THE PATTERN ELECTRORETINOGRAM IN PARKINSONS-DISEASE REVEALS LACK OF RETINAL SPATIAL TUNING

Spatio-temporal visual abnormalities, involving processing of medium c oarse stimuli, are known to occur in Parkinson's disease (PD). While t hese deficits have been related to retinal dopaminergic deficiency, pr evious ERG studies in PD patients have provided conflicting results, p robably due to differences in stimulus conditions. The influence of pa ttern element size (spatial frequency, SF) on the pattern electroretin ogram (PERG) in PD has not been systematically studied. We recorded st eady-state PERG to sinusoidal gratings of 50% contrast, counterphase m odulated at 7.5 Hz with a series of SFs ranging from 0.5 to 6.9 c/deg in 20 PD patients and 20 healthy volunteers, subdivided in 10 ''young' ' and 10 ''age-matched'' (AM) subjects. The PERG was analyzed by means of Fast Fourier Transform and the amplitude and the phase of the seco nd harmonic response (15 Hz) were taken into account. We evaluated the medium-to-low SF amplitude ratio and termed it ''PERG tuning ratio'' (TR). The results indicate that aging affects all the studied SF, but the pattern of age-related loss differs from that observed in PD. Comp ared to AM subjects, PD patients show a specific deficit at medium SF, with a distorted PERG SF response function. Consequently, all PD pati ents show an attenuated PERG TR and 17 of them (85%) have an inverted TR. A significant TR decrease is correlated with the clinical stage of PD. There is a marked TR difference between patients receiving and no t receiving L-DOPA. We conclude that stimulus SF is a crucial variable of the PERG in PD. PERG measurements and the derived PERG TR may prov ide a simple tool to evaluate retinal dopaminergic mechanisms and coul d contribute to the clinical assessment and monitoring of dopaminergic therapy in PD.