Timing in Perceptual and Motor Tasks in Parkinson’s Disease

Abstract

The symptomatic manifestations of Parkinson’s disease (PD) are mainly motor. The meaning and origin of these clinical motor symptoms are poorly understood at present. Adequate perception of the external world (time and space parameters) and of one’s body are required to achieve normal motor performance. Timing is a primary aspect of any movement since very few real life acts can be characterized by force attainment criteria independent of time. Deficits in timing are evident in most abnormalities reported in PD such as, increased reaction time and movement time, greater inter-onset latency in sequential movements and slowness in the performance of repetitive movements. There is some evidence indicative that the timing alteration of patients with PD may be not limited to the motor domain. Furthermore, motor disturbances may be either secondary to inadequate temporal perception (time discrimination and estimation) or to the dysfunction of the internal generator responsible to trigger the movement. Various aspects of timing were examined in 44 patients with PD. Compared to age-matched controls the patients had higher temporal discrimination thresholds (the minimal time interval required for a pair of stimuli to be felt separated in time) in the auditory, visual and somaesthetic sensory modalities. In time estimation and reproduction tasks, PD patients overestimated the duration of time intervals reproduced by internal counting compared to normals. The patients showed greater variability in the performance of repetitive alternating flexion-extension movements of the wrist at various frequencies. The results suggest that the timing deficits of patients with PD involve motor and perceptual tasks. The role of the internal time-keeper in these dysfunctions would be discussed.