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Balazs L. Varsanyi, Balazs V. Nagy, András Magyar, Agnes Farkas, Janos Nemeth; Clinical Verification of Input-Lag Correction for Comparison of pVEP signals acquired using CRT and TFT displays. Invest. Ophthalmol. Vis. Sci. 2012;53(14):5711.
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In our previous studies we have reported a significant delay of the visual evoked potential (VEP) responses when the stimulation was performed on TFT displays compared to the "gold standard" cathode ray tube (CRT) screens. In a normal control group, we have detected a 10 ms delay, which was in accordance with the "input lag" characteristic of the TFT display, measurable by simple photometric test. In this recent study, we aimed to check the clinical usefulness and reliability of these results in a patient cohort.
To record VEP signals a Roland RetiPort electrophysiological system was used. The pattern VEP tests were carried out according to ISCEV protocols using 1° and 15’ check sizes on a CRT and a TFT monitor consecutively. Achromatic checkerboard pattern was used at maximal different contrast level (97%); with maximal luminance of 106 cd/m2 and 96 cd/m, average luminance of 55 and 50 cd/m2, respectively. Both CRT and TFT displays were luminance and contrast matched, according to the gamma functions based on measurements at several DAC values. Spatial luminance inhomogeneities and other monitor specific luminance effects were measured by means of spectroradiometric instruments. Temporal differences between the displays’ electronic and radiometric signals were also measured .We tested 16 patients with monocular pathology (neuritis retrobulbaris, AION, etc) typically affecting the latencies of the VEP responses, especially P100. Visual acuity was 20/20 on the unaffected fellow eye and between 20/60 and 20/40 on the affected eye. The tests were repeated on each patient using both stimulators.
We found significant differences between the CRT and TFT stimulations at both affected and unaffected eyes. Implicit times were in average 11.8 ± 2.61 ms longer with LCD monitor (p<0,05).We found no significant difference between the delays observed in the affected or unaffected eyes.Using the input-lag correction obtained by photometric measurements (10.1 ms) for the responses of TFT stimulation, the two groups became highly comparable, no significant difference was present (mean: 1,2 ms, range: -2,8 - 3,2), either at affected or not affected eyes.
There are several electrophysiological systems available commercially. To compare their results usually control groups are required, due to the differences between display types. According to our results, using a photometric temporal signal measurement, VEP responses obtained on different monitors became comparable for both normal and affected subjects.
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