Pattern adaptation affects the phase of the PERG

Purpose. The aim of our study was to evaluate the effect of pattern adaptation on the simultaneously recorded PERG and VEP. Methods. Each eye of 2 healthy subjects (1 female, 29, 1 male, 26) were tested 2-4 different times. ERG and VEP were recorded simultaneously by routine methods. Test stimuli were presented on a CRT display (10° of visual field at 130 cm viewing distance) as horizontal sinusoidal gratings reversed at 7.6 Hz. The spatial frequency of gratings was 4 cycles/degree, contrasts were 30%, 40% and 60%, respectively. Adaptation was performed by pre-exposure to either a blank screen of the same mean luminance or to a steady sinusoidal grating of identical spatial frequency and orientation with the test grating. Contrast values of adapting patterns were 0% (blank), 20% and 60%. After averaging the records, off-line artifact removal and fast Fourier transformation were performed. The amplitudes and phases of second harmonics (15.2 Hz) of both PERG and VEP were considered. Data from different eyes and times were averaged and compared by means of student's T-test. Results. 1) The VEP amplitude to any test contrast decreased to 60% adaptation. The PERG amplitude remained constant or might have slightly increased. 2) The VEP phase remained constant independently of the adapting contrast. The PERG phase on the other hand, shifted as a function of adapting contrast. The effect of adapting contrast was different for different test contrasts; it was least pronounced for 60% test. Conclusions. As opposed to psychophysical and VEP studies on pattern adaptation there are few and sometimes controversial PERG adaptation data reported Most studies evaluated only PERG amplitude. Our study demonstrates that pattern adaptation causes a significant phase shift of PERG. The phase of the simultaneously recorded VEP did not change suggesting the lack of passive coupling between PERG and VEP. Future studies should place emphasis on the possible and interesting connection between the PERG phase shift and VEP amplitude reduction elicited by pattern adaptation.