The relationship between abnormalities of saccadic and manual response times in Parkinson's disease

Clinicians normally use subjective rating scales to estimate the impairment of patients with Parkinson's disease (PD). More objective and quantitative methods of assessment would greatly aid our understanding of the disease. One promising approach is to measure reaction time: the large amount of data recorded in a short period provides precise, reproducible evaluation of the underlying neural decision processes. Manual evoked reaction times and repetitive tapping speed are often used, but differences of experimental design and analysis tend to obscure their interpretation. Saccadic latency, in many ways a simpler and more standardised task, is also normally affected in PD, but its relation to the kind of movement impairment that affects patients' quality of life is less obvious. OBJESTIVE: The aim of this study was to evaluate these tasks in detail and also see whether their use in combination could provide a better measure than each in isolation.

We compared three reaction time tasks: saccadometry, and evoked and spontaneous tapping, using protocols as similar as possible, correlating the measurements within a group of PD patients and of age-matched controls.

Surprisingly, manual and saccadic performance is uncorrelated in the normal population; but both are similarly affected by PD. The differences between groups are strengthened when the three measures are combined.

Saccadic latency can be regarded as an appropriate surrogate for more general kinds of motor impairment. The combination of saccadic and manual parameters enhances their potential use in quantifying disease status and evaluating treatments.