tracking tasks , in normal subjects and in patients with multiple sclerosis (MS) with or without motor deficits. The effects of withdrawing visual feedback of either the target or the cursor were then investigated. Visually c ued simple reaction times (SRTs) were also measured. The effects of th alamotomy on motor performance in these tasks were evaluated in seven patients. In the MS patients with tremor, there was no correlation bet ween AT and impairment in control of MV, but the latter was highly cor related with an increased delay in SRT. Withdrawal of visually guiding cues increased the error significantly in MV, but reduced AT by simil ar to 30% in magnitude. Frequency analysis indicated that the AT had t wo components: (a) non-visual-dependent. oscillatory movements, mainly at 4 Hz; and (2) visual-dependent, repetitive movements, with signifi cant power at 1-2 #z. Thalamotomy significantly reduced AT but hardly improved accuracy in MV. These results suggest that visual feedback of a spatial mismatch signal may provoke a visually dependent repetitive movement contributing to AT. Conduction delays along either the corti co-cerebello-cortical or the proprioceptive pathways and impaired work ing memory caused by MS may be responsible for the movement disorders in these patients. Key Words: Multiple sclerosis-Tremor-Movement veloc ity-Visual guidance-Tracking, wrist.