Visual-evoked potentials to onset of chromatic red-green and blue-yellow gratings in Parkinson's disease never treated with L-dopa

Abstract

The differential dysfunction of chromatic and achromatic visual pathways in early Parkinson's disease (PD) was evaluated by means of visual-evoked potentials (VEPs) recorded in 12 patients (mean age 60.1 +/- 8.3 years; range 46 to 74 years) in the early stages of PD and not yet undergoing treatment with L-dopa, and in 12 age-matched controls. Visual stimuli were full-field (14 deg) equiluminant red-green (R-G), blue-yellow (B-Y), and black-white (B-W) sinusoidal gratings of two cycles per degree, presented in onset (300 milliseconds)--offset (700 milliseconds) mode, at two contrast (K) levels (90% and 25%). The VEP mean latencies were significantly more delayed in PD patients than in controls for chromatic than for luminance stimuli, in particular for B-Y stimuli of low contrast (K90%: B-W = 6.6 milliseconds, R-G = 3.34 milliseconds, B-Y = 15.48 milliseconds; K25%: B-W = 7.8 milliseconds, R-G = 14.8 milliseconds, B-Y = 28.9). Latencies of chromatic VEPs were more variable that achromatic VEP latencies in both normal subjects and PD patients. Therefore, the frequency of latency abnormalities (within 30%) was not significantly different for the three visual stimuli. Our results show that, in addition to achromatic VEPs, chromatic VEPs are impaired in early PD patients not yet undergoing L-dopa therapy, indicating an acquired color deficiency in these patients. The greater delay for the B-Y VEPs suggests a higher vulnerability of visual blue-cone pathway in the early stages of the disease. However, the overall sensitivity of chromatic VEPs in detecting early visual impairment in PD is comparable with that of achromatic VEPs.

FIGURE 1

Representative examples of VEP waveform to onset (300 milliseconds) – offset (700 milliseconds) horizontal sinusoidal gratings (spatial frequency two cycles per degree, field size 14 ×16 degrees) with different levels of pure chromatic contrast (RG: red-green; BY: blue-yellow; BW: black-white); two traces are superimposed for each stimulating pattern to ensure reliability. The upper line show responses to 90% contrast and the lower line to 25% contrast for all the three types of stimuli. Note that chromatic responses are represented mainly by a negative-positive complex (N1-P1;) whose amplitude and latency change substantially with contrast. Vertical lines indicate upper normal limits using 2 SD. Positivity is up.

FIGURE 2

Amplitude versus latency scatterplots of chromatic VEPs, measured in individual eyes of PD patients. Filled circles indicate PD patients; empty circles indicate controls. It can be seen that scatter increases by reducing stimulus contrast, and that the data overlap considerably.