Parkinsonʹs disease changes the balance of onset and offset visual responses: an evoked potential study

Objectives: We investigated whether the transient pattern onset and offset visual evoked potential (VEP) can distinguish between patients with Parkinsonʹs disease (PD) and normal subjects. Methods: Two horizontal sinusoidal gratings differing in spatial frequency, i.e. 1 and 4 cycles per degree, were presented to 17 patients with PD and 16 age-matched control subjects. We analyzed the responses in the time-domain and measured the latencies and amplitudes of N1 and P1 to the onset and the offset of the stimulus; we also derived the measures of offset N1 and P1 amplitude responses ‘normalized’ to onset N1 and P1 amplitude values, respectively (amplitude ratios). Results: Absolute and normalized offset P1 amplitude is a distinguishing feature of PD patients from controls. Offset P1 amplitude was significantly larger in PD patients than in controls, particularly to the lower spatial frequency stimulus (P<0.01 for absolute and P<0.001 for normalized values, respectively). Conclusions: We conclude that the pattern onset/offset VEP amplitude provides a simple measure to evaluate visual processing deficits in PD and could contribute to an understanding of the pathophysiology of these changes.